# 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.