# CellNaut - full text Complete English text of every lesson, illustrated topic and glossary term on https://cellnaut.com. Index: https://cellnaut.com/llms.txt --- # What Is a Cell? > Meet the basic unit of life, the three ideas of cell theory, and the big divide between prokaryotic and eukaryotic cells. Level: Introductory | Reading time: 10 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/what-is-a-cell **Quick answer:** A cell is the smallest unit of life that can take in energy, build and repair itself, respond to its surroundings and reproduce. Cell theory says all living things are made of cells, the cell is the basic unit of structure and function, and every cell comes from an existing cell. ## What you will learn - State the three parts of cell theory. - List the structures that every cell has in common. - Compare prokaryotic and eukaryotic cells. - Name three organelles found in eukaryotic cells and say what each does. ## Key takeaways - Cell theory has three parts: living things are made of cells, the cell is the basic unit of life, and all cells come from existing cells. - Every cell has a plasma membrane, cytoplasm, DNA and ribosomes. - Prokaryotic cells have no nucleus; eukaryotic cells keep their DNA in a nucleus and have membrane-bound organelles. - Viruses are not cells. ## The smallest unit of life Look at your hand, a leaf, or a drop of pond water through a strong enough microscope and you will see the same thing: tiny compartments packed side by side. These are cells. A cell is the smallest unit that can do everything we call living: take in energy, build and repair itself, respond to its surroundings, and reproduce. The word "cell" was coined by Robert Hooke in 1665, when he looked at a thin slice of cork and saw rows of small box-like chambers that reminded him of monks' rooms. Those were the empty walls of dead plant cells. Within a few decades, Antonie van Leeuwenhoek was watching living cells swim in water drops. ## Cell theory By the middle of the 1800s, a simple but powerful idea had taken shape. Today we call it the cell theory: 1. All living things are made of one or more cells. 2. The cell is the basic unit of structure and function in living things. 3. All cells come from other cells that already exist. The third point, usually credited to Rudolf Virchow, ended the old belief that living cells could appear out of non-living matter. Every cell in your body descends from a single fertilized egg, and that cell descends from cells in your parents, all the way back through billions of years. > **Note** > > Viruses are not cells. They cannot make their own energy or reproduce by themselves, which is why most biologists treat them as lying at the edge of the definition of life. ## What every cell has Bacteria, oak trees, and humans look nothing alike, yet all of their cells share four things: - A plasma membrane, the thin boundary that separates the inside of the cell from the outside. - Cytoplasm, the jelly-like interior in which chemical reactions take place. - DNA, the molecule that stores the instructions for building and running the cell. - Ribosomes, the molecular machines that build proteins. Having these four parts in common is one of the strongest pieces of evidence that all life shares a common ancestor. ## Two kinds of cells Cells fall into two big groups, based on how they store their DNA. A prokaryote such as a bacterium has no nucleus. Its DNA sits in a region of the cytoplasm called the nucleoid, which is not wrapped in a membrane. Prokaryotic cells are small, typically 1 to 5 µm across, and most have a cell wall made of peptidoglycan. Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/nucleoid) - A bacterium. Drag to rotate, then pick a structure below the model. Interactive 3D model: [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria/nucleoid) - A cyanobacterium is also a prokaryote: its DNA floats free too, but it carries membranes for photosynthesis. A eukaryote has a true nucleus, plus other internal compartments called organelles, each wrapped in its own membrane. Animals, plants, fungi, and protists are all eukaryotes. Their cells are usually 10 to 100 µm across, around ten times wider than a typical bacterium. | | Prokaryotic cell | Eukaryotic cell | | ------------------------- | -------------------------- | ---------------------------------------- | | Nucleus | No, DNA is in the nucleoid | Yes, DNA is inside a nuclear envelope | | Membrane-bound organelles | None | Many (mitochondria, Golgi, ER, and more) | | Typical size | 1 to 5 µm | 10 to 100 µm | | Examples | Bacteria, archaea | Animals, plants, fungi, protists | > **Common misconception** > > **"Prokaryotes have no organelles."** Prokaryotes do have ribosomes and often have specialized regions inside the cell. What they lack are **membrane-bound** organelles. That is a more precise, and more accurate, statement. **Check your understanding:** Which structure is found in both prokaryotic and eukaryotic cells? - A. Nucleus - B. Ribosomes - C. Mitochondria - D. Golgi apparatus **Answer:** B. Ribosomes Ribosomes build proteins in every kind of cell. The other three are membrane-bound organelles, which prokaryotes do not have. ## A tour of a eukaryotic cell An animal cell is a good place to start exploring. The nucleus stores most of the cell's DNA. Mitochondria convert fuel into usable energy. The Golgi apparatus packages proteins for delivery. Click through the labels in the viewer to see where each sits. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - An animal cell, the standard eukaryotic example. Interactive 3D model: [Yeast Cell](https://cellnaut.com/studio/yeast/nucleus) - Yeast is a single cell, yet it is a eukaryote with a true nucleus. Want a labelled picture? Compare [animal and plant cells](https://cellnaut.com/topics/animal-vs-plant-cell) or [prokaryotic and eukaryotic cells](https://cellnaut.com/topics/prokaryote-vs-eukaryote) side by side in the illustrated topics. The rest of this course takes these organelles one at a time. Before that, a question that sounds simple: just how big is a cell? That is the subject of the next lesson. **Check your understanding:** Where is the DNA of a bacterium located? - A. In a nucleus - B. In the nucleoid region of the cytoplasm - C. In the cell wall - D. In mitochondria **Answer:** B. In the nucleoid region of the cytoplasm Bacteria have no nucleus. Their DNA is concentrated in the nucleoid, a region with no membrane around it. **Check your understanding:** According to cell theory, where do new cells come from? - A. From non-living matter - B. From existing cells - C. From viruses - D. From the nucleus only **Answer:** B. From existing cells The third part of cell theory states that all cells arise from pre-existing cells. ## Sources - [Biology 2e, Chapter 4: Cell Structure (OpenStax)](https://openstax.org/books/biology-2e/pages/4-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [NIH 3D: Animal cell model](https://3d.nih.gov/entries/3DPX-015797/2) - See entry page and docs/ASSETS.md --- # Cell Size and Scale > Learn the units of the microscopic world and why most cells stay small, with a little geometry. Level: Introductory | Reading time: 9 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/cell-size-and-scale **Quick answer:** Most cells are only 1 to 100 micrometres across because a cell takes in nutrients and gets rid of waste through its surface, and as a cell grows its volume rises faster than its surface area. A small cell has a higher surface-area-to-volume ratio, and diffusion across it is quick. ## What you will learn - Convert between millimetres, micrometres, and nanometres. - Place a virus, a bacterium, an animal cell, and a human egg on a size scale. - Explain why a small cell has a higher surface-area-to-volume ratio. - Describe two ways cells overcome the limits of size. ## Key takeaways - 1 millimetre is 1,000 micrometres, and 1 micrometre is 1,000 nanometres. - Typical bacteria are 1 to 5 µm across and typical eukaryotic cells 10 to 100 µm. - Doubling a cell's side length cuts its surface-area-to-volume ratio in half. - Cells beat the size limit by folding their surface, becoming long and thin, or using internal membranes. ## Units for the very small Everyday units such as the metre or centimetre are far too large to talk about cells. Biologists use three smaller ones: | Unit | Symbol | Size | Example | | ---------- | ------ | ------------------------------ | ----------------------------------- | | Millimetre | mm | one thousandth of a metre | A grain of sand is about 1 mm | | Micrometre | µm | one thousandth of a millimetre | A red blood cell is about 7 to 8 µm | | Nanometre | nm | one thousandth of a micrometre | A ribosome is about 25 nm | Each step down is a factor of 1,000. So 1 mm = 1,000 µm, and 1 µm = 1,000 nm. ## A map of the microscopic world Here are some typical sizes, from largest to smallest. The values are rounded, because real cells vary. - **Human egg cell:** about 100 µm. This is just visible to the naked eye as a speck. - **Typical animal cell:** 10 to 30 µm. - **Red blood cell:** 7 to 8 µm. - **Mitochondrion:** 1 to 10 µm long. - **Typical bacterium:** 1 to 5 µm. - **Virus:** roughly 20 to 300 nm, much smaller than most cells. - **Ribosome:** about 25 nm. - **DNA double helix:** about 2 nm wide. A useful anchor: you could line up about ten average animal cells across the width of a human hair, which is around 70 to 100 µm thick. Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/flagellum) - A bacterium is often only 1 to 5 µm long, around one tenth the width of an animal cell. ## Some cells are giants A few cells break the rules by being long, not wide. - A single **neuron** can have an axon that reaches over a metre in length, running from the base of the spine to the foot. The cell body is still only tens of micrometres wide. - A **skeletal muscle fiber** can grow to many centimetres long. It manages this by fusing together many cells, so one fiber contains hundreds of nuclei. Interactive 3D model: [Neuron](https://cellnaut.com/studio/neuron/axon) - A neuron: tiny cell body, extremely long axon. Notice what these cells have in common. They are long and thin, so every part of the cell stays close to a surface. That brings us to the key reason cells are small. ## Why most cells stay small A cell takes in food and oxygen, and gets rid of waste, through its surface. The amount of material it needs depends on its volume, the amount of living stuff inside. As a cell grows, its volume grows faster than its surface. To see this, imagine a cell as a cube: | Side length | Surface area (6 × side²) | Volume (side³) | Surface-area-to-volume ratio | | ----------- | ------------------------ | -------------- | ---------------------------- | | 1 µm | 6 µm² | 1 µm³ | 6 | | 2 µm | 24 µm² | 8 µm³ | 3 | | 10 µm | 600 µm² | 1,000 µm³ | 0.6 | Doubling the side makes the surface-area-to-volume ratio fall by half. A large cell has proportionally less membrane to feed each part of its interior. There is a second problem. Molecules spread inside a cell by diffusion, and diffusion is quick over short distances but slow over long ones. The time it takes grows with the **square** of the distance. If you make a cell ten times wider, diffusion across it takes about a hundred times longer. > **Common misconception** > > **"Bigger animals have bigger cells."** Not really. An elephant's cells are about the same size as a mouse's. Bigger animals have **more** cells, not larger ones. **Check your understanding:** A cube-shaped cell doubles its side length. What happens to its surface-area-to-volume ratio? - A. It doubles - B. It stays the same - C. It falls by half - D. It becomes zero **Answer:** C. It falls by half Surface area grows by 4 times but volume by 8 times, so the ratio halves (from 6 to 3 in the table above). ## How cells beat the limit Cells that need a lot of surface have found clever tricks. - **Folding the surface.** Cells lining the small intestine have thousands of finger-like microvilli that multiply the area available for absorbing nutrients. - **Becoming long and thin.** Neurons and muscle fibers keep a high ratio by stretching out in one direction. - **Internal membranes.** Organelles such as the endoplasmic reticulum and mitochondria pack huge membrane area into small spaces. Interactive 3D model: [Epithelial Cell](https://cellnaut.com/studio/epithelial/microvilli) - Epithelial cells use microvilli to expand their absorbing surface. **Check your understanding:** What is the main benefit of microvilli on intestinal cells? - A. They move the cell - B. They store DNA - C. They increase surface area for absorption - D. They make the cell wall **Answer:** C. They increase surface area for absorption Microvilli fold the membrane into many tiny projections, greatly increasing the surface area available for absorbing nutrients. **Check your understanding:** How many micrometres are there in 1 millimetre? - A. 10 - B. 100 - C. 1,000 - D. 1,000,000 **Answer:** C. 1,000 A micrometre is one thousandth of a millimetre, so 1 mm = 1,000 µm. ## Sources - [Biology 2e, Chapter 4: Cell Structure (OpenStax)](https://openstax.org/books/biology-2e/pages/4-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [BioNumbers, the database of useful biological numbers](https://bionumbers.hms.harvard.edu/) - Numerical facts cited for reference; no text or tables reproduced - [NIH 3D: Neuron model](https://3d.nih.gov/entries/3DPX-015796/2) - See entry page and docs/ASSETS.md --- # The Plasma Membrane and Transport > How a membrane only a few molecules thick controls what enters and leaves, from simple diffusion to pumps and vesicles. Level: High school | Reading time: 12 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/plasma-membrane-and-transport **Quick answer:** The plasma membrane is a fluid double layer of phospholipids with embedded proteins that lets a cell control what enters and leaves. Water and small molecules cross by passive transport without energy, pumps use ATP for active transport, and vesicles carry large cargo in and out. ## What you will learn - Describe the fluid mosaic model of the plasma membrane. - Distinguish passive transport from active transport. - Predict which way water moves when a cell is placed in a salty or pure-water solution. - Explain how endocytosis and exocytosis move large cargo. ## Key takeaways - The fluid mosaic model describes proteins floating in and across a flexible phospholipid bilayer. - Passive transport (diffusion, osmosis, facilitated diffusion) moves substances down their gradient and needs no energy. - Active transport uses energy, usually ATP, to move substances against their gradient. - Water moves by osmosis toward the side with more dissolved solute; endocytosis and exocytosis move large cargo in vesicles. ## A boundary that thinks Every cell is wrapped in a plasma membrane only about 7 to 10 nm thick, thinner than a thousandth of the width of a human hair. Yet it does a remarkable job. It keeps the cell's contents in, lets nutrients and signals through, and keeps the wrong things out. ## The fluid mosaic model The membrane is built from phospholipids. Each has a **head** that likes water and two **tails** that avoid it. In water, phospholipids spontaneously arrange themselves into a double layer, with the heads facing the watery inside and outside of the cell and the tails hidden in the middle. Proteins are embedded in this double layer. Some are channels, some are carriers, some are receptors, and some anchor the cell to its neighbors. Because the lipids and many proteins can drift sideways, the membrane behaves like a flexible film, not a rigid wall. This is the fluid mosaic model. In animal cells, cholesterol tucked between the phospholipids keeps the membrane from becoming too stiff in the cold or too runny when warm. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - Every structure you see here sits inside a plasma membrane. Interactive 3D model: [Red Blood Cell](https://cellnaut.com/studio/redBlood/spectrin) - A red blood cell's membrane is reinforced from the inside by a protein skeleton. The center of the membrane is oily, so it is easy for small, uncharged molecules such as oxygen and carbon dioxide to slip through, and hard for ions and large polar molecules to cross. This property is called selective permeability. ## Passive transport: no energy needed Passive transport moves substances down their concentration gradient, from where there is more to where there is less. The cell does not spend energy, because the movement is driven by the random motion of the particles themselves. - **Simple diffusion.** Small, nonpolar molecules such as O₂ and CO₂ slide directly through the lipid layer. - **Facilitated diffusion.** Ions and polar molecules such as glucose cross through channel or carrier proteins. Still no ATP is needed, because they travel down their gradient. - **Osmosis.