Transport across membranesEdexcel A-Level Biology A: Revision notes

Osmosis is the net movement of free water molecules from a region of higher concentration of free water molecules (a dilute solution) to a region of lower concentration of free water molecules (a more concentrated solution) through a partially permeable membrane.

Solutes bind some water molecules, so a solution with more solute has fewer free water molecules. Water molecules cross in both directions, but there is a net movement down the concentration gradient.

• Animal cell in pure water: net movement of water in, cell swells and bursts (no cell wall).
• Animal cell in a concentrated solution: net movement of water out, cell shrinks.
• Same concentration: no net movement.

Diffusion is the net movement of particles from a region of higher concentration to lower concentration. It is passive: it does not need ATP.

• Simple diffusion through the bilayer: small, non-polar molecules such as oxygen and carbon dioxide.
• Facilitated diffusion through channel proteins (water-filled pores) or carrier proteins (which change shape): for ions and polar molecules such as glucose. It is still passive and goes down the concentration gradient.

The rate of facilitated diffusion levels off at high concentration because all the protein molecules are in use (saturated).

Active transport moves substances against their concentration gradient, from low to high concentration, using carrier proteins and energy from ATP.

1. The molecule binds to a carrier protein.
2. ATP is hydrolysed, releasing energy (ATP is the immediate source of energy).
3. The carrier protein changes shape and releases the molecule on the other side.

Active transport stops if respiration is inhibited, which is evidence that it needs ATP. Examples include the uptake of mineral ions by root hair cells and the pumping of sodium ions out of cells.

Large molecules and particles cross membranes in vesicles, using energy from ATP.

• Endocytosis: the cell surface membrane folds inwards around the material and pinches off as a vesicle inside the cell, as when a phagocyte engulfs a bacterium.
• Exocytosis: vesicles (for example from the Golgi apparatus) move to the cell surface membrane, fuse with it and release their contents outside, as when insulin is secreted.

Endocytosis removes membrane from the cell surface and exocytosis adds it back.

• Simple diffusion: down the gradient, no ATP, no protein.
• Facilitated diffusion: down the gradient, no ATP, channel or carrier proteins.
• Osmosis: down the gradient of free water molecules, no ATP, partially permeable membrane.
• Active transport: against the gradient, needs ATP, carrier proteins.
• Endocytosis and exocytosis: bulk transport in vesicles, needs ATP.

Proteins are involved in facilitated diffusion and active transport, so the rate of these can be limited by the number of protein molecules (saturation).