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Every cell is enclosed by a plasma membrane roughly 7–8 nm thick. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes this membrane as a phospholipid bilayer in which protein molecules are embedded and free to move laterally.

Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In an aqueous environment the molecules arrange themselves spontaneously into a bilayer, heads outward and tails inward. This arrangement is the basis of the membrane's selective permeability: small non-polar molecules such as oxygen and carbon dioxide cross the hydrophobic core freely, while ions and large polar molecules such as glucose cannot.

Cholesterol molecules sit between the phospholipids and regulate fluidity. At high temperatures they restrain movement; at low temperatures they prevent the fatty acid tails packing too closely and the membrane solidifying.

Passive transport moves substances down a concentration gradient and requires no metabolic energy.

Simple diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, resulting from their random kinetic motion. Net movement continues until equilibrium is reached, at which point particles still move but there is no further net change in distribution.

The rate of diffusion across a membrane is described by Fick's law: the rate is proportional to surface area multiplied by the concentration difference, divided by the diffusion distance. Adaptations such as the folded epithelium of the alveoli and the flattened biconcave shape of a red blood cell all act to increase surface area or decrease diffusion distance.

Facilitated diffusion uses channel and carrier proteins to move polar molecules and ions that cannot cross the bilayer unaided. It remains passive — no ATP is consumed — but the rate plateaus once every available transport protein is saturated.

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane.

Water potential, given the symbol Ψ and measured in kilopascals, is the tendency of water to move out of a solution. Pure water at atmospheric pressure has a water potential of zero. Adding any solute lowers the water potential, so every solution has a negative water potential.

A cell placed in a solution of higher water potential than its own gains water. An animal cell, having no cell wall, may continue to take in water until it bursts — lysis. A plant cell in the same solution becomes turgid instead: the cellulose cell wall resists further expansion and the pressure potential rises until net water movement stops.

A cell placed in a solution of lower water potential loses water. An animal cell crenates, shrinking and becoming wrinkled. A plant cell becomes plasmolysed as the protoplast pulls away from the cell wall.

Active transport moves substances against a concentration gradient, from a lower to a higher concentration, and therefore requires energy released by the hydrolysis of ATP.

Carrier proteins act as pumps. The substance binds to a specific receptor site on the protein, ATP is hydrolysed to ADP and phosphate, and the resulting change in the protein's shape carries the substance across the membrane and releases it on the far side. Each carrier protein is specific to one substance or group of substances.

The sodium-potassium pump illustrates the mechanism. For every molecule of ATP hydrolysed, three sodium ions are moved out of the cell and two potassium ions are moved in. Because unequal numbers of ions cross in each direction, the pump is electrogenic: it generates a potential difference across the membrane, which is essential to the transmission of nerve impulses.

Cells heavily engaged in active transport, such as those lining the proximal convoluted tubule of the kidney, contain unusually large numbers of mitochondria.

Molecules too large to cross the membrane by any transport protein are moved in bulk, enclosed within vesicles.

In endocytosis the plasma membrane invaginates around the material and pinches off to form a vesicle inside the cell. Phagocytosis describes the uptake of solid material, such as a macrophage engulfing a bacterium; pinocytosis describes the uptake of liquid.

In exocytosis a vesicle formed by the Golgi apparatus fuses with the plasma membrane and releases its contents outside the cell. Digestive enzymes and neurotransmitters are both released in this way.

Both processes require ATP, and both alter the total surface area of the plasma membrane.