** Water moves across the membrane toward the side that has more dissolved particles. ### Osmosis in real cells Whether a cell swells or shrinks depends on how its surroundings compare with its inside. | Solution outside the cell | Water movement | Animal cell | Plant cell | | -------------------------------------- | --------------- | --------------------- | ----------------------------------------------------- | | Less solute than the cell (hypotonic) | Into the cell | Swells, and may burst | Becomes firm and turgid, which is healthy | | Same solute as the cell (isotonic) | No net change | Stays normal | Slightly limp | | More solute than the cell (hypertonic) | Out of the cell | Shrivels | Membrane pulls away from the wall and the plant wilts | A plant cell's cell wall stops it from bursting. The pressure of water pushing out against the wall is called turgor pressure, and it is what keeps lettuce crisp and stems upright. The big central vacuole is where much of that water is stored. > **Common misconception** > > **"Osmosis moves the dissolved particles."** In osmosis it is the **water** that moves across the membrane, toward the side with more dissolved solutes. The solutes often cannot cross the membrane at all. Interactive 3D model: [Guard Cell](https://cellnaut.com/studio/guard/stoma) - Guard cells use osmosis as a valve: when water flows in they swell and the pore opens. Interactive 3D model: [Paramecium](https://cellnaut.com/studio/paramecium/contractileVacuole) - A freshwater Paramecium has to pump out the water that osmosis keeps pushing in. **Check your understanding:** A red blood cell is placed in pure water. What is most likely to happen? - A. It shrinks - B. It swells and may burst - C. Nothing, the membrane blocks water - D. It makes a cell wall **Answer:** B. It swells and may burst Pure water has less solute than the inside of the cell, so water flows in by osmosis. An animal cell has no wall to resist, so it can swell until it bursts. ## Active transport: working against the gradient Sometimes a cell must move something **uphill**, toward the side where it is already more concentrated. That takes energy. In active transport, a protein pump uses ATP to push the substance across. The classic example is the sodium-potassium pump, found in nearly all animal cells. For each ATP it uses, it moves 3 sodium ions out and 2 potassium ions in. In a resting neuron, this pump alone uses a large share of the cell's energy supply. It keeps the electrical charge across the membrane that nerve signals depend on. Interactive 3D model: [Neuron](https://cellnaut.com/studio/neuron/axon) - Neurons rely on pumps in the membrane to keep the charge needed for signals. ## Moving big cargo: vesicles Proteins, bacteria, and other large particles cannot squeeze through membrane proteins. Cells move them in bubbles of membrane called vesicles. - Endocytosis brings material in. The membrane folds around the cargo, then pinches off as a vesicle. A white blood cell uses a form of endocytosis called phagocytosis to engulf bacteria. - Exocytosis sends material out. A vesicle fuses with the membrane and empties its contents outside, which is how neurons release signals and how cells secrete hormones. Interactive 3D model: [White Blood Cell](https://cellnaut.com/studio/whiteBlood/lysosome) - White blood cells swallow invaders by endocytosis, then digest them. **Check your understanding:** Which kind of transport directly requires energy from ATP? - A. Simple diffusion - B. Osmosis - C. Facilitated diffusion - D. Active transport **Answer:** D. Active transport Active transport moves substances against their gradient and is powered by ATP. The other three all move substances down a gradient. **Check your understanding:** Why can oxygen pass through the membrane without help from a protein? - A. It is a small, nonpolar molecule - B. It is positively charged - C. It is very large - D. ATP pushes it through **Answer:** A. It is a small, nonpolar molecule The oily core of the membrane lets small, uncharged molecules such as O₂ dissolve in and slip across. ## Sources - [Biology 2e, Chapter 5: Structure and Function of Plasma Membranes (OpenStax)](https://openstax.org/books/biology-2e/pages/5-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. --- # The Nucleus and the Endomembrane System > Follow a protein from the gene that codes for it, through the ER and Golgi, to its destination, and meet the lysosome clean-up crew. Level: High school | Reading time: 12 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/nucleus-and-endomembrane **Quick answer:** The nucleus stores the cell's DNA and controls gene activity, while the endomembrane system builds, folds, sorts and ships proteins and lipids. A secreted protein is made on the rough ER, modified and packaged in the Golgi apparatus, and sent to the cell surface in vesicles. Lysosomes digest worn-out material. ## What you will learn - Describe the structure of the nucleus and the role of the nuclear pores. - Distinguish rough ER from smooth ER. - Trace the path a secreted protein takes through the cell. - Explain what lysosomes do and why white blood cells rely on them. ## Key takeaways - The nuclear envelope has pores that control traffic between the nucleus and the cytoplasm. - Rough ER carries ribosomes and makes proteins; smooth ER makes lipids and helps detoxify. - Secreted proteins travel ER, then Golgi, then a vesicle, then the plasma membrane. - Lysosomes hold digestive enzymes, which is why white blood cells use them to destroy engulfed microbes. ## The nucleus: the genome's vault The nucleus is usually the largest organelle in an animal cell, about 5 to 10 µm across. It holds nearly all of the cell's DNA, keeps it safe, and controls when each gene is used. Three features are worth knowing: - The nuclear envelope is a double membrane. It is dotted with thousands of **nuclear pores**, channels that let messenger molecules out and regulatory proteins in, while keeping the DNA itself inside. - Inside, the DNA is wound around proteins to form chromatin. When a cell is not dividing, chromatin is spread out so genes can be read. - A dense region called the **nucleolus** assembles the parts of ribosomes. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - The nucleus of an animal cell. For a labelled diagram of these parts, open the topic [The Cell Nucleus](https://cellnaut.com/topics/nucleus). > **Common misconception** > > **"Every cell has a nucleus."** Prokaryotes do not, as you saw in lesson 1. Even within the human body, mature red blood cells push out their nucleus to make more room for oxygen-carrying hemoglobin. ## The endomembrane system Most proteins and lipids the cell exports, or builds into its membranes, go through a connected network of compartments called the endomembrane system. Think of it as a factory with a post office attached. ### Endoplasmic reticulum: the factory floor The endoplasmic reticulum (ER) is a maze of flattened sacs and tubes that is directly joined to the nuclear envelope. It comes in two types: - **Rough ER** is studded with ribosomes. Proteins made here are threaded into the ER interior as they are built. It is especially large in cells that secrete a lot of protein. - **Smooth ER** has no ribosomes. It makes lipids, helps break down toxins in liver cells, and stores calcium ions. ### Golgi apparatus: sorting and shipping Proteins leave the ER wrapped in small vesicles and travel to the Golgi apparatus, a stack of flattened membrane sacs. Inside, enzymes trim, tag, and modify them, such as by adding sugar chains. At the far face, the finished products are packaged into new vesicles with a destination label. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/golgi) - The Golgi apparatus sits close to the nucleus. ### A protein's journey Here is the route a protein takes when the cell is going to secrete it, for example an antibody or a digestive enzyme: 1. The gene is copied into messenger RNA in the nucleus. 2. The mRNA leaves through a nuclear pore, and a ribosome on the rough ER builds the protein. 3. A transport vesicle carries the protein to the Golgi. 4. The Golgi modifies and sorts it into a secretory vesicle. 5. The vesicle moves to the plasma membrane and releases its contents by exocytosis. **Check your understanding:** Where are proteins that will be exported from the cell first made? - A. Ribosomes on the rough ER - B. Smooth ER - C. Lysosomes - D. The nucleolus **Answer:** A. Ribosomes on the rough ER Ribosomes attached to the rough ER build proteins destined for secretion or for the cell membrane. **Check your understanding:** Which organelle modifies, sorts, and packages proteins into vesicles for delivery? - A. Mitochondrion - B. Golgi apparatus - C. Nucleus - D. Ribosome **Answer:** B. Golgi apparatus The Golgi apparatus receives proteins from the ER, modifies them, and sends them to their destinations. ## Lysosomes: the recycling center A lysosome is a small, acidic vesicle filled with about 50 different digestive enzymes. It does two jobs: it digests material the cell has taken in, and it recycles the cell's own worn-out parts. The acid inside is a safety feature. The enzymes only work well at low pH, so if a lysosome leaks into the neutral cytoplasm, they are mostly inactive and the cell is not digested from within. White blood cells make heavy use of lysosomes. A neutrophil engulfs a bacterium in a vesicle, which then fuses with lysosomes. The invader is broken down inside a sealed compartment, where it cannot harm the cell. Interactive 3D model: [White Blood Cell](https://cellnaut.com/studio/whiteBlood/lysosome) - A white blood cell, with lysosomes and a lobed nucleus. **Check your understanding:** Why can lysosomes digest material without harming the rest of the cell? - A. Their enzymes work best in an acidic compartment enclosed by a membrane - B. They only digest water - C. They sit inside the nucleus - D. They have no membrane **Answer:** A. Their enzymes work best in an acidic compartment enclosed by a membrane Lysosomal enzymes need acidic conditions, and the lysosome keeps them sealed away from the cytoplasm. ## Sources - [Biology 2e, Chapter 4: Cell Structure (OpenStax)](https://openstax.org/books/biology-2e/pages/4-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [NIH 3D: Animal cell model](https://3d.nih.gov/entries/3DPX-015797/2) - See entry page and docs/ASSETS.md --- # Mitochondria and Chloroplasts > How cells make ATP and capture sunlight, and why these two organelles look like bacteria that moved in. Level: High school | Reading time: 13 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/energy-organelles **Quick answer:** Mitochondria release usable energy by breaking down food molecules to make ATP, and chloroplasts capture sunlight to build sugar by photosynthesis. Both have double membranes, their own DNA and ribosomes, and divide on their own, which supports the endosymbiotic theory that they descend from engulfed bacteria. ## What you will learn - Explain the role of ATP in the cell. - Describe the structure of a mitochondrion and what it produces. - Describe the structure of a chloroplast and what it produces. - List the evidence for the endosymbiotic origin of both organelles. ## Key takeaways - ATP is the cell's energy currency: cells spend it to power work and rebuild it from food or light. - A mitochondrion's inner membrane is folded into cristae, which increases the surface available for making ATP. - Chloroplasts stack thylakoids into grana and turn light energy into chemical energy stored in sugar. - Evidence for endosymbiosis: their own circular DNA, bacteria-like ribosomes, double membranes and division by splitting. ## ATP: the energy currency Cells constantly need energy to move molecules, build proteins, and contract muscles. They pay for most of this with a single molecule, ATP. When a phosphate group is cut off from ATP, a small packet of energy is released and used to drive a reaction. The cell then rebuilds ATP from the leftover pieces. A working human cell recycles its whole stock of ATP in a minute or so, and an adult turns over an amount of ATP close to their own body weight in a day. Two organelles are responsible for most of the ATP supply in eukaryotic cells: mitochondria, which release energy from food, and chloroplasts, which capture it from sunlight. ## Mitochondria: powerhouses with a double wall A mitochondrion (plural: mitochondria) is a bean-shaped organelle 1 to 10 µm long, with two membranes. The smooth outer membrane encloses the organelle. The inner membrane is folded into shelves called **cristae**, which multiply the surface area on which ATP is made. The space inside is the **matrix**. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/mitochondrion) - A mitochondrion in an animal cell. Mitochondria carry out most of cellular respiration. In summary: > glucose + oxygen → carbon dioxide + water + ATP First, glucose is split into smaller pieces in the cytoplasm. These pieces are then broken down completely inside the mitochondrion, and the energy released is used to pump protons across the inner membrane. The protons flow back through a molecular turbine called ATP synthase, and as they do, the turbine's rotation joins phosphate onto ADP to make ATP. One glucose molecule can yield about 30 ATP. Cells with high energy demands have many mitochondria. A muscle fiber can hold thousands, packed between its contractile filaments. Endurance training increases their number. Interactive 3D model: [Muscle Cell](https://cellnaut.com/studio/muscle/mitochondria) - Muscle cells need many mitochondria for a steady ATP supply. **Check your understanding:** Which organelle makes most of the ATP in an animal cell? - A. Golgi apparatus - B. Lysosome - C. Mitochondrion - D. Nucleus **Answer:** C. Mitochondrion Mitochondria carry out most of cellular respiration and produce the bulk of a cell's ATP. ## Chloroplasts: solar panels with stacks A chloroplast is a lens-shaped organelle, typically 3 to 10 µm long, found in plant and algal cells. Like a mitochondrion it has a double membrane, but it also contains a third membrane system: flattened sacs called thylakoids, stacked into piles called **grana**. The thylakoid membranes contain the green pigment chlorophyll. The fluid around them is the **stroma**. Photosynthesis happens in two stages: 1. **Light reactions** (thylakoid membranes): chlorophyll absorbs light, splits water, releases oxygen, and produces ATP and an energy-carrying molecule. 2. **Calvin cycle** (stroma): the ATP and energy carriers are used to build sugar from carbon dioxide. > carbon dioxide + water + light → glucose + oxygen ![Stylized 3D illustration of a chloroplast showing stacked thylakoids, with numbered markers.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/chloroplast-photosynthesis-teaching.jpg) *A stylized teaching illustration of a chloroplast. Credit: Project-generated teaching illustration (AI-generated; simplified)* [Open the interactive, labelled version](https://cellnaut.com/topics/chloroplast-photosynthesis) Interactive 3D model: [Plant Cell](https://cellnaut.com/studio/plant/chloroplast) - A plant cell with chloroplasts, a central vacuole, and a cell wall. Interactive 3D model: [Chlamydomonas](https://cellnaut.com/studio/chlamydomonas/chloroplast) - Chlamydomonas is a single-celled alga with one large, cup-shaped chloroplast. > **Common misconception** > > **"Plants do photosynthesis, and animals do respiration."