Glossary

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.

Appears in: Specialized Cells and TissuesSee in 3D: Sperm Cell

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.

Appears in: The Cytoskeleton and Cell MovementSee in 3D: Muscle Cell, Epithelial Cell

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.

Appears in: Cell Communication and the NeuronSee in 3D: Neuron, Muscle Cell

Movement of a substance across a membrane against its concentration gradient. It needs energy, usually from ATP, and is carried out by protein pumps.

Appears in: The Plasma Membrane and TransportSee in 3D: Animal Cell, Epithelial Cell

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.

Appears in: The Plasma Membrane and Transport, Mitochondria and ChloroplastsSee in 3D: Animal Cell, Muscle Cell

The long, thin extension of a neuron that carries electrical signals away from the cell body toward other cells.

Appears in: Cell Size and Scale, Cell Communication and the NeuronSee in 3D: Neuron

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.

Appears in: Specialized Cells and TissuesSee in 3D: Yeast Cell

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.

Appears in: The Cell Cycle and Mitosis

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.

Appears in: Specialized Cells and TissuesSee in 3D: Cyanobacterium

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.

Appears in: What Is a Cell?See in 3D: Animal Cell, Plant Cell, Bacteria Cell

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.

Appears in: The Cell Cycle and MitosisSee in 3D: Plant Cell, Animal Cell

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.

Appears in: What Is a Cell?

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.

Appears in: What Is a Cell?, The Plasma Membrane and Transport, Specialized Cells and TissuesSee in 3D: Plant Cell, Bacteria Cell

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.

Appears in: Mitochondria and ChloroplastsSee in 3D: Animal Cell, Muscle Cell

A double-membrane organelle in plants and algae where photosynthesis takes place. It contains green chlorophyll and its own small DNA genome.

Appears in: Mitochondria and ChloroplastsSee in 3D: Plant Cell

DNA wrapped around proteins, as it appears in the nucleus when the cell is not dividing.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: Animal Cell

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.

Appears in: From DNA to Protein, The Cell Cycle and MitosisSee in 3D: Animal Cell

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.

Appears in: The Cytoskeleton and Cell MovementSee in 3D: Epithelial Cell

A group of three nucleotides in messenger RNA that specifies one amino acid (or a signal to stop) during translation.

Appears in: From DNA to Protein

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.

Appears in: The Plasma Membrane and Transport

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.

Appears in: Specialized Cells and TissuesSee in 3D: Paramecium

The final step of cell division, in which the cytoplasm is divided and two separate daughter cells form.

Appears in: The Cell Cycle and MitosisSee in 3D: Plant Cell, Animal Cell

Everything inside the plasma membrane except the nucleus: the watery cytosol together with the organelles suspended in it.

Appears in: What Is a Cell?See in 3D: Animal Cell

A network of protein filaments inside the cell that gives it shape, anchors organelles, and provides tracks and motors for movement.

Appears in: The Cytoskeleton and Cell MovementSee in 3D: Muscle Cell, Epithelial Cell, Animal Cell

A branched extension of a neuron that receives signals from other cells and passes them toward the cell body.

Appears in: Cell Communication and the NeuronSee in 3D: Neuron

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.

Appears in: Specialized Cells and TissuesSee in 3D: Neuron, Muscle Cell, Epithelial Cell, White Blood Cell

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.

Appears in: Cell Size and Scale, The Plasma Membrane and Transport

The molecule that stores genetic information as a sequence of four chemical bases (A, T, G, C) along a double helix.

Appears in: What Is a Cell?, The Nucleus and the Endomembrane System, From DNA to ProteinSee in 3D: Animal Cell, Bacteria Cell

A process in which the plasma membrane folds inward and pinches off to bring material into the cell inside a vesicle.

Appears in: The Plasma Membrane and TransportSee in 3D: White Blood Cell

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.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: Animal Cell

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.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: Animal Cell

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.

Appears in: Mitochondria and ChloroplastsSee in 3D: Animal Cell, Plant Cell

A protein that speeds up a specific chemical reaction without being used up.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: White Blood Cell

An organism whose cells have a nucleus and other membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes.

Appears in: What Is a Cell?See in 3D: Animal Cell, Plant Cell

A process in which a vesicle fuses with the plasma membrane and releases its contents outside the cell.

