Active transport, vesicles and ATPEdexcel A-Level Biology B: Revision notes

Active transport is the movement of molecules or ions across a membrane against their concentration gradient, using energy from the hydrolysis of ATP. It needs specific carrier proteins (sometimes called pumps).

1. The ion binds to a specific site on the carrier protein.
2. ATP is hydrolysed to ADP and phosphate, and the energy released changes the shape of the carrier.
3. The ion is released on the other side of the membrane.

Evidence for active transport: uptake against a concentration gradient; uptake reduced by respiratory inhibitors or lack of oxygen; many mitochondria in the cells concerned; uptake reaches a maximum when all carriers are in use.

Large molecules, and whole cells, are moved by vesicles, which need ATP.

• Endocytosis: the cell surface membrane folds inwards, surrounds the material and pinches off to form a vesicle in the cytoplasm. Phagocytosis takes in solid particles such as bacteria; pinocytosis takes in liquid.
• Exocytosis: vesicles (e.g. from the Golgi apparatus) move to the cell surface and fuse with the membrane, releasing their contents outside. The membrane of the vesicle becomes part of the cell surface membrane.

Examples: secretion of digestive enzymes by pancreatic cells (exocytosis); a neutrophil engulfing bacteria (endocytosis).

ATP (adenosine triphosphate) is a nucleotide made of adenine, ribose and three phosphate groups. It is the immediate source of energy for cell processes such as active transport, muscle contraction, synthesis and vesicle movement.

Hydrolysis: ATP + H₂O → ADP + Pᵢ, catalysed by ATP hydrolase. A small amount of energy is released (about 30 kJ mol⁻¹), which is enough to drive a reaction without wasting much as heat.

ATP is suitable because the hydrolysis is a single step so energy is released quickly, ATP is small and soluble so it moves where it is needed, and it can be used in many different reactions.

Cells hold only a small store of ATP, so it is continually regenerated. Phosphorylation is the addition of a phosphate group. ADP + Pᵢ → ATP needs energy, which comes from respiration (in mitochondria, using ATP synthase). Energy released by the oxidation of respiratory substrates is stored in the new bond.

The ATP–ADP cycle: ATP is hydrolysed where energy is needed, and ADP and Pᵢ return to be phosphorylated again.

• Simple diffusion: small, non-polar molecules; down gradient; no ATP; no protein.
• Facilitated diffusion: polar molecules and ions