** Plant cells have mitochondria too, and they carry out cellular respiration all the time. Photosynthesis makes the sugar; respiration is how plants release its energy as ATP, day and night. ## Two organelles that were once bacteria Mitochondria and chloroplasts are strange organelles. They have their own DNA, their own ribosomes, and a double membrane, and they divide on their own by splitting in two. These are bacterial traits. The endosymbiotic theory explains why: more than a billion years ago, an ancestral cell engulfed an oxygen-using bacterium, but instead of digesting it, the two began to live together. The bacterium became the mitochondrion. Later, a similar partnership with a photosynthetic bacterium gave rise to the chloroplast. The evidence: - Both organelles have a **circular DNA genome**, like bacteria. - Their **ribosomes** are more similar to bacterial ribosomes than to the ones in the cytoplasm. - They are **surrounded by two membranes**, consistent with an engulfed cell. - They reproduce by a process like **binary fission**. - Gene sequences place them close to certain groups of living bacteria. Interactive 3D model: [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria/thylakoids) - A cyanobacterium photosynthesizes with thylakoid membranes but has no chloroplast at all. **Check your understanding:** Which observation supports the idea that mitochondria were once free-living bacteria? - A. They have their own circular DNA - B. They are found in every prokaryote - C. They contain chlorophyll - D. They have no membranes **Answer:** A. They have their own circular DNA Having its own circular DNA and bacteria-like ribosomes is strong evidence that mitochondria descend from engulfed bacteria. **Check your understanding:** Which statement about plant cells is correct? - A. They have chloroplasts but no mitochondria - B. They have both chloroplasts and mitochondria - C. They have mitochondria but no chloroplasts - D. They have neither **Answer:** B. They have both chloroplasts and mitochondria Most plant cells that photosynthesize have both organelles. Chloroplasts make sugar; mitochondria use it to make ATP. ## Sources - [Biology 2e, Chapter 7: Cellular Respiration (OpenStax)](https://openstax.org/books/biology-2e/pages/7-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Biology 2e, Chapter 8: Photosynthesis (OpenStax)](https://openstax.org/books/biology-2e/pages/8-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Project teaching illustration: chloroplast photosynthesis (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset) --- # The Cytoskeleton and Cell Movement > The protein scaffolding that gives cells their shape, moves cargo, and powers everything from crawling immune cells to flexing muscles. Level: High school | Reading time: 11 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/cytoskeleton-and-movement **Quick answer:** The cytoskeleton is a network of protein filaments that gives a cell its shape, moves cargo and powers movement. Microtubules form transport tracks, actin filaments work with myosin to drive crawling and muscle contraction, and intermediate filaments give mechanical strength. ## What you will learn - Name the three types of cytoskeletal filaments and give one job for each. - Explain how microtubules act as tracks for transport. - Describe how actin and myosin shorten a sarcomere. - Contrast bacterial flagella with eukaryotic cilia and flagella. ## Key takeaways - The three filament types are microtubules, actin filaments (microfilaments) and intermediate filaments. - Motor proteins such as kinesin and dynein walk along microtubules carrying vesicles. - A muscle contracts when myosin pulls actin filaments past each other and shortens the sarcomere. - Bacterial flagella are rotating protein filaments, while eukaryotic cilia and flagella are built from microtubules and bend to beat. ## A skeleton that rebuilds itself Your skeleton gives your body shape, but it never changes on the timescale of minutes. The cytoskeleton is different. It is a network of protein filaments that gives a cell its shape and holds organelles in place, and it can be taken apart and reassembled in seconds. That is how cells crawl, divide, and change shape. There are three main types of filament. | Filament | Width | Built from | Main jobs | | --------------------- | ----------- | ---------------------------- | ------------------------------------------------------- | | Actin filament | about 7 nm | actin | Shape of the cell surface, crawling, muscle contraction | | Intermediate filament | 8 to 12 nm | keratin and related proteins | Mechanical strength; anchoring the nucleus | | Microtubule | about 25 nm | tubulin | Transport tracks, cell division, cilia and flagella | ## Microtubules: highways inside the cell A microtubule is a hollow tube assembled from tubulin proteins. In most animal cells it radiates out from a region near the nucleus, like the spokes of a wheel. Motor proteins use these tubes as roads. A motor called kinesin walks toward the cell edge carrying vesicles, while dynein walks the other way. Each step is powered by one ATP. In a neuron, this "axonal transport" carries cargo along an axon that can be a metre long. Interactive 3D model: [Neuron](https://cellnaut.com/studio/neuron/axon) - In neurons, microtubules carry cargo along the whole length of the axon. During cell division, microtubules also form the spindle that pulls chromosomes apart, which you will see in lesson 8. **Check your understanding:** Which cytoskeletal filament forms the spindle that separates chromosomes? - A. Actin filaments - B. Intermediate filaments - C. Microtubules - D. Phospholipids **Answer:** C. Microtubules The mitotic spindle is built from microtubules. ## Actin and muscle contraction Actin filaments sit just under the plasma membrane, giving the cell its outline. In crawling cells such as white blood cells, new actin filaments grow at the front edge and push the membrane forward. Actin does its most dramatic work in muscle. A muscle fiber is packed with myofibrils, thread-like bundles that run its entire length. Each myofibril is a chain of repeating units called sarcomeres. In a sarcomere, thin actin filaments overlap with thick filaments made of the motor protein myosin. When a nerve signal arrives, calcium is released inside the fiber. Myosin heads grab the actin filaments, bend, and let go, again and again, using ATP at every cycle. The effect is that the actin filaments slide inward past the myosin ones, the sarcomere shortens, and the muscle contracts. This is the sliding filament model. Interactive 3D model: [Muscle Cell](https://cellnaut.com/studio/muscle/myofibril) - Striped myofibrils in a muscle fiber. The stripes are the repeating sarcomeres. > **Common misconception** > > **"Muscle filaments get shorter when a muscle contracts."** The filaments keep the same length. They **slide** over each other, so the sarcomere as a whole becomes shorter. **Check your understanding:** What happens to actin and myosin filaments when a sarcomere shortens? - A. Both filaments shrink - B. They slide past each other - C. They break and reform - D. Myosin turns into actin **Answer:** B. They slide past each other In the sliding filament model, the filaments keep their length and slide past each other. ## Intermediate filaments: cellular rope Intermediate filaments are the toughest and least dynamic. They twist together into rope-like fibers that hold up well under tension. Skin cells are filled with keratin, the protein of hair and nails. Epithelial cells also use intermediate filaments to anchor into junctions between neighbors, so that a sheet of cells can resist being stretched or torn. Interactive 3D model: [Epithelial Cell](https://cellnaut.com/studio/epithelial/junctions) - Epithelial cells link their internal ropes across junctions to act as one sheet. ## Swimming and sweeping Cells can also move by beating extensions of their surface. - A cilium is a short projection with a core of microtubules. Cilia usually occur in large numbers and beat in waves. The cells lining your airways have cilia that sweep mucus and trapped dust up and out of your lungs. - A flagellum is longer and usually occurs singly or in pairs. A sperm cell is propelled by a eukaryotic flagellum, which bends in a whip-like motion. Bacterial flagella look similar in a picture but work in a completely different way. A bacterial flagellum is made of a protein called flagellin and spins around like a propeller, driven by a tiny rotary motor in the cell wall. The motor is powered by the flow of ions across the membrane, not by ATP directly. Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/flagellum) - A bacterial flagellum is a rotary motor, not a beating whip. Interactive 3D model: [Sperm Cell](https://cellnaut.com/studio/sperm/flagellum) - A sperm tail is a eukaryotic flagellum: a bundle of microtubules that slide against each other to make it bend. Interactive 3D model: [Paramecium](https://cellnaut.com/studio/paramecium/cilia) - A Paramecium is covered in cilia that beat in coordinated waves. **Check your understanding:** How does a bacterial flagellum move a cell? - A. It bends back and forth like a sperm tail - B. It rotates like a propeller - C. It contracts like a muscle - D. It pulls on the nucleus **Answer:** B. It rotates like a propeller Bacterial flagella rotate. This is different from the bending motion of eukaryotic flagella. ## Sources - [Biology 2e, Chapter 4: Cell Structure (OpenStax)](https://openstax.org/books/biology-2e/pages/4-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. --- # From DNA to Protein > How a gene stored in DNA is copied into RNA and then read by a ribosome to build a protein. Level: High school | Reading time: 14 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/dna-to-protein **Quick answer:** A gene is a stretch of DNA that codes for a protein or a functional RNA. In transcription, RNA polymerase copies the gene into mRNA; in translation, a ribosome reads the mRNA three bases (one codon) at a time and joins amino acids into a protein. ## What you will learn - Describe the structure of DNA and what a gene is. - Explain transcription and where it takes place. - Use a codon table idea to read a short mRNA message. - Explain why all the cells in your body can have the same DNA but different proteins. ## Key takeaways - DNA is a double helix built from four bases (A, T, G, C); RNA uses U in place of T. - The central dogma runs DNA to RNA to protein. - Each codon of three bases specifies one amino acid or a stop signal. - Cells differ because they switch on different genes, not because they contain different DNA. ## The cell's instruction manual Everything a cell does depends on proteins: they build structures, speed up reactions, pump ions, and send signals. The instructions for every protein are stored in DNA. DNA is a double helix, like a twisted ladder. Each rung is a pair of chemical bases: **A** always pairs with **T**, and **G** always pairs with **C**. The order of bases along one strand is a code, much like the order of letters in a sentence. A gene is a stretch of this code that contains the recipe for a particular RNA or protein. A human cell holds around 20,000 protein-coding genes spread across 46 chromosomes. ![Stylized 3D illustration of a DNA replication fork with the double helix unwinding.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/dna-replication-fork-teaching.jpg) *A stylized illustration of DNA being unwound and copied. Replication happens before cell division; the same double helix is what gets read during transcription. Credit: Project-generated teaching illustration (AI-generated; simplified)* [Open the interactive, labelled version](https://cellnaut.com/topics/dna-replication-fork) ## The central dogma Information moves in one main direction inside cells: > DNA → RNA → protein The two steps are called transcription and translation. ## Step 1: transcription Transcription copies a gene into a molecule of RNA. An enzyme called RNA polymerase attaches at the start of the gene, unzips a short section of the double helix, and builds a matching RNA strand using one DNA strand as a template. RNA differs from DNA in three ways: it is single-stranded, it uses the sugar ribose, and it uses the base **U** (uracil) in place of T. The result is a messenger RNA (mRNA), a portable copy of the gene. In eukaryotic cells transcription takes place in the nucleus. The new mRNA is processed (non-coding sections are cut out, a protective cap and a tail are added) and then leaves through a nuclear pore. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - Transcription happens in the nucleus; the mRNA then travels out to the cytoplasm. ## Step 2: translation In translation, a ribosome grabs the mRNA and reads it three letters at a time. Each three-letter group is a codon, and each codon specifies one amino acid. Adapter molecules called transfer RNAs (tRNAs) bring in the right amino acid for each codon. The ribosome joins the amino acids into a chain, which folds into a finished protein. The genetic code has a few special codons: - **AUG** is the start codon and codes for the amino acid methionine. - **UAA, UAG, and UGA** are stop codons. They signal the ribosome to release the finished chain. There are 64 possible codons but only 20 amino acids, so several codons mean the same thing. The code is nearly universal: bacteria, plants, and humans all read it the same way. ### A tiny worked example Suppose a gene is transcribed into this message: > AUG GCU UAC UAA Reading it three letters at a time, the ribosome starts at the start codon AUG (methionine), adds the amino acids that GCU (alanine) and UAC (tyrosine) specify, and stops at UAA. The result is a very short chain: Met – Ala – Tyr. > **Note** > > In bacteria there is no nucleus, so translation can begin while the mRNA is still being made. In eukaryotic cells the two steps are separated in space and time, which gives the cell an extra chance to process the message. Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/nucleoid) - In a bacterium, DNA lies in the open cytoplasm, so ribosomes can start reading the message as soon as it appears. **Check your understanding:** What is the product of transcription? - A. A protein - B. A messenger RNA - C. A new DNA molecule - D. A ribosome **Answer:** B. A messenger RNA Transcription copies a gene's DNA sequence into messenger RNA. Translation then turns that mRNA into a protein. **Check your understanding:** How many nucleotides make up one codon? - A. 1 - B. 2 - C. 3 - D. 4 **Answer:** C. 3 A codon is a group of three nucleotides, and each codon specifies one amino acid or a stop signal. ## One genome, many cell types > **Common misconception** > > **"A muscle cell and a neuron have different DNA."** Nearly every cell in your body carries the same genome. What differs is **which genes are transcribed**. A neuron makes proteins for neurotransmitter receptors, and a muscle cell makes large amounts of myosin, using the same instruction manual with different pages opened. This idea, selective gene expression, is the key to understanding how a single fertilized egg can build hundreds of cell types. You will return to it in lesson 10. **Check your understanding:** Where does translation take place? - A. On the ribosome - B. Inside the nucleolus only - C. On the cell wall - D. Inside the Golgi apparatus **Answer:** A. On the ribosome Ribosomes read mRNA and assemble amino acids into proteins. ## Sources - [Biology 2e, Chapter 14: DNA Structure and Function (OpenStax)](https://openstax.org/books/biology-2e/pages/14-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Biology 2e, Chapter 15: Genes and Proteins (OpenStax)](https://openstax.org/books/biology-2e/pages/15-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Project teaching illustration: DNA replication fork (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset) --- # The Cell Cycle and Mitosis > How a cell copies its DNA, divides it equally between two daughters, and what happens when the controls fail. Level: High school | Reading time: 13 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/cell-division **Quick answer:** The cell cycle runs from interphase (G1, S and G2), when the cell grows and copies its DNA, through mitosis and cytokinesis, which split it into two identical daughter cells. Mitosis has four stages in order: prophase, metaphase, anaphase and telophase. ## What you will learn - List the phases of the cell cycle and say what happens in each. - Name the four stages of mitosis in order. - Compare cytokinesis in animal and plant cells. - Explain how mutations in cell-cycle control can lead to cancer. ## Key takeaways - Interphase has three parts: G1 growth, S phase DNA replication and G2 preparation for division. - The four stages of mitosis are prophase, metaphase, anaphase and telophase. - Animal cells divide by pinching in a cleavage furrow; plant cells build a cell plate. - Checkpoints stop damaged cells from dividing, and mutations in the genes that control them can lead to cancer. ## Why cells divide Your body makes millions of new cells every second. They replace those that wear out, repair wounds, and, in a growing child, increase the total number. Each new cell must receive a full and accurate copy of the DNA. The cell does this through a repeating sequence of events called the cell cycle. ## The phases of the cell cycle Most of a cell's life is spent in interphase, the long stretch between divisions. Interphase has three parts. | Phase | Name | What happens | | ----- | ---------- | ------------------------------------------------------------------- | | G1 | First gap | The cell grows and does its normal work. | | S | Synthesis | The DNA is copied, so each chromosome is duplicated. | | G2 | Second gap | The cell grows more, checks the copied DNA, and prepares to divide. | After interphase comes the M phase, in which the cell divides. M phase has two jobs: mitosis, which splits up the nucleus, and cytokinesis, which splits up the rest of the cell. A fast-dividing human cell in culture may complete the full cycle in about a day, with mitosis itself taking around an hour. Other cells, such as most neurons, stop cycling altogether. > **Common misconception** > > **"The DNA is copied during mitosis."** DNA copying happens earlier, in the S phase of interphase. By the time mitosis begins, each chromosome already consists of two identical copies, called sister chromatids, joined together. ## The stages of mitosis Mitosis is a continuous process, but it is easier to understand as four stages. 1. **Prophase.** The chromatin condenses into visible, X-shaped chromosomes. The nuclear envelope breaks down, and a spindle made of microtubules begins to form. 2. **Metaphase.