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.
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.
A boundary that thinks
Every cell is wrapped in a plasma membranePlasma membraneThe thin, flexible boundary that surrounds every cell and controls what enters and leaves. 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 phospholipidsPhospholipidA 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.. 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 modelFluid mosaic modelThe accepted picture of the plasma membrane: a flexible double layer of phospholipids in which proteins float and drift like tiles in a changing mosaic..
In animal cells, cholesterol tucked between the phospholipids keeps the membrane from becoming too stiff in the cold or too runny when warm.
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 permeabilitySelective permeabilityThe property of a membrane that lets some substances cross easily, others slowly, and some not at all..
Passive transport: no energy needed
Passive transportPassive transportAny movement across a membrane that needs no energy from the cell, because substances move down their concentration gradient. Diffusion and osmosis are examples. moves substances down their concentration gradientConcentration gradientA 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., 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 diffusionDiffusionThe 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.. Small, nonpolar molecules such as O₂ and CO₂ slide directly through the lipid layer.
- Facilitated diffusionFacilitated diffusionDiffusion across a membrane through a channel or carrier protein. Substances still move down their concentration gradient, so no ATP is needed.. Ions and polar molecules such as glucose cross through channel or carrier proteins. Still no ATP is needed, because they travel down their gradient.
- OsmosisOsmosisThe diffusion of water across a selectively permeable membrane toward the side with the higher concentration of dissolved solutes.. 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 wallCell wallA rigid layer outside the plasma membrane of plants, fungi, and most bacteria. It protects the cell and resists the pressure of water pushing outward. stops it from bursting. The pressure of water pushing out against the wall is called turgor pressureTurgor pressureThe 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., and it is what keeps lettuce crisp and stems upright. The big central vacuoleVacuoleA fluid-filled, membrane-bound compartment. In mature plant cells a large central vacuole stores water and helps maintain turgor pressure. is where much of that water is stored.
A red blood cell is placed in pure water. What is most likely to happen?
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 transportActive transportMovement of a substance across a membrane against its concentration gradient. It needs energy, usually from ATP, and is carried out by protein pumps., a protein pump uses ATPATP (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. 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.
Moving big cargo: vesicles
Proteins, bacteria, and other large particles cannot squeeze through membrane proteins. Cells move them in bubbles of membrane called vesicles.
- EndocytosisEndocytosisA process in which the plasma membrane folds inward and pinches off to bring material into the cell inside a vesicle. 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.
- ExocytosisExocytosisA process in which a vesicle fuses with the plasma membrane and releases its contents outside the cell. 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.
Which kind of transport directly requires energy from ATP?
Why can oxygen pass through the membrane without help from a protein?
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.
Sources
- Biology 2e, Chapter 5: Structure and Function of Plasma Membranes (OpenStax) Concepts adapted and rewritten; no text reproduced verbatim.
Last reviewed October 1, 2026