Appears in: The Plasma Membrane and Transport, The Nucleus and the Endomembrane System, Cell Communication and the NeuronSee in 3D: Neuron

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.

Appears in: Specialized Cells and TissuesSee in 3D: Chlamydomonas

Diffusion across a membrane through a channel or carrier protein. Substances still move down their concentration gradient, so no ATP is needed.

Appears in: The Plasma Membrane and Transport

A long, whip-like projection that moves a cell. Bacterial flagella spin like a propeller; eukaryotic flagella bend back and forth.

Appears in: The Cytoskeleton and Cell MovementSee in 3D: Bacteria Cell

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.

Appears in: The Plasma Membrane and TransportSee in 3D: Animal Cell

A stretch of DNA that contains the instructions for making a particular RNA or protein.

Appears in: From DNA to Protein, Specialized Cells and Tissues

A six-carbon sugar that is the main fuel for most cells and the main product of photosynthesis.

Appears in: Mitochondria and ChloroplastsSee in 3D: Plant Cell, Muscle Cell

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.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: Animal 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.

Appears in: Specialized Cells and TissuesSee in 3D: Guard Cell

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.

Appears in: Specialized Cells and TissuesSee in 3D: Red Blood Cell

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.

Appears in: Specialized Cells and TissuesSee in 3D: Cardiac Muscle Cell

A rope-like protein fiber (8 to 12 nm wide) that gives cells mechanical strength. Keratin in skin cells is one example.

Appears in: The Cytoskeleton and Cell MovementSee in 3D: Epithelial Cell

The long part of the cell cycle between divisions, when the cell grows, carries out its normal work, and copies its DNA.

Appears in: The Cell Cycle and Mitosis

A small chamber in the bone matrix that houses one osteocyte. Thin canals called canaliculi link neighboring lacunae.

Appears in: Specialized Cells and TissuesSee in 3D: Osteocyte

A signaling molecule that binds to a specific receptor protein, like a key fitting a lock.

Appears in: Cell Communication and the Neuron

A storage body made of fats such as triglycerides, wrapped in a single layer of phospholipid. Fat cells hold one very large droplet.

Appears in: Specialized Cells and TissuesSee in 3D: Fat Cell

An acidic, membrane-bound organelle filled with digestive enzymes that breaks down worn-out cell parts and material the cell has engulfed.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: White Blood Cell, Animal Cell

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.

Appears in: Cell Size and Scale

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.

Appears in: The Cytoskeleton and Cell Movement, The Cell Cycle and MitosisSee in 3D: Neuron

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.

Appears in: Cell Size and Scale, Specialized Cells and TissuesSee in 3D: Epithelial Cell

A double-membrane organelle, folded inside into cristae, where most of a eukaryotic cell's ATP is made. Plural: mitochondria.

Appears in: Mitochondria and ChloroplastsSee in 3D: Animal Cell, Muscle Cell

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.

Appears in: The Cell Cycle and MitosisSee in 3D: Plant Cell, Animal Cell

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.

Appears in: From DNA to Protein

A long, thread-like bundle of contractile proteins inside a muscle fiber, made of repeating units called sarcomeres.

Appears in: The Cytoskeleton and Cell MovementSee in 3D: Muscle Cell

A chemical released by a neuron at a synapse that crosses the gap and binds to receptors on the next cell.

Appears in: Cell Communication and the NeuronSee in 3D: Neuron

The double membrane around the nucleus. It is perforated by nuclear pores that control which molecules enter and leave.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: Animal Cell, Plant Cell

The region of a prokaryotic cell where the DNA is concentrated. It is not enclosed by a membrane.

Appears in: What Is a Cell?See in 3D: Bacteria Cell

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.

Appears in: What Is a Cell?, The Nucleus and the Endomembrane System, From DNA to ProteinSee in 3D: Animal Cell, Plant Cell, White Blood Cell

A specialized structure inside a cell that performs a specific job, such as the nucleus, mitochondria, or Golgi apparatus.

Appears in: What Is a Cell?See in 3D: Animal Cell

The diffusion of water across a selectively permeable membrane toward the side with the higher concentration of dissolved solutes.

Appears in: The Plasma Membrane and TransportSee in 3D: Plant Cell

Any movement across a membrane that needs no energy from the cell, because substances move down their concentration gradient. Diffusion and osmosis are examples.