** The chromosomes line up along the middle of the cell, each attached to spindle fibers from both poles. 3. **Anaphase.** The sister chromatids are pulled apart toward opposite ends of the cell. Each is now an independent chromosome. 4. **Telophase.** Two new nuclear envelopes form around the two sets of chromosomes, and the chromosomes relax back into chromatin. The outcome: two nuclei with identical sets of chromosomes. A human cell starts with 46 chromosomes, and each of the two daughter nuclei ends with 46. ![Stylized 3D illustration of a plant root tip in which many cells show chromosomes at different stages of mitosis.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/root-tip-mitosis-teaching.jpg) *A stylized plant root tip, a classic place to find cells in every stage of mitosis at once. Credit: Project-generated teaching illustration (AI-generated; simplified)* [Open the interactive, labelled version](https://cellnaut.com/topics/root-tip-mitosis) **Check your understanding:** During which phase of the cell cycle is DNA copied? - A. G1 - B. S phase - C. Metaphase - D. Cytokinesis **Answer:** B. S phase The S (synthesis) phase of interphase is when each chromosome is duplicated. **Check your understanding:** What happens during anaphase? - A. DNA is copied - B. Sister chromatids are pulled to opposite poles - C. The nuclear envelope reforms - D. The cell wall forms **Answer:** B. Sister chromatids are pulled to opposite poles In anaphase the spindle pulls the sister chromatids apart toward opposite ends of the cell. ## Dividing the cytoplasm Cytokinesis differs between plants and animals. - In an **animal cell**, a ring of actin and myosin around the middle tightens like a drawstring, pinching the cell in two. The dent is called the cleavage furrow. - In a **plant cell**, the rigid wall prevents pinching. Instead, vesicles gather in the middle and fuse to build a new wall from the inside out. This is the cell plate. Interactive 3D model: [Plant Cell](https://cellnaut.com/studio/plant/cellWall) - A plant cell builds a new wall between its daughters. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - An animal cell pinches in two. Its nucleus has to divide first. Interactive 3D model: [Stem Cell](https://cellnaut.com/studio/stem/nucleus) - Stem cells keep dividing, and can also leave the cycle to become specialized. Interactive 3D model: [Yeast Cell](https://cellnaut.com/studio/yeast/bud) - Yeast divides by budding, an unequal division that makes a small daughter cell. ## Checkpoints and cancer Division is risky, so the cycle has built-in checkpoints, where proteins check whether the cell is big enough, the DNA is undamaged, and the chromosomes are properly attached to the spindle. If something is wrong, the cycle halts for repair or the cell is instructed to self-destruct. Cancer develops when mutations break these controls. Two kinds of genes are especially important. Genes that act like an accelerator pedal can become stuck in the "on" position, and genes that act like a brake, such as the well-known p53 gene, can be lost. A cell that has accumulated several such changes ignores the stop signals and divides without control. It is not a single change, but a build-up over time, which is why the risk rises with age. ## Meiosis in one paragraph Cells that make eggs and sperm divide differently, through a process called meiosis. It has two rounds of division and produces four cells, each with half the number of chromosomes. When egg and sperm fuse, the full number of 46 is restored in the fertilized egg. Mitosis keeps the chromosome number the same; meiosis halves it. **Check your understanding:** Why is cancer described as a disease of lost cell-cycle control? - A. Cancer cells have no DNA - B. Mutations let cells ignore checkpoints and keep dividing - C. Cancer cells skip interphase and never grow - D. Cancer cells only divide by meiosis **Answer:** B. Mutations let cells ignore checkpoints and keep dividing Mutations in genes that regulate the cell cycle let cells bypass the normal checkpoints and divide when they should not. ## Sources - [Biology 2e, Chapter 10: Cell Reproduction (OpenStax)](https://openstax.org/books/biology-2e/pages/10-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Biology 2e, Chapter 11: Meiosis and Sexual Reproduction (OpenStax)](https://openstax.org/books/biology-2e/pages/11-introduction) - CC BY-NC-SA 4.0. Used for the short comparison with meiosis. - [Project teaching illustration: root-tip mitosis (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset) --- # Cell Communication and the Neuron > How cells send, receive, and respond to signals, and how a nerve cell passes a message across a synapse. Level: High school | Reading time: 12 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/cell-communication **Quick answer:** Cells communicate by releasing signal molecules that bind receptors on target cells, which turn the signal into a response. A neuron sends an electrical action potential along its axon, then releases neurotransmitters across the synapse to pass the message to the next cell. ## What you will learn - Describe the three stages of cell signaling: reception, transduction, and response. - Explain why only some cells respond to a given signal. - Describe how an action potential travels along an axon. - Outline the steps of chemical transmission at a synapse. ## Key takeaways - Only cells that have the matching receptor respond to a given signal. - Signaling has three steps: receptor, transducer and response. - An action potential is a wave of ion channels opening that travels along the axon without fading. - At the synapse, calcium entry makes vesicles release neurotransmitter, which crosses the gap and binds receptors on the next cell. ## Cells talk to each other A single cell in a multicellular body needs to know what its neighbors are doing. It must learn when to divide, when to release a hormone, when to contract, and when to die. It does so by exchanging chemical and electrical signals. Signals can travel in several ways: - **Direct contact.** Neighboring cells exchange small molecules through channels in their membranes (gap junctions in animals, and plasmodesmata in plants). - **Local signaling.** A cell releases molecules that act on cells nearby. - **Long-distance signaling.** Hormones are carried in the blood to targets across the body. - **Synaptic signaling.** Neurons release neurotransmitters across a tiny gap onto the next cell. ## Receptor, transducer, response Most signaling follows the same three-stage plan. 1. **Reception.** The signaling molecule, called a ligand, binds to a specific receptor protein, typically on the cell surface. The fit is like a key in a lock. 2. **Transduction.** The receptor changes shape and starts a chain of events inside the cell. This relay is called signal transduction. Often a single ligand sets off a cascade in which each step activates many molecules in the next, so a faint signal is amplified into a big response. 3. **Response.** The cell does something: opens an ion channel, turns a gene on or off, changes its shape, or starts to secrete. Only cells with the right receptor respond to a given ligand. That is how a hormone can travel through the whole body yet change only its target tissues. **Check your understanding:** What is the correct order of the stages of cell signaling? - A. Response, reception, transduction - B. Reception, transduction, response - C. Transduction, response, reception - D. Reception, response, transduction **Answer:** B. Reception, transduction, response A ligand is received by a receptor, the signal is transduced inside the cell, and the cell gives a response. Interactive 3D model: [White Blood Cell](https://cellnaut.com/studio/whiteBlood/granules) - Immune cells constantly sense chemical signals from infected tissue and respond by moving and releasing granule contents. Interactive 3D model: [Rod Photoreceptor](https://cellnaut.com/studio/rod/outerSegment) - A rod cell is a sensor: light striking rhodopsin in its outer segment starts the signal. ## The neuron: a cell built for signaling A dendrite is a branched extension that receives signals. The cell body (soma) combines them. If the combined input is strong enough, the neuron fires an electrical signal that runs along its axon to the far end. Interactive 3D model: [Neuron](https://cellnaut.com/studio/neuron/dendrites) - Dendrites receive thousands of inputs; the single axon carries the output. ## The action potential A resting neuron is electrically charged: the inside of the membrane is about 70 millivolts negative compared with the outside. This is the result of the sodium-potassium pump and of channels that let potassium leak out. When the neuron is stimulated past a threshold, sodium channels open. Sodium ions rush in and the inside briefly becomes positive. This swing is the action potential. It opens the neighboring channels, which open the next ones, and so the pulse travels down the axon like a row of falling dominoes. A moment later, potassium channels open and the membrane resets. An action potential is all-or-nothing: it has the same size no matter how strong the stimulus was. Many axons are wrapped in a fatty insulating sheath called myelin, which makes the pulse jump from gap to gap and speeds it to over 100 metres per second. ## Crossing the synapse An axon does not touch the next cell. At the synapse, a gap about 20 nm wide separates the two. The message must be translated to a chemical one to cross it. ![Stylized 3D illustration of a chemical synapse showing vesicles releasing neurotransmitters toward receptors on the next cell.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/neuron-synapse-transmission-teaching.jpg) *A stylized chemical synapse: vesicles in the presynaptic terminal release neurotransmitter onto receptors on the next cell. Credit: Project-generated teaching illustration (AI-generated; simplified)* [Open the interactive, labelled version](https://cellnaut.com/topics/neuron-synapse-transmission) 1. The action potential reaches the axon terminal and opens calcium channels. Calcium ions flow in. 2. The calcium triggers vesicles full of neurotransmitter to fuse with the membrane and release their contents by exocytosis. 3. The neurotransmitter diffuses across the gap. 4. It binds to receptors on the next cell, which opens ion channels there. The next cell is either excited or inhibited. 5. The neurotransmitter is rapidly removed, either by enzymes that destroy it or by pumps that take it back up, so the signal ends cleanly. Many medicines and drugs act at this step. For example, some antidepressants slow the re-uptake of neurotransmitters so that they stay longer in the gap. > **Common misconception** > > **"A nerve signal is electricity flowing all the way from the brain to the muscle."** The electrical pulse is regenerated along every axon, and at most synapses it is converted into a chemical signal and back. Nerve signals are not like current in a wire. **Check your understanding:** What triggers synaptic vesicles to release neurotransmitter? - A. An influx of calcium ions - B. Sodium leaving the cell - C. DNA replication - D. A cell wall forming **Answer:** A. An influx of calcium ions When the action potential arrives, calcium channels open, and the incoming calcium makes the vesicles fuse with the membrane. **Check your understanding:** Why does a hormone in the blood affect only certain cells? - A. Only those cells have the matching receptor - B. The hormone is too big for other cells - C. Other cells have no membranes - D. Hormones only travel to the brain **Answer:** A. Only those cells have the matching receptor A signal needs a matching receptor. Cells without it do not respond even though the hormone passes by. ## Sources - [Biology 2e, Chapter 9: Cell Communication (OpenStax)](https://openstax.org/books/biology-2e/pages/9-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [NIH 3D: Neuron model](https://3d.nih.gov/entries/3DPX-015796/2) - See entry page and docs/ASSETS.md - [Project teaching illustration: neuron synapse transmission (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset) --- # Specialized Cells and Tissues > A tour of the Studio's specimens in four groups (animal, plant, protist and fungus, and prokaryote), and how one genome builds cells with very different shapes and jobs. Level: Introductory | Reading time: 12 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/specialized-cells-and-tissues **Quick answer:** Cells with the same DNA become different types through differentiation: each type switches on a different set of genes, so it makes different proteins and takes a shape that fits its job. Stem cells have not yet committed and can give rise to specialized cells. ## What you will learn - Explain how cells with the same DNA become different cell types. - Match each specimen's key structure to its function. - Name the four basic animal tissue types. - Describe how stem cells differ from specialized cells. ## Key takeaways - A human body has roughly 200 recognized cell types, all built from one genome. - Specialized cells keep all of their genes and simply use some of them. - Animals have four basic tissues (epithelial, connective, muscle, nervous); plants have dermal, vascular and ground tissue systems. - Structure matches function: microvilli absorb, axons conduct signals and chloroplasts photosynthesize. ## Form follows function A human body contains tens of trillions of cells of roughly 200 recognized types. A neuron looks nothing like a red blood cell, and neither looks like the cells in your skin. Yet nearly all of them carry the same DNA that was in the fertilized egg. The answer is differentiation. As an embryo develops, cells switch on different sets of genes in response to signals from their neighbors. A cell that expresses the genes for myosin and actin becomes a muscle cell, and one that expresses neurotransmitter receptors and ion channels becomes a neuron. Once switched, the pattern is usually stable. A stem cell is a cell that has not yet committed. It can divide to make more stem cells and can also give rise to specialized cells. Bone marrow stem cells, for instance, produce all the different cells in your blood. > **Common misconception** > > **"Specialized cells have lost the genes they do not use."** They still have them. Cloning experiments, in which the nucleus of a mature cell was placed into an egg and developed into a whole animal, showed that a specialized nucleus still holds the complete genome. ## Animal and human cells Each cell in the Studio is a case study in structure and function. Pick out the feature that gives each one its job. We start with the animal and human cells, which share the standard eukaryotic toolkit but exaggerate different parts of it. ### Animal cell: the general-purpose model The animal cell has the standard eukaryotic toolkit: nucleus, mitochondria, ER, Golgi, lysosomes, and a flexible membrane. Specialized animal cells add or exaggerate parts of this basic plan. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/mitochondrion) ### Neuron: long and branched The shape of a neuron is its function. Branching dendrites collect inputs, and a long axon delivers the output to other cells, sometimes more than a metre away. Interactive 3D model: [Neuron](https://cellnaut.com/studio/neuron/soma) ### Muscle fiber: packed with machinery A skeletal muscle fiber is filled with myofibrils, and mitochondria sit between them to supply ATP. It is multinucleated, because it formed by fusion of many precursor cells. Interactive 3D model: [Muscle Cell](https://cellnaut.com/studio/muscle/myofibril) ### Cardiac muscle cell: wired together Heart cells are branched and joined end to end by intercalated discs. These contain gap junctions that pass electrical signals from cell to cell, and strong anchors that carry the force of each beat, so the heart contracts as one unit. Roughly a third of the cell's volume is mitochondria, because the heart never rests. Interactive 3D model: [Cardiac Muscle Cell](https://cellnaut.com/studio/cardiac/intercalatedDisc) ### Epithelial cell: a living barrier Epithelial cells line the skin, the gut, and the airways. They are joined side by side by tight junctions that seal the gaps between cells, and many carry microvilli on the exposed surface to absorb nutrients. Interactive 3D model: [Epithelial Cell](https://cellnaut.com/studio/epithelial/junctions) ### White blood cell: a mobile defender A white blood cell crawls through tissue, engulfs invaders, and digests them using lysosomes. Its lobed nucleus is flexible enough to squeeze through narrow gaps. Interactive 3D model: [White Blood Cell](https://cellnaut.com/studio/whiteBlood/nucleus) ### Red blood cell: an oxygen carrier A mature red blood cell has thrown away its nucleus and most of its organelles to make room for hemoglobin, the iron-containing protein that carries oxygen. The biconcave shape gives a large surface area for gas exchange, and a protein skeleton under the membrane lets the cell fold its way through the narrowest capillaries. Interactive 3D model: [Red Blood Cell](https://cellnaut.com/studio/redBlood/hemoglobin) ### Sperm cell: stripped down for one journey A sperm cell keeps only what the trip needs. The head holds a compact nucleus capped by the acrosome, a sac of enzymes that helps the cell penetrate the egg. A midpiece packed with mitochondria powers the long flagellum. Interactive 3D model: [Sperm Cell](https://cellnaut.com/studio/sperm/acrosome) ### Egg cell: a supply depot The egg is one of the largest cells in the human body, and it is mostly a store of materials for the early embryo. It is wrapped in the zona pellucida, a glycoprotein coat that sperm must bind to. After fertilization, cortical granules release enzymes that harden this coat so that only one sperm gets in. Interactive 3D model: [Egg Cell](https://cellnaut.com/studio/egg/zonaPellucida) ### Stem cell: not yet committed A stem cell looks plain on purpose. It has a relatively large nucleus with loosely packed chromatin, so many genes can still be switched on, and few small mitochondria. When the right signals arrive it begins to specialize and its organelles change to match. Interactive 3D model: [Stem Cell](https://cellnaut.com/studio/stem/nucleus) ### Osteocyte: the cell inside bone Bone looks inert, but it is alive. Osteocytes sit in small chambers called lacunae inside the hard mineral matrix, and reach toward each other through thin canals. They sense the load on the skeleton and help decide where bone should be rebuilt. Interactive 3D model: [Osteocyte](https://cellnaut.com/studio/osteocyte/lacuna) ### Fat cell: built around a droplet An adipocyte is built around one huge lipid droplet that stores energy as triglyceride. The droplet pushes the nucleus and a thin rim of cytoplasm against the membrane, so under the microscope the cell looks like a ring. Interactive 3D model: [Fat Cell](https://cellnaut.com/studio/adipocyte/lipidDroplet) ### Rod cell: a sensor for dim light Rod cells in the retina detect very dim light. Stacks of membrane discs in the outer segment are packed with the light-sensitive pigment rhodopsin, and even a single photon can start a signal. Mitochondria in the inner segment supply the energy, and the synaptic terminal passes the message on to the next neuron. Interactive 3D model: [Rod Photoreceptor](https://cellnaut.com/studio/rod/outerSegment) ## Plant cells Plant cells share the eukaryotic toolkit, and add a wall, a large vacuole, and chloroplasts. Their specializations mostly serve two jobs: collecting light and managing water. ### Plant cell: wall, vacuole, chloroplast A plant cell is boxed in by a rigid cellulose cell wall. A large central vacuole pushes water against that wall, creating turgor pressure that keeps soft tissue firm. Chloroplasts make sugar from sunlight. Interactive 3D model: [Plant Cell](https://cellnaut.com/studio/plant/vacuole) ### Guard cell: a living valve A pair of guard cells surrounds each stoma, the tiny pore in a leaf where carbon dioxide enters and water vapour leaves. The wall is thicker on the pore side, so when the cells take up water they bow apart and the pore opens, and when they lose water it closes. Osmosis works the valve. Interactive 3D model: [Guard Cell](https://cellnaut.com/studio/guard/stoma) ### Root hair cell: more surface to absorb A root hair is a long outgrowth of a single root cell. It greatly increases the surface that takes in water and minerals, the same strategy that microvilli use in the gut. Interactive 3D model: [Root Hair Cell](https://cellnaut.com/studio/rootHair/hair) ## Protists and fungi Not every eukaryote is a plant or an animal. Many are single cells that do every job of life on their own. ### Yeast: a fungus in one cell Yeast is a fungus that lives as one eukaryotic cell. It divides by budding: a small daughter grows out of the mother, receives a nucleus, and separates. Because yeast is easy to grow and has a nucleus and organelles like ours, it is one of the workhorses of cell biology. Interactive 3D model: [Yeast Cell](https://cellnaut.com/studio/yeast/bud) ### Paramecium: a whole organism in one cell Thousands of cilia propel a Paramecium and sweep bacteria into its oral groove. Because it lives in fresh water, osmosis keeps pushing water in, and two contractile vacuoles pump it back out. Interactive 3D model: [Paramecium](https://cellnaut.com/studio/paramecium/cilia) ### Chlamydomonas: a swimming alga Chlamydomonas has one cup-shaped chloroplast and two flagella. An eyespot senses the direction of light, so the cell can steer toward the best place to photosynthesize. Interactive 3D model: [Chlamydomonas](https://cellnaut.com/studio/chlamydomonas/eyespot) ## Prokaryotes ### Bacterium: small, sturdy, and fast A bacterial cell has a peptidoglycan wall, a nucleoid in place of a nucleus, and often a flagellum for swimming. Its small size gives a very high surface-area-to-volume ratio, so it can take in food and divide quickly. Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/cellWall) ### Cyanobacterium: photosynthesis without chloroplasts Cyanobacteria photosynthesize, but they have no chloroplasts. Their thylakoid membranes lie free in the cytoplasm, and protein-shelled carboxysomes concentrate the enzyme that fixes carbon dioxide. Chloroplasts are thought to descend from ancient cyanobacteria. Interactive 3D model: [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria/carboxysome) **Check your understanding:** How can a neuron and a muscle cell have the same DNA but look so different? - A. They express different sets of genes - B. One of them lost most of its genes - C. Neurons have no DNA - D. Muscle cells use RNA instead of DNA **Answer:** A. They express different sets of genes Differentiation depends on which genes are switched on, not on which genes are present. ## From cells to tissues A tissue is a group of similar cells that perform a shared function. Animal bodies are built from four basic types: | Tissue | Main job | Example | | ---------- | ------------------------------------------------------ | ------------------------------ | | Epithelial | Cover surfaces and line cavities; absorb and secrete | Skin, gut lining | | Connective | Support, connect, and protect; includes blood and bone | Blood, cartilage, bone | | Muscle | Contract to produce movement | Skeletal, heart, smooth muscle | | Nervous | Sense, process, and send signals | Brain, nerves | Plants have their own three tissue systems: dermal (the outer covering), vascular (the pipes that carry water and sugar), and ground (everything else, including photosynthesis and storage). Tissues combine into organs, organs into organ systems, and these into an organism. At each level, the property of the whole depends on the cooperation of its specialized parts. **Check your understanding:** Which feature helps intestinal epithelial cells absorb nutrients efficiently? - A. Microvilli - B. A chloroplast - C. A cell wall - D. A flagellum **Answer:** A. Microvilli Microvilli greatly increase the membrane area exposed to the contents of the gut. **Check your understanding:** Which of these is NOT one of the four basic types of animal tissue? - A. Epithelial tissue - B. Connective tissue - C. Dermal tissue - D. Nervous tissue **Answer:** C. Dermal tissue Dermal tissue is one of the three plant tissue systems. Animals have epithelial, connective, muscle, and nervous tissue. ## What to explore next You have now covered the main structures, processes, and specializations of cells. Return to the Studio to rotate each specimen, take the quiz, or look up any term in the glossary. ## Sources - [Biology 2e, Chapter 4: Cell Structure (OpenStax)](https://openstax.org/books/biology-2e/pages/4-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Biology 2e, Chapter 15: Genes and Proteins (OpenStax)](https://openstax.org/books/biology-2e/pages/15-introduction) - CC BY-NC-SA 4.0. Used for gene expression and cell differentiation. - [NIH 3D: Gram-positive cell wall model](https://3d.nih.gov/entries/3DPX-010752/2) - See entry page and docs/ASSETS.md --- # Neuron Synapse and Neurotransmitter Release > A synapse is the junction where one neuron passes a message to the next cell. An electrical signal reaching the axon terminal triggers vesicles to release chemical messengers, which cross a tiny gap and open channels on the receiving cell. Web page: https://cellnaut.com/topics/neuron-synapse-transmission ![Teaching illustration: Neuron Synapse and Neurotransmitter Release](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/neuron-synapse-transmission-teaching.jpg) ## Labelled parts 1. **Presynaptic axon terminal**: The swollen end of the sending neuron's axon, packed with vesicles. 2. **Synaptic vesicles**: Small membrane sacs that store neurotransmitter molecules. 3. **Mitochondrion**: Supplies the ATP needed to refill vesicles and run ion pumps. 4. **Vesicle fusion site**: Where a vesicle merges with the membrane and releases its contents by exocytosis. 5. **Synaptic cleft**: The narrow gap, about 20 nm wide, between the two cells. 6. **Neurotransmitters**: Chemical messengers that diffuse across the cleft in a fraction of a millisecond. 7. **Postsynaptic membrane**: The receiving cell's membrane, which carries the receptors. 8. **Receptor channels**: When the transmitter binds, these ion channels open and change the receiving cell's voltage. ## Key points - Calcium entering the terminal makes vesicles fuse with the membrane and release neurotransmitter. - Neurotransmitters cross the synaptic cleft by diffusion, which is fast because the gap is only about 20 nm wide. - Binding to receptor channels changes the voltage of the receiving cell, so the signal can continue or be inhibited. ## Check yourself **1. What triggers synaptic vesicles to release their contents?** - A. Calcium ions entering the terminal - B. Oxygen leaving the cell - C. DNA replication - D. Light **Answer:** A. Calcium ions entering the terminal - An arriving action potential opens calcium channels, and calcium triggers vesicle fusion. **2. How do neurotransmitters cross the synaptic cleft?** - A. Active transport by pumps - B. Diffusion - C. They are carried by red blood cells - D. They are not released **Answer:** B. Diffusion - The gap is so narrow that simple diffusion is fast enough. ## Related lessons - [Cell Communication and the Neuron](https://cellnaut.com/learn/cell-communication) ## See it in 3D - [Neuron](https://cellnaut.com/studio/neuron) --- # Chloroplast and Photosynthetic Structures > A chloroplast turns light into sugar. Light reactions in the thylakoid membranes capture energy and release oxygen, and the Calvin cycle in the stroma uses that energy to build sugar from carbon dioxide. Web page: https://cellnaut.com/topics/chloroplast-photosynthesis ![Teaching illustration: Chloroplast and Photosynthetic Structures](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/chloroplast-photosynthesis-teaching.jpg) ## Labelled parts 1. **Light direction**: Light arrives from the sun and is absorbed by chlorophyll in the thylakoids. 2. **Outer chloroplast membrane**: The outer boundary of the double-membrane envelope; it lets small molecules pass. 3. **Double-membrane envelope**: Two membranes surround the organelle, a clue to its bacterial origin. 4. **Granum**: A stack of thylakoid discs, like a pile of coins, where the light reactions happen. 5. **Thylakoid stack**: Stacking packs a large area of membrane and chlorophyll into a small volume. 6. **Stroma lamellae**: Unstacked thylakoid membranes that connect one granum to the next. 7. **Stroma**: The fluid where the Calvin cycle builds sugar from carbon dioxide. 8. **Circular chloroplast DNA**: The organelle's own small circular genome, much like a bacterium's. 9. **Starch granule**: Temporary storage of sugar made during the day. 10. **Oxygen release**: Oxygen is a by-product, released when water is split in the light reactions. ## Key points - Thylakoid membranes hold chlorophyll and carry out the light reactions; stacks of thylakoids are called grana. - The stroma is the fluid around the thylakoids, where the Calvin cycle makes sugar. - Oxygen is a by-product of splitting water, and excess sugar is stored as starch. ## Check yourself **1. Where do the light reactions take place?** - A. Thylakoid membranes - B. Stroma - C. Outer membrane - D. Starch granule **Answer:** A. Thylakoid membranes - Chlorophyll sits in the thylakoid membranes, so light is captured there. **2. The oxygen released by photosynthesis comes from which molecule?** - A. Carbon dioxide - B. Water - C. Glucose - D. Chlorophyll **Answer:** B. Water - The light reactions split water, releasing O₂. ## Related lessons - [Mitochondria and Chloroplasts](https://cellnaut.com/learn/energy-organelles) ## See it in 3D - [Plant Cell](https://cellnaut.com/studio/plant) - [Chlamydomonas](https://cellnaut.com/studio/chlamydomonas) - [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria) --- # Antibody and Antigen Binding > An IgG antibody is a Y-shaped protein. The tips of its two arms fit a specific part of an antigen, and its tail signals other immune cells to deal with whatever is bound. Web page: https://cellnaut.com/topics/antibody-antigen-binding ![Teaching illustration: Antibody and Antigen Binding](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/antibody-antigen-binding-teaching.jpg) ## Labelled parts 1. **Antigen particle**: A foreign particle, such as part of a virus or bacterium, that the immune system can recognise. 2. **Surface antigen**: A molecule on the particle's surface that antibodies can bind. 3. **Antigen epitope**: The small part of an antigen that an antibody actually recognises. 4. **Fab arm**: Each arm of the Y (fragment antigen-binding) ends in a variable tip. 5. **Antigen-binding site**: The tip's shape and charge match one epitope. 6. **Hinge region**: A flexible joint that lets the arms swing to reach two antigens. 7. **Heavy and light chains**: An IgG is built from two heavy and two light chains joined by disulfide bonds. 8. **Fc region**: The tail, which signals immune cells such as macrophages to engulf the bound target. ## Key points - Binding is specific: the shape and charge of the tip match one epitope, like a key in a lock. - Each IgG has two identical antigen-binding sites, so it can link two antigens together. - The Fc tail is recognised by immune cells such as macrophages, which then engulf the target. ## Check yourself **1. Which part of an antibody recognises the antigen?** - A. The Fc region - B. The tips of the Fab arms - C. The hinge - D. The disulfide bonds **Answer:** B. The tips of the Fab arms - The variable tips of the Fab arms form the antigen-binding sites. **2. How many antigen-binding sites does one IgG antibody have?** - A. 1 - B. 2 - C. 3 - D. 4 **Answer:** B. 2 - One at the tip of each arm. ## Related lessons - [Cell Communication and the Neuron](https://cellnaut.com/learn/cell-communication) ## See it in 3D - [White Blood Cell](https://cellnaut.com/studio/whiteBlood) --- # Alveoli and Gas Exchange > The lungs end in millions of tiny air sacs called alveoli. Each is wrapped in capillaries, and the wall between air and blood is so thin that oxygen and carbon dioxide diffuse across in a fraction of a second. Web page: https://cellnaut.com/topics/alveoli-gas-exchange ![Teaching illustration: Alveoli and Gas Exchange](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/alveoli-gas-exchange-teaching.jpg) ## Labelled parts 1. **Alveolar lumen**: The air-filled space where inhaled air meets the wall. 2. **Alveolar wall**: A wall only one cell thick, so gases cross a very short distance. 3. **Type I pneumocyte**: Thin, flat cells that cover most of the surface and form the exchange barrier. 4. **Type II pneumocyte**: Rounder cells that make surfactant. 5. **Surfactant film**: A thin film that lowers surface tension and keeps the alveoli open. 6. **Capillary network**: A dense mesh of tiny vessels wrapped around each alveolus. 7. **Red blood cell**: Carries oxygen on hemoglobin and brings CO₂ back from the tissues. 8. **Gas molecules**: O₂ diffuses into the blood and CO₂ diffuses out. 9. **Blood-air barrier**: The alveolar cell, a shared basement membrane and the capillary cell together form the thin barrier gases cross. ## Key points - Gases cross by simple diffusion, down their concentration gradients: O₂ into the blood, CO₂ out. - The blood-air barrier is often only about half a micrometre thick. - Surfactant from type II cells reduces surface tension so the alveoli do not collapse. ## Check yourself **1. How do O₂ and CO₂ cross the blood-air barrier?** - A. Simple diffusion - B. Active transport by pumps - C. Endocytosis - D. Cell division **Answer:** A. Simple diffusion - Small nonpolar gases slide through, down their concentration gradients. **2. What is the job of surfactant?