Appears in: The Plasma Membrane and Transport

A mesh-like polymer of sugars and short peptides that forms the bacterial cell wall.

Appears in: What Is a Cell?, Specialized Cells and TissuesSee in 3D: Bacteria Cell

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.

Appears in: The Plasma Membrane and Transport

The process by which plants, algae, and some bacteria use light energy to turn carbon dioxide and water into glucose and oxygen.

Appears in: Mitochondria and ChloroplastsSee in 3D: Plant Cell

The thin, flexible boundary that surrounds every cell and controls what enters and leaves.

Appears in: What Is a Cell?, The Plasma Membrane and TransportSee in 3D: Animal Cell

An organism, such as a bacterium, whose cells have no nucleus or other membrane-bound organelles.

Appears in: What Is a Cell?See in 3D: Bacteria Cell

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.

Appears in: From DNA to Protein

A protein, usually in the plasma membrane, that binds a specific signaling molecule and triggers a response inside the cell.

Appears in: Cell Communication and the NeuronSee in 3D: Neuron, White Blood Cell

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.

Appears in: Specialized Cells and TissuesSee in 3D: Rod Photoreceptor

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.

Appears in: What Is a Cell?, The Nucleus and the Endomembrane System, From DNA to ProteinSee in 3D: Animal Cell, Bacteria Cell

A single-stranded nucleic acid that carries or helps carry out genetic instructions. Messenger, transfer, and ribosomal RNA all take part in making proteins.

Appears in: From DNA to Protein

A long, thin outgrowth of a root surface cell. It greatly increases the surface area for absorbing water and minerals from soil.

Appears in: Specialized Cells and TissuesSee in 3D: Root Hair Cell

The repeating contractile unit of a myofibril, in which actin and myosin filaments slide past each other to shorten the muscle.

Appears in: The Cytoskeleton and Cell MovementSee in 3D: Muscle Cell

The property of a membrane that lets some substances cross easily, others slowly, and some not at all.

Appears in: The Plasma Membrane and Transport

The chain of steps by which a signal that reaches a receptor at the cell surface is converted into a response inside the cell.

Appears in: Cell Communication and the Neuron

A cell that can divide to make more of itself and can also give rise to one or more specialized cell types.

Appears in: Specialized Cells and Tissues

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).

Appears in: Specialized Cells and TissuesSee in 3D: Guard Cell

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.

Appears in: Cell Size and ScaleSee in 3D: Epithelial Cell, Bacteria Cell

The junction where a neuron passes a signal to another cell, usually by releasing neurotransmitters across a tiny gap.

Appears in: Cell Communication and the NeuronSee in 3D: Neuron

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.

Appears in: Mitochondria and Chloroplasts, Specialized Cells and TissuesSee in 3D: Cyanobacterium, Chlamydomonas

A seal between neighboring epithelial cells that blocks most substances from leaking between them.

Appears in: Specialized Cells and TissuesSee in 3D: Epithelial Cell

A group of similar cells that work together to perform a function, such as muscle tissue or epithelial tissue.

Appears in: Specialized Cells and TissuesSee in 3D: Epithelial Cell, Muscle Cell

The first step of gene expression: an enzyme called RNA polymerase copies a gene's DNA sequence into a messenger RNA.

Appears in: From DNA to Protein

The second step of gene expression: a ribosome reads the codons of a messenger RNA and assembles the matching chain of amino acids.

Appears in: From DNA to Protein

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.

Appears in: The Plasma Membrane and Transport, Specialized Cells and TissuesSee in 3D: Plant Cell

A fluid-filled, membrane-bound compartment. In mature plant cells a large central vacuole stores water and helps maintain turgor pressure.

Appears in: The Plasma Membrane and Transport, Specialized Cells and TissuesSee in 3D: Plant Cell

A small membrane-bound bubble that carries cargo between compartments, to the plasma membrane, or into the cell.

Appears in: The Nucleus and the Endomembrane SystemSee in 3D: Animal Cell, Neuron

The glycoprotein coat surrounding a mammalian egg. Sperm must bind to it, and after fertilization it hardens to block other sperm.

Appears in: Specialized Cells and TissuesSee in 3D: Egg Cell