** - A. Carry oxygen - B. Lower surface tension so alveoli do not collapse - C. Make the wall thicker - D. Kill all bacteria **Answer:** B. Lower surface tension so alveoli do not collapse - Without it the wet inner surface would pull the alveoli shut. ## Related lessons - [The Plasma Membrane and Transport](https://cellnaut.com/learn/plasma-membrane-and-transport) ## See it in 3D - [Red Blood Cell](https://cellnaut.com/studio/redBlood) - [Epithelial Cell](https://cellnaut.com/studio/epithelial) --- # Glomerulus and Filtration > In the kidney, blood is filtered in a ball of capillaries called the glomerulus. Water and small solutes are forced through a three-layer filter into Bowman's capsule, while blood cells and large proteins stay behind. Web page: https://cellnaut.com/topics/nephron-glomerulus-filtration ![Teaching illustration: Glomerulus and Filtration](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/nephron-glomerulus-filtration-teaching.jpg) ## Labelled parts 1. **Afferent arteriole**: Brings blood into the glomerulus. 2. **Efferent arteriole**: Carries filtered blood away; being narrower, it keeps pressure high inside the glomerulus. 3. **Bowman's capsule**: A cup-shaped capsule that surrounds the glomerulus and collects the filtrate. 4. **Glomerular capillary tuft**: A ball of leaky capillaries where blood is filtered under pressure. 5. **Podocyte**: A specialized cell that wraps around the capillaries. 6. **Foot processes and filtration slits**: Interlocking foot processes leave narrow slits that block large proteins. 7. **Red blood cell**: Too large to pass the filter, so normally stays in the blood. 8. **Bowman's space**: The space where the filtrate collects. 9. **Start of proximal tubule**: The filtrate flows on here, where most water and nutrients are reabsorbed. ## Key points - The efferent arteriole is narrower than the afferent one, which keeps the pressure high enough to filter. - Podocyte foot processes leave filtration slits that act as the final sieve. - The filtrate then flows to the proximal tubule, where most water and nutrients are reabsorbed. ## Check yourself **1. Why do red blood cells normally stay in the blood instead of entering the filtrate?** - A. They are too large to pass the filtration barrier - B. They are destroyed in the glomerulus - C. They are attracted to the podocytes - D. There is no pressure **Answer:** A. They are too large to pass the filtration barrier - Blood cells and large proteins cannot fit through the slits. **2. What structure forms the filtration slits?** - A. Red blood cells - B. Podocyte foot processes - C. The proximal tubule - D. The afferent arteriole **Answer:** B. Podocyte foot processes - Neighbouring foot processes interlock and leave slits between them. ## Related lessons - [The Plasma Membrane and Transport](https://cellnaut.com/learn/plasma-membrane-and-transport) ## See it in 3D - [Epithelial Cell](https://cellnaut.com/studio/epithelial) --- # Root Tip and Mitosis > A root grows from its tip. Cells divide in the meristem, lengthen in the elongation zone and take on their jobs in the differentiation zone, while the root cap protects the tip as it pushes through soil. Web page: https://cellnaut.com/topics/root-tip-mitosis ![Teaching illustration: Root Tip and Mitosis](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/root-tip-mitosis-teaching.jpg) ## Labelled parts 1. **Root cap**: Protects the growing tip as it pushes through soil. 2. **Meristematic zone**: Small, densely packed cells that keep dividing. 3. **Mitotic cell**: A cell caught in the middle of dividing. 4. **Chromosomes**: Condensed during mitosis so the copies can be separated cleanly. 5. **Cell wall**: Each plant cell is boxed in; a new wall forms between the daughters. 6. **Nucleus**: In cells that are not dividing, the intact nucleus holds the DNA. 7. **Elongation zone**: Cells stop dividing and grow longer, pushing the root forward. 8. **Differentiation zone**: Cells take on specific roles such as transport or absorption. 9. **Root hair**: Outgrowths that increase the surface for absorbing water and minerals. ## Key points - Mitosis happens mainly in the meristematic zone, just behind the root cap. - Growth in length comes mostly from cell elongation just above the meristem. - Root hairs appear in the differentiation zone and increase the area for absorption. ## Check yourself **1. Where in a root do cells divide most actively?** - A. Meristematic zone - B. Root cap - C. Elongation zone - D. Root hair zone **Answer:** A. Meristematic zone - The meristem is where new cells are made. **2. What mainly happens in the elongation zone?** - A. Cells divide rapidly - B. Cells grow longer - C. Root hairs are shed - D. Chloroplasts form **Answer:** B. Cells grow longer - Cells lengthen and push the root tip forward. ## Related lessons - [The Cell Cycle and Mitosis](https://cellnaut.com/learn/cell-division) ## See it in 3D - [Root Hair Cell](https://cellnaut.com/studio/rootHair) - [Plant Cell](https://cellnaut.com/studio/plant) --- # DNA Replication Fork > Before a cell divides it copies its DNA. At the replication fork the double helix is unwound and each strand serves as a template. One new strand is made continuously, and the other in short pieces. Web page: https://cellnaut.com/topics/dna-replication-fork ![Teaching illustration: DNA Replication Fork](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/dna-replication-fork-teaching.jpg) ## Labelled parts 1. **Replication fork**: The Y-shaped region where the double helix is being opened and copied. 2. **Helicase**: Breaks the hydrogen bonds between the strands to unwind the helix. 3. **Single-strand binding proteins**: Coat the exposed single strands so they do not pair up again. 4. **Leading strand**: Copied continuously in the same direction as the fork opens. 5. **DNA polymerase**: Adds nucleotides to the growing strand, following the template. 6. **Sliding clamp**: A ring-shaped protein that holds the polymerase on the DNA. 7. **Lagging strand**: Copied in short pieces, because polymerase can only build in one direction. 8. **Okazaki fragment**: One of the short pieces on the lagging strand, later joined together by ligase. 9. **RNA primer**: A short RNA starter that polymerase needs to begin each new piece. ## Key points - Helicase unwinds the helix and single-strand binding proteins keep the strands apart. - DNA polymerase can only add nucleotides in one direction, so the lagging strand is made as Okazaki fragments. - Each daughter DNA molecule has one old strand and one new strand (semi-conservative replication). ## Check yourself **1. Which enzyme unwinds the DNA double helix?** - A. DNA ligase - B. Helicase - C. RNA polymerase - D. Ribosome **Answer:** B. Helicase - Helicase breaks the hydrogen bonds between the strands. **2. Why is the lagging strand made in fragments?** - A. Polymerase works in one direction, away from the fork on this strand - B. It has a different sequence - C. It is made by ribosomes - D. It is not copied at all **Answer:** A. Polymerase works in one direction, away from the fork on this strand - The short pieces are Okazaki fragments, joined later. ## Related lessons - [From DNA to Protein](https://cellnaut.com/learn/dna-to-protein) - [The Cell Cycle and Mitosis](https://cellnaut.com/learn/cell-division) ## See it in 3D - [Animal Cell](https://cellnaut.com/studio/animal) --- # Meiosis and Homologous Chromosome Crossing Over > In prophase I of meiosis, homologous chromosomes pair up and swap segments. This crossing over mixes the alleles inherited from the two parents, so every egg or sperm carries a unique combination. Web page: https://cellnaut.com/topics/meiosis-crossing-over ![Teaching illustration: Meiosis and Homologous Chromosome Crossing Over](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/meiosis-crossing-over-teaching.jpg) ## Labelled parts 1. **Nucleus**: Holds the chromosomes; its envelope breaks down by the end of prophase I. 2. **Nuclear envelope**: Breaks down as the cell prepares to build the spindle. 3. **Homologous chromosomes**: One from each parent, matching in size and genes, and they pair up. 4. **Sister chromatids**: Identical copies made by replication, still joined at the centromere. 5. **Tetrad**: A pair of homologous chromosomes, four chromatids in all, held together. 6. **Chiasma crossing point**: Where non-sister chromatids have crossed and swapped segments. 7. **Exchanged segment**: The DNA swapped between homologs creates new combinations of alleles. 8. **Centrosome**: Organizes the spindle microtubules. 9. **Spindle fibers**: Microtubules that will pull the homologs apart. ## Key points - Homologous chromosomes are the matching pair, one from each parent. - A tetrad contains four chromatids, and crossing over happens between non-sister chromatids. - Crossing over, plus random assortment of chromosomes, creates genetic variation. ## Check yourself **1. When does crossing over occur?** - A. Prophase I of meiosis - B. Anaphase II - C. S phase - D. Cytokinesis **Answer:** A. Prophase I of meiosis - Homologs pair up in prophase I and exchange segments. **2. Crossing over exchanges segments between which chromatids?** - A. Two sister chromatids - B. Non-sister chromatids of homologous chromosomes - C. Chromatids of different species - D. Chromatids in different cells **Answer:** B. Non-sister chromatids of homologous chromosomes - Sister chromatids are identical, so swapping them would change nothing. ## Related lessons - [The Cell Cycle and Mitosis](https://cellnaut.com/learn/cell-division) ## See it in 3D - [Egg Cell](https://cellnaut.com/studio/egg) - [Sperm Cell](https://cellnaut.com/studio/sperm) --- # Enzyme Catalysis and Active Site > Enzymes are protein catalysts. A substrate binds in the active site, the enzyme makes the reaction easier, and the products leave so the enzyme can work again. Web page: https://cellnaut.com/topics/enzyme-catalysis-active-site ![Teaching illustration: Enzyme Catalysis and Active Site](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/enzyme-catalysis-active-site-teaching.jpg) ## Labelled parts 1. **Enzyme surface**: The folded protein; its three-dimensional shape determines what it does. 2. **Active site**: The pocket where the substrate binds and the reaction happens. 3. **Substrate**: The molecule the enzyme acts on. 4. **Catalytic residues**: Amino acids in the site that directly help break or form bonds. 5. **Noncovalent interactions**: Hydrogen bonds and charge attractions hold the substrate in place without permanent bonds. 6. **Enzyme-substrate complex**: The temporary complex in which the reaction is speeded up. 7. **Product release**: Products no longer fit the site well, so they leave. 8. **Product molecules**: The new molecules formed; the enzyme itself is not used up. ## Key points - The active site fits particular substrates, which is why enzymes are specific. - Enzymes lower the activation energy, so the reaction runs faster without being used up. - Temperature and pH change an enzyme's shape, so they affect its activity. ## Check yourself **1. What does an enzyme do to the activation energy of a reaction?** - A. Raises it - B. Lowers it - C. Removes the need for substrates - D. Changes the products into reactants **Answer:** B. Lowers it - A lower energy barrier means the reaction runs faster. **2. What happens to an enzyme after the reaction?** - A. It is used up - B. It is released unchanged and can be reused - C. It becomes the product - D. It turns into DNA **Answer:** B. It is released unchanged and can be reused - Catalysts are not consumed. ## Related lessons - [Mitochondria and Chloroplasts](https://cellnaut.com/learn/energy-organelles) ## See it in 3D - [Animal Cell](https://cellnaut.com/studio/animal) --- # Bacterial Cell Structure > A bacterium is a prokaryote: it has no membrane-bound nucleus. Its DNA sits in the nucleoid region, and the cell is protected by a wall and often a capsule, with pili and flagella on the outside. Web page: https://cellnaut.com/topics/bacterial-cell-structure ![Teaching illustration: Bacterial Cell Structure](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/bacterial-cell-structure-teaching.jpg) ## Labelled parts 1. **Capsule**: A sticky outer layer that helps resist drying and immune attack. 2. **Cell wall**: A peptidoglycan wall that holds the cell's shape. 3. **Cell membrane**: Controls what enters and leaves; many reactions happen on it in bacteria. 4. **Cytoplasm**: The gel-like fluid where metabolism takes place. 5. **Nucleoid region**: The region where DNA is concentrated, with no membrane around it. 6. **Circular chromosome DNA**: Most bacteria have a single circular chromosome. 7. **Plasmid**: A small extra circle of DNA that often carries useful genes. 8. **Ribosomes**: Smaller than eukaryotic ribosomes; they build proteins. 9. **Pili**: Hair-like projections for attaching to surfaces and exchanging DNA. 10. **Flagellum**: A rotary motor with a long propeller for swimming. ## Key points - DNA is a single circular chromosome in the nucleoid, with no nuclear envelope. - Plasmids are small extra circles of DNA that can carry genes such as antibiotic resistance. - The peptidoglycan cell wall gives shape and is the target of many antibiotics. ## Check yourself **1. Where is the DNA of a bacterium found?** - A. In a nucleus - B. In the nucleoid region - C. In mitochondria - D. In the cell wall **Answer:** B. In the nucleoid region - Bacteria have no nucleus; the DNA sits in the nucleoid. **2. What do plasmids often carry?** - A. The whole genome - B. Extra genes such as antibiotic resistance - C. Ribosomes - D. Chloroplasts **Answer:** B. Extra genes such as antibiotic resistance - They are small extra loops of DNA that can be shared between bacteria. ## Related lessons - [What Is a Cell?](https://cellnaut.com/learn/what-is-a-cell) - [The Cytoskeleton and Cell Movement](https://cellnaut.com/learn/cytoskeleton-and-movement) ## See it in 3D - [Bacteria Cell](https://cellnaut.com/studio/bacteria) - [Cyanobacterium](https://cellnaut.com/studio/cyanobacteria) --- # CRISPR-Cas9 Gene Editing > Cas9 is a protein that cuts DNA, and a guide RNA tells it where. When the guide pairs with a matching sequence, Cas9 cuts both strands, and the cell's own repair machinery then fixes the break, which can switch off a gene or let a new sequence be added. Web page: https://cellnaut.com/topics/crispr-cas9-gene-editing ![Teaching illustration: CRISPR-Cas9 Gene Editing](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/crispr-cas9-gene-editing-teaching.jpg) ## Labelled parts 1. **Cas9 protein**: A nuclease that cuts both strands of DNA. 2. **Target DNA**: The sequence that is to be edited. 3. **Guide RNA**: A short RNA that directs Cas9 to a matching DNA sequence. 4. **RNA-DNA pairing region**: Base pairing between guide and target provides the specificity. 5. **Cleavage site**: Cas9 cuts here, near a short PAM sequence. 6. **Broken DNA ends**: The cell repairs the break, which can disable a gene or add new DNA. 7. **Metal ion cofactor**: Magnesium ions help the nuclease domains cut. 8. **Double-stranded DNA**: The full helix, cut across both strands. ## Key points - The guide RNA gives specificity: about 20 bases pair with the target DNA. - Cas9 cuts next to a short sequence called a PAM, using magnesium ions in its nuclease domains. - The outcome depends on how the cell repairs the break. ## Check yourself **1. What guides Cas9 to the right place in the genome?** - A. A ribosome - B. The guide RNA - C. A mitochondrion - D. A lysosome **Answer:** B. The guide RNA - The guide RNA base-pairs with the target sequence. **2. What happens after Cas9 cuts the DNA?** - A. The cell's repair machinery fixes the break - B. The cell always dies - C. The DNA turns into RNA - D. Nothing, the cut seals itself instantly with no effect **Answer:** A. The cell's repair machinery fixes the break - How the break is repaired decides whether a gene is disabled or edited. ## Related lessons - [From DNA to Protein](https://cellnaut.com/learn/dna-to-protein) ## See it in 3D - [Stem Cell](https://cellnaut.com/studio/stem) --- # Plant Vascular Bundle Transport > A stem cross-section shows the plant's plumbing. Xylem carries water and minerals upward, phloem carries sugars to where they are needed, and the cambium between them makes new cells. Web page: https://cellnaut.com/topics/plant-vascular-bundle-transport ![Teaching illustration: Plant Vascular Bundle Transport](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/plant-vascular-bundle-transport-teaching.jpg) ## Labelled parts 1. **Epidermis**: The outer protective layer of the stem. 2. **Cortex**: Ground tissue that stores food and gives support. 3. **Ring of vascular bundles**: In many dicot stems the bundles are arranged in a ring. 4. **Pith**: Central storage tissue. 5. **Xylem vessel**: Hollow, lignified tubes that carry water and minerals upward. 6. **Water transport**: Water moves up, pulled by evaporation from the leaves. 7. **Cambium**: A thin layer of dividing cells that adds new xylem and phloem. 8. **Phloem sieve tube**: Living tubes with perforated end walls that carry sugars. 9. **Companion cell**: Supports a sieve tube cell, supplying energy and proteins. 10. **Organic solute transport**: Sugar flows from sources such as leaves to sinks such as roots and fruit. ## Key points - Xylem vessels are dead, hollow tubes; water is pulled up by evaporation from the leaves. - Phloem sieve tubes are living cells helped by companion cells, and carry sugar from sources to sinks. - The cambium divides to add xylem and phloem, thickening the stem. ## Check yourself **1. Which tissue carries water upward?** - A. Xylem - B. Phloem - C. Cambium - D. Epidermis **Answer:** A. Xylem - Xylem vessels carry water and minerals from the roots. **2. What do companion cells do?** - A. Carry water up the stem - B. Support the sieve tube cells - C. Make new bark - D. Absorb light **Answer:** B. Support the sieve tube cells - Sieve tube cells have lost much of their contents and rely on their companions. ## Related lessons - [Specialized Cells and Tissues](https://cellnaut.com/learn/specialized-cells-and-tissues) ## See it in 3D - [Guard Cell](https://cellnaut.com/studio/guard) - [Root Hair Cell](https://cellnaut.com/studio/rootHair) - [Plant Cell](https://cellnaut.com/studio/plant) --- # Animal Cell vs Plant Cell > Animal and plant cells are both eukaryotic, so they share a nucleus, mitochondria and a membrane system. A plant cell adds a rigid wall, chloroplasts and a large central vacuole, which together suit it to standing still and making its own food. Web page: https://cellnaut.com/topics/animal-vs-plant-cell ![Teaching illustration: Animal Cell vs Plant Cell](https://cellnaut.com/topic-illustrations/animal-vs-plant-cell-teaching.jpg) ## Labelled parts 1. **Animal plasma membrane**: The only outer boundary of an animal cell, so the cell can change shape. 2. **Animal nucleus**: Holds the DNA and sits near the middle of the cell. 3. **Endoplasmic reticulum**: Folded membranes that make and carry proteins and lipids; both cell types have them. 4. **Mitochondrion**: Releases usable energy from sugar in both animals and plants. 5. **Cell wall**: A stiff layer of cellulose outside the membrane that gives the plant cell its box-like shape. 6. **Central vacuole**: Stores water and ions and pushes the cytoplasm against the wall, keeping the cell firm. 7. **Chloroplast**: Captures light and builds sugar by photosynthesis; animal cells have none. 8. **Plant nucleus**: Often pushed to the edge by the large vacuole. ## How they differ | Feature | Animal cell | Plant cell | | --- | --- | --- | | Outer boundary | Plasma membrane only | Cell wall outside the plasma membrane | | Shape | Round or irregular, can change | Fixed, often box-like | | Chloroplasts | Absent | Present in green tissues | | Vacuole | Small and temporary, if any | One large central vacuole | | Centrioles | Present in the centrosome | Absent in flowering plants | | Stored sugar | Glycogen | Starch | **What both have in common:** Nucleus, Mitochondria, Endoplasmic reticulum, Golgi apparatus, Ribosomes, Plasma membrane ## Key points - Only plant cells have a cell wall, chloroplasts and a large central vacuole. - Both cell types have a nucleus, mitochondria, endoplasmic reticulum and a Golgi apparatus. - Plants make sugar in chloroplasts, yet their mitochondria still burn it for energy. ## Check yourself **1. Which structure is found in plant cells but not in animal cells?** - A. Mitochondrion - B. Chloroplast - C. Nucleus - D. Ribosome **Answer:** B. Chloroplast - Chloroplasts carry out photosynthesis, which animal cells do not do. **2. What does the large central vacuole do for a plant cell?** - A. Makes ATP - B. Keeps the cell firm by holding water - C. Copies DNA - D. Builds the cell wall **Answer:** B. Keeps the cell firm by holding water - Water pressing outward against the wall keeps the cell rigid. ## Related lessons - [What Is a Cell?](https://cellnaut.com/learn/what-is-a-cell) - [Mitochondria and Chloroplasts](https://cellnaut.com/learn/energy-organelles) ## See it in 3D - [Animal Cell](https://cellnaut.com/studio/animal) - [Plant Cell](https://cellnaut.com/studio/plant) Explanations rewritten in our own words from Biology 2e, Chapter 4: Cell Structure (OpenStax), licensed CC BY-NC-SA 4.0. https://openstax.org/books/biology-2e/pages/4-introduction --- # Prokaryotic vs Eukaryotic Cells > The biggest divide in biology is whether a cell keeps its DNA inside a nucleus. Prokaryotes such as bacteria are small and have no membrane-bound organelles; eukaryotes are larger and divide their work among many compartments. Web page: https://cellnaut.com/topics/prokaryote-vs-eukaryote ![Teaching illustration: Prokaryotic vs Eukaryotic Cells](https://cellnaut.com/topic-illustrations/prokaryote-vs-eukaryote-teaching.jpg) ## Labelled parts 1. **Nucleoid**: The tangled region where the bacterial chromosome lies, with no membrane around it. 2. **Cell wall and capsule**: A tough outer layer that protects the cell and keeps its shape. 3. **Nucleus**: Keeps the chromosomes inside a double membrane studded with pores. 4. **Endoplasmic reticulum**: A network of membranes next to the nucleus that builds proteins and lipids. 5. **Mitochondrion**: A membrane-bound organelle that produces ATP; prokaryotes make ATP at their plasma membrane instead. 6. **Eukaryotic plasma membrane**: Surrounds the cell and also helps it import and export material in vesicles. ## How they differ | Feature | Prokaryotic cell | Eukaryotic cell | | --- | --- | --- | | Nucleus | None; DNA sits in the nucleoid | Yes, enclosed by a double envelope | | Membrane-bound organelles | None | Many | | Typical size | About 1 to 10 micrometres | About 10 to 100 micrometres | | DNA | Usually one circular chromosome, plus small plasmids | Several linear chromosomes | | Ribosomes | Smaller | Larger | | Division | Binary fission | Mitosis followed by cytokinesis | **What both have in common:** Plasma membrane, Cytoplasm, Ribosomes, DNA as genetic material ## Key points - A prokaryote's DNA floats in a region called the nucleoid, with no nuclear envelope around it. - Eukaryotes have organelles such as mitochondria and the endoplasmic reticulum, each wrapped in membrane. - Both kinds of cell have a plasma membrane, cytoplasm, ribosomes and DNA. ## Check yourself **1. Where is the DNA of a bacterium kept?** - A. In a nucleus - B. In the nucleoid region - C. In mitochondria - D. Outside the cell **Answer:** B. In the nucleoid region - Prokaryotes have no nucleus; their DNA lies in an unwrapped region called the nucleoid. **2. Which feature do prokaryotic and eukaryotic cells both have?** - A. Mitochondria - B. A nuclear envelope - C. Ribosomes - D. An endoplasmic reticulum **Answer:** C. Ribosomes - Every cell needs ribosomes to build proteins. ## Related lessons - [What Is a Cell?](https://cellnaut.com/learn/what-is-a-cell) - [Cell Size and Scale](https://cellnaut.com/learn/cell-size-and-scale) ## See it in 3D - [Bacteria Cell](https://cellnaut.com/studio/bacteria) - [Animal Cell](https://cellnaut.com/studio/animal) Explanations rewritten in our own words from Biology 2e, Chapter 4: Cell Structure (OpenStax), licensed CC BY-NC-SA 4.0. https://openstax.org/books/biology-2e/pages/4-introduction --- # The Cell Cycle, Stage by Stage > A dividing animal cell moves through a fixed order: interphase to prepare, then four stages of mitosis that share out the chromosomes, and finally cytokinesis that splits the cell in two. Step through the markers in order. Web page: https://cellnaut.com/topics/cell-cycle-stages ![Teaching illustration: The Cell Cycle, Stage by Stage](https://cellnaut.com/topic-illustrations/cell-cycle-stages-teaching.jpg) ## Labelled parts 1. **1. Interphase**: The nucleus is intact and the DNA, loosely spread out, is copied before division. 2. **2. Prophase**: Chromosomes condense into X-shaped rods, the nuclear envelope breaks down and the spindle begins to form. 3. **3. Metaphase**: Chromosomes line up across the middle of the cell, each attached to spindle fibres from both poles. 4. **4. Anaphase**: The two copies of each chromosome are pulled apart toward opposite ends of the cell. 5. **5. Telophase**: New nuclear envelopes form around each set of chromosomes, which begin to spread out again. 6. **6. Cytokinesis**: The membrane pinches in at the middle, splitting the cell into two daughter cells. ## Key points - Interphase is the longest part of the cycle; the DNA is copied before any visible division starts. - Mitosis runs prophase, metaphase, anaphase, telophase, and each copy of a chromosome ends up in a different daughter nucleus. - Cytokinesis divides the cytoplasm, so two separate cells result. ## Check yourself **1. During which stage do the chromosomes line up in the middle of the cell?** - A. Prophase - B. Metaphase - C. Anaphase - D. Telophase **Answer:** B. Metaphase - Metaphase means the chromosomes sit on the cell's equator, ready to be pulled apart. **2. When is the DNA copied?** - A. During interphase - B. During anaphase - C. During cytokinesis - D. It is never copied **Answer:** A. During interphase - The copying happens in interphase, so every chromosome is already doubled when mitosis begins. ## Related lessons - [The Cell Cycle and Mitosis](https://cellnaut.com/learn/cell-division) ## See it in 3D - [Stem Cell](https://cellnaut.com/studio/stem) - [Animal Cell](https://cellnaut.com/studio/animal) Explanations rewritten in our own words from Biology 2e, Chapter 10: Cell Reproduction (OpenStax), licensed CC BY-NC-SA 4.0. https://openstax.org/books/biology-2e/pages/10-introduction --- # The Cell Nucleus > The nucleus is the cell's control room. A double membrane with pores wraps the chromatin, and inside sits the nucleolus where ribosome parts are built. It connects directly to the rough endoplasmic reticulum. Web page: https://cellnaut.com/topics/nucleus ![Teaching illustration: The Cell Nucleus](https://cellnaut.com/topic-illustrations/nucleus-teaching.jpg) ## Labelled parts 1. **Nuclear envelope**: Two membranes that separate the DNA from the cytoplasm; the outer one joins the endoplasmic reticulum. 2. **Nuclear pores**: Protein-lined channels that control what goes in and out, such as messenger RNA leaving. 3. **Nucleolus**: A dense region that builds ribosome subunits. 4. **Chromatin**: Thread-like DNA and protein; tightly packed parts are less active, loose parts are being read. 5. **Rough endoplasmic reticulum**: Folded membrane sheets continuous with the outer nuclear membrane. 6. **Ribosomes**: Small particles on the rough ER that build proteins from messenger RNA. ## Key points - The nuclear envelope has two membranes and many pores that let RNA and proteins pass. - Chromatin is DNA wrapped around proteins; it coils up into chromosomes before division. - The nucleolus is where ribosomal RNA is made and combined with proteins into ribosome subunits. ## Check yourself **1. What is the job of the nucleolus?** - A. Make ATP - B. Build ribosome subunits - C. Digest waste - D. Store water **Answer:** B. Build ribosome subunits - Ribosomal RNA is made in the nucleolus and joins proteins there. **2. What lets messenger RNA leave the nucleus?** - A. Nuclear pores - B. Mitochondria - C. Lysosomes - D. The cell wall **Answer:** A. Nuclear pores - Messenger RNA travels out through nuclear pores to meet ribosomes. ## Related lessons - [The Nucleus and the Endomembrane System](https://cellnaut.com/learn/nucleus-and-endomembrane) - [From DNA to Protein](https://cellnaut.com/learn/dna-to-protein) ## See it in 3D - [Animal Cell](https://cellnaut.com/studio/animal) - [Stem Cell](https://cellnaut.com/studio/stem) Explanations rewritten in our own words from Biology 2e, Chapter 4: Cell Structure (OpenStax), licensed CC BY-NC-SA 4.0. https://openstax.org/books/biology-2e/pages/4-introduction --- # The Plasma Membrane > The plasma membrane is a double layer of lipids with proteins floating in it, a design called the fluid mosaic model. It decides what enters and leaves the cell while letting the cell sense its surroundings. Web page: https://cellnaut.com/topics/plasma-membrane ![Teaching illustration: The Plasma Membrane](https://cellnaut.com/topic-illustrations/plasma-membrane-teaching.jpg) ## Labelled parts 1. **Phospholipid heads**: Water-loving heads that face the watery outside and inside of the cell. 2. **Fatty acid tails**: Oily tails that form a barrier most water-soluble molecules cannot cross. 3. **Channel protein**: A pore that lets specific ions or small molecules cross down their concentration gradient. 4. **Carrier protein**: Changes shape to carry a molecule across, sometimes using energy. 5. **Glycoprotein**: A protein with a sugar chain, used in cell recognition. 6. **Cholesterol**: Tucks between the tails and keeps the membrane from becoming too stiff or too runny. ## Key points - Phospholipids face their water-loving heads outward and hide their oily tails in the middle. - Channel and carrier proteins let ions and larger molecules cross when they could not slip through the lipids. - Sugar chains on the outer surface act as identity tags that other cells can recognise. ## Check yourself **1. Which part of a phospholipid is water-loving?** - A. The head - B. The tails - C. Both - D. Neither **Answer:** A. The head - The charged head attracts water; the fatty acid tails avoid it. **2. Why does a charged ion need a channel to cross the membrane?** - A. The oily middle blocks charged particles - B. Ions are too big - C. The membrane has no lipids - D. Ions are alive **Answer:** A. The oily middle blocks charged particles - Charged particles cannot dissolve in the oily core, so proteins offer a path. ## Related lessons - [The Plasma Membrane and Transport](https://cellnaut.com/learn/plasma-membrane-and-transport) ## See it in 3D - [Animal Cell](https://cellnaut.com/studio/animal) - [Epithelial Cell](https://cellnaut.com/studio/epithelial) Explanations rewritten in our own words from Biology 2e, Chapter 5: Structure and Function of Plasma Membranes (OpenStax), licensed CC BY-NC-SA 4.0. https://openstax.org/books/biology-2e/pages/5-introduction --- # The Plant Cell Wall > A plant cell wall is a layered coat of cellulose fibres in a gel-like matrix, with a thin shared layer between neighbours. It resists the push of water inside the cell, supports the plant, and has channels that still link neighbouring cells. Web page: https://cellnaut.com/topics/cell-wall ![Teaching illustration: The Plant Cell Wall](https://cellnaut.com/topic-illustrations/cell-wall-teaching.jpg) ## Labelled parts 1. **Cell wall**: The tough outer layer that protects the cell and sets its shape. 2. **Cellulose fibres**: Long, strong threads laid in crisscross layers. 3. **Plasmodesma**: A channel that crosses both walls and links the cytoplasm of neighbouring cells. 4. **Vacuole**: Water inside pushes the membrane against the wall; the wall pushes back. 5. **Chloroplast**: Sits inside the wall, close to the light that passes through it. ## Key points - Cellulose fibres give the wall strength, like steel rods inside concrete. - Neighbouring cells are glued together by the middle lamella. - Plasmodesmata are narrow channels through the wall that let water and small molecules pass between cells. ## Check yourself **1. What is the main material of a plant cell wall?** - A. Cellulose - B. Cholesterol - C. Glycogen - D. Keratin **Answer:** A. Cellulose - Cellulose fibres make up most of the wall. **2. What do plasmodesmata do?** - A. Make light - B. Connect neighbouring cells - C. Copy DNA - D. Break down proteins **Answer:** B. Connect neighbouring cells - They are channels through the walls that let neighbouring cells share water and small molecules. ## Related lessons - [What Is a Cell?](https://cellnaut.com/learn/what-is-a-cell) - [Specialized Cells and Tissues](https://cellnaut.com/learn/specialized-cells-and-tissues) ## See it in 3D - [Plant Cell](https://cellnaut.com/studio/plant) - [Guard Cell](https://cellnaut.com/studio/guard) Explanations rewritten in our own words from Biology 2e, Chapter 4: Cell Structure (OpenStax), licensed CC BY-NC-SA 4.0. https://openstax.org/books/biology-2e/pages/4-introduction --- ## Glossary Web page: https://cellnaut.com/glossary - **Acrosome**: A cap-like sac of digestive enzymes at the front of a sperm head. The enzymes help the sperm penetrate the layers around the egg. (https://cellnaut.com/glossary#acrosome) - **Actin filament**: A thin, flexible protein thread (about 7 nm wide) in the cytoskeleton. Actin filaments shape the cell surface, drive crawling movement, and slide past myosin to make muscles contract. (https://cellnaut.com/glossary#actin-filament) - **Action potential**: A brief, self-propagating wave of electrical change along a cell membrane. Neurons and muscle cells use action potentials to carry signals quickly over long distances. (https://cellnaut.com/glossary#action-potential) - **Active transport**: Movement of a substance across a membrane against its concentration gradient. It needs energy, usually from ATP, and is carried out by protein pumps. (https://cellnaut.com/glossary#active-transport) - **ATP (adenosine triphosphate)**: The small molecule cells use as an energy currency. Breaking off one of its phosphate groups releases energy that powers pumps, motors, and chemical reactions. (https://cellnaut.com/glossary#atp) - **Axon**: The long, thin extension of a neuron that carries electrical signals away from the cell body toward other cells. (https://cellnaut.com/glossary#axon) - **Budding**: A form of reproduction in which a small daughter cell grows out of the parent, receives a copy of the nucleus, and then separates. Yeast divides this way. (https://cellnaut.com/glossary#budding) - **Cancer**: A group of diseases in which cells divide without the normal controls, usually because of accumulated changes (mutations) in genes that regulate the cell cycle. (https://cellnaut.com/glossary#cancer) - **Carboxysome**: A polyhedral compartment with a protein shell, found in cyanobacteria. It packs the enzyme rubisco together with carbon dioxide so that carbon fixation works faster. (https://cellnaut.com/glossary#carboxysome) - **Cell**: The smallest unit of life that can carry out all the activities of a living thing: taking in energy, using information stored in DNA, and reproducing. (https://cellnaut.com/glossary#cell) - **Cell cycle**: The ordered series of events in a cell's life: a long interphase in which the cell grows and copies its DNA, followed by mitosis and cytokinesis. (https://cellnaut.com/glossary#cell-cycle) - **Cell theory**: The foundation of biology: all living things are made of one or more cells, the cell is the basic unit of life, and all cells come from existing cells. (https://cellnaut.com/glossary#cell-theory) - **Cell wall**: A rigid layer outside the plasma membrane of plants, fungi, and most bacteria. It protects the cell and resists the pressure of water pushing outward. (https://cellnaut.com/glossary#cell-wall) - **Cellular respiration**: The set of reactions that break down fuel molecules such as glucose, using oxygen, to make ATP. Most of the ATP is made in the mitochondria. (https://cellnaut.com/glossary#cellular-respiration) - **Chloroplast**: A double-membrane organelle in plants and algae where photosynthesis takes place. It contains green chlorophyll and its own small DNA genome. (https://cellnaut.com/glossary#chloroplast) - **Chromatin**: DNA wrapped around proteins, as it appears in the nucleus when the cell is not dividing. (https://cellnaut.com/glossary#chromatin) - **Chromosome**: One long DNA molecule packaged with proteins. Human cells have 46 chromosomes. Before division each chromosome is copied and condensed into a visible X shape. (https://cellnaut.com/glossary#chromosome) - **Cilium**: A short, hair-like projection from the cell surface built around microtubules. Cilia beat to move fluid across a surface or to move the cell itself. (https://cellnaut.com/glossary#cilium) - **Codon**: A group of three nucleotides in messenger RNA that specifies one amino acid (or a signal to stop) during translation. (https://cellnaut.com/glossary#codon) - **Concentration gradient**: A difference in the amount of a substance between two regions. Substances tend to move from where they are more concentrated to where they are less concentrated. (https://cellnaut.com/glossary#concentration-gradient) - **Contractile vacuole**: A vacuole in many freshwater protists that collects excess water and periodically pumps it out of the cell, which stops the cell from swelling and bursting. (https://cellnaut.com/glossary#contractile-vacuole) - **Cytokinesis**: The final step of cell division, in which the cytoplasm is divided and two separate daughter cells form. (https://cellnaut.com/glossary#cytokinesis) - **Cytoplasm**: Everything inside the plasma membrane except the nucleus: the watery cytosol together with the organelles suspended in it. (https://cellnaut.com/glossary#cytoplasm) - **Cytoskeleton**: A network of protein filaments inside the cell that gives it shape, anchors organelles, and provides tracks and motors for movement. (https://cellnaut.com/glossary#cytoskeleton) - **Dendrite**: A branched extension of a neuron that receives signals from other cells and passes them toward the cell body. (https://cellnaut.com/glossary#dendrite) - **Differentiation**: The process by which a less specialized cell turns on a particular set of genes and becomes a specific cell type, such as a neuron or a muscle fiber. (https://cellnaut.com/glossary#differentiation) - **Diffusion**: The spreading of particles from a region of higher concentration to a region of lower concentration through random motion. It needs no energy input from the cell. (https://cellnaut.com/glossary#diffusion) - **DNA (deoxyribonucleic acid)**: The molecule that stores genetic information as a sequence of four chemical bases (A, T, G, C) along a double helix. (https://cellnaut.com/glossary#dna) - **Endocytosis**: A process in which the plasma membrane folds inward and pinches off to bring material into the cell inside a vesicle. (https://cellnaut.com/glossary#endocytosis) - **Endomembrane system**: The connected set of membrane-bound compartments in eukaryotic cells: the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and the vesicles that shuttle between them. (https://cellnaut.com/glossary#endomembrane-system) - **Endoplasmic reticulum (ER)**: A network of membrane tubes and sheets continuous with the nuclear envelope. Rough ER, studded with ribosomes, makes proteins for export; smooth ER makes lipids and detoxifies chemicals. (https://cellnaut.com/glossary#endoplasmic-reticulum) - **Endosymbiosis**: A partnership in which one cell lives inside another. Mitochondria and chloroplasts are thought to descend from free-living bacteria that were engulfed by ancestral cells. (https://cellnaut.com/glossary#endosymbiosis) - **Enzyme**: A protein that speeds up a specific chemical reaction without being used up. (https://cellnaut.com/glossary#enzyme) - **Eukaryote**: An organism whose cells have a nucleus and other membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes. (https://cellnaut.com/glossary#eukaryote) - **Exocytosis**: A process in which a vesicle fuses with the plasma membrane and releases its contents outside the cell. (https://cellnaut.com/glossary#exocytosis) - **Eyespot**: A small, pigmented patch with light-sensitive proteins in some algae. It lets a swimming cell detect the direction of light and steer toward it. (https://cellnaut.com/glossary#eyespot) - **Facilitated diffusion**: Diffusion across a membrane through a channel or carrier protein. Substances still move down their concentration gradient, so no ATP is needed. (https://cellnaut.com/glossary#facilitated-diffusion) - **Flagellum**: A long, whip-like projection that moves a cell. Bacterial flagella spin like a propeller; eukaryotic flagella bend back and forth. (https://cellnaut.com/glossary#flagellum) - **Fluid mosaic model**: The accepted picture of the plasma membrane: a flexible double layer of phospholipids in which proteins float and drift like tiles in a changing mosaic. (https://cellnaut.com/glossary#fluid-mosaic-model) - **Gene**: A stretch of DNA that contains the instructions for making a particular RNA or protein. (https://cellnaut.com/glossary#gene) - **Glucose**: A six-carbon sugar that is the main fuel for most cells and the main product of photosynthesis. (https://cellnaut.com/glossary#glucose) - **Golgi apparatus**: A stack of flattened membrane sacs that modifies, sorts, and packages proteins and lipids received from the ER, then sends them to their destinations in vesicles. (https://cellnaut.com/glossary#golgi-apparatus) - **Guard cell**: One of a pair of plant cells that surround a stoma. By taking up or losing water they change shape and so open or close the pore. (https://cellnaut.com/glossary#guard-cell) - **Hemoglobin**: The iron-containing protein in red blood cells that binds oxygen in the lungs and releases it in the tissues. It gives blood its red colour. (https://cellnaut.com/glossary#hemoglobin) - **Intercalated disc**: The specialized junction between two neighboring heart muscle cells. It contains gap junctions that pass electrical signals and anchoring junctions that transmit the force of contraction. (https://cellnaut.com/glossary#intercalated-disc) - **Intermediate filament**: A rope-like protein fiber (8 to 12 nm wide) that gives cells mechanical strength. Keratin in skin cells is one example. (https://cellnaut.com/glossary#intermediate-filament) - **Interphase**: The long part of the cell cycle between divisions, when the cell grows, carries out its normal work, and copies its DNA. (https://cellnaut.com/glossary#interphase) - **Lacuna**: A small chamber in the bone matrix that houses one osteocyte. Thin canals called canaliculi link neighboring lacunae. (https://cellnaut.com/glossary#lacuna) - **Ligand**: A signaling molecule that binds to a specific receptor protein, like a key fitting a lock. (https://cellnaut.com/glossary#ligand) - **Lipid droplet**: A storage body made of fats such as triglycerides, wrapped in a single layer of phospholipid. Fat cells hold one very large droplet. (https://cellnaut.com/glossary#lipid-droplet) - **Lysosome**: An acidic, membrane-bound organelle filled with digestive enzymes that breaks down worn-out cell parts and material the cell has engulfed. (https://cellnaut.com/glossary#lysosome) - **Micrometer (µm)**: One millionth of a metre, or one thousandth of a millimetre. A typical animal cell is about 10 to 30 µm across and a typical bacterium about 1 to 5 µm. (https://cellnaut.com/glossary#micrometer) - **Microtubule**: A hollow protein tube about 25 nm wide. Microtubules act as tracks for transport, form the mitotic spindle, and make up the core of cilia and eukaryotic flagella. (https://cellnaut.com/glossary#microtubule) - **Microvilli**: Tiny finger-like folds on the surface of a cell that greatly increase its surface area for absorption, as in the lining of the intestine. (https://cellnaut.com/glossary#microvilli) - **Mitochondrion**: A double-membrane organelle, folded inside into cristae, where most of a eukaryotic cell's ATP is made. Plural: mitochondria. (https://cellnaut.com/glossary#mitochondrion) - **Mitosis**: The part of the cell cycle in which a copied set of chromosomes is separated into two identical nuclei. Its stages are prophase, metaphase, anaphase, and telophase. (https://cellnaut.com/glossary#mitosis) - **mRNA (messenger RNA)**: A single-stranded copy of a gene. It carries the instructions from the DNA to the ribosome, where they are read to build a protein. (https://cellnaut.com/glossary#mrna) - **Myofibril**: A long, thread-like bundle of contractile proteins inside a muscle fiber, made of repeating units called sarcomeres. (https://cellnaut.com/glossary#myofibril) - **Neurotransmitter**: A chemical released by a neuron at a synapse that crosses the gap and binds to receptors on the next cell. (https://cellnaut.com/glossary#neurotransmitter) - **Nuclear envelope**: The double membrane around the nucleus. It is perforated by nuclear pores that control which molecules enter and leave. (https://cellnaut.com/glossary#nuclear-envelope) - **Nucleoid**: The region of a prokaryotic cell where the DNA is concentrated. It is not enclosed by a membrane. (https://cellnaut.com/glossary#nucleoid) - **Nucleus**: The membrane-bound organelle of eukaryotic cells that holds most of the cell's DNA and is the site where genes are copied into RNA. (https://cellnaut.com/glossary#nucleus) - **Organelle**: A specialized structure inside a cell that performs a specific job, such as the nucleus, mitochondria, or Golgi apparatus. (https://cellnaut.com/glossary#organelle) - **Osmosis**: The diffusion of water across a selectively permeable membrane toward the side with the higher concentration of dissolved solutes. (https://cellnaut.com/glossary#osmosis) - **Passive transport**: Any movement across a membrane that needs no energy from the cell, because substances move down their concentration gradient. Diffusion and osmosis are examples. (https://cellnaut.com/glossary#passive-transport) - **Peptidoglycan**: A mesh-like polymer of sugars and short peptides that forms the bacterial cell wall. (https://cellnaut.com/glossary#peptidoglycan) - **Phospholipid**: A lipid with a water-loving head and two water-fearing tails. Phospholipids line up in two layers to form the basic structure of cell membranes. (https://cellnaut.com/glossary#phospholipid) - **Photosynthesis**: The process by which plants, algae, and some bacteria use light energy to turn carbon dioxide and water into glucose and oxygen. (https://cellnaut.com/glossary#photosynthesis) - **Plasma membrane**: The thin, flexible boundary that surrounds every cell and controls what enters and leaves. (https://cellnaut.com/glossary#plasma-membrane) - **Prokaryote**: An organism, such as a bacterium, whose cells have no nucleus or other membrane-bound organelles. (https://cellnaut.com/glossary#prokaryote) - **Protein**: A chain of amino acids folded into a specific three-dimensional shape. Proteins do most of the work in cells: building structures, catalyzing reactions, moving molecules, and sending signals. (https://cellnaut.com/glossary#protein) - **Receptor**: A protein, usually in the plasma membrane, that binds a specific signaling molecule and triggers a response inside the cell. (https://cellnaut.com/glossary#receptor) - **Rhodopsin**: The light-sensitive pigment in rod cells of the retina. A single photon can change its shape and start the signal that we experience as vision in dim light. (https://cellnaut.com/glossary#rhodopsin) - **Ribosome**: A molecular machine made of RNA and protein that reads messenger RNA and links amino acids together to build a protein. It is found in all cells. (https://cellnaut.com/glossary#ribosome) - **RNA (ribonucleic acid)**: A single-stranded nucleic acid that carries or helps carry out genetic instructions. Messenger, transfer, and ribosomal RNA all take part in making proteins. (https://cellnaut.com/glossary#rna) - **Root hair**: A long, thin outgrowth of a root surface cell. It greatly increases the surface area for absorbing water and minerals from soil. (https://cellnaut.com/glossary#root-hair) - **Sarcomere**: The repeating contractile unit of a myofibril, in which actin and myosin filaments slide past each other to shorten the muscle. (https://cellnaut.com/glossary#sarcomere) - **Selective permeability**: The property of a membrane that lets some substances cross easily, others slowly, and some not at all. (https://cellnaut.com/glossary#selective-permeability) - **Signal transduction**: The chain of steps by which a signal that reaches a receptor at the cell surface is converted into a response inside the cell. (https://cellnaut.com/glossary#signal-transduction) - **Stem cell**: A cell that can divide to make more of itself and can also give rise to one or more specialized cell types. (https://cellnaut.com/glossary#stem-cell) - **Stoma**: A tiny pore in the surface of a leaf, surrounded by two guard cells. Carbon dioxide enters and water vapour leaves through it (plural: stomata). (https://cellnaut.com/glossary#stoma) - **Surface-area-to-volume ratio**: The amount of surface a cell has for each unit of its volume. As a cell grows, volume increases faster than surface area, so the ratio falls. (https://cellnaut.com/glossary#surface-area-to-volume-ratio) - **Synapse**: The junction where a neuron passes a signal to another cell, usually by releasing neurotransmitters across a tiny gap. (https://cellnaut.com/glossary#synapse) - **Thylakoid**: A flattened, membrane-bound sac that holds chlorophyll and carries out the light reactions of photosynthesis. In chloroplasts thylakoids are stacked; in cyanobacteria they lie free in the cell. (https://cellnaut.com/glossary#thylakoid) - **Tight junction**: A seal between neighboring epithelial cells that blocks most substances from leaking between them. (https://cellnaut.com/glossary#tight-junction) - **Tissue**: A group of similar cells that work together to perform a function, such as muscle tissue or epithelial tissue. (https://cellnaut.com/glossary#tissue) - **Transcription**: The first step of gene expression: an enzyme called RNA polymerase copies a gene's DNA sequence into a messenger RNA. (https://cellnaut.com/glossary#transcription) - **Translation**: The second step of gene expression: a ribosome reads the codons of a messenger RNA and assembles the matching chain of amino acids. (https://cellnaut.com/glossary#translation) - **Turgor pressure**: The outward pressure of water against the cell wall in a plant cell. It keeps non-woody plant tissue firm; when it drops, the plant wilts. (https://cellnaut.com/glossary#turgor-pressure) - **Vacuole**: A fluid-filled, membrane-bound compartment. In mature plant cells a large central vacuole stores water and helps maintain turgor pressure. (https://cellnaut.com/glossary#vacuole) - **Vesicle**: A small membrane-bound bubble that carries cargo between compartments, to the plasma membrane, or into the cell. (https://cellnaut.com/glossary#vesicle) - **Zona pellucida**: The glycoprotein coat surrounding a mammalian egg. Sperm must bind to it, and after fertilization it hardens to block other sperm. (https://cellnaut.com/glossary#zona-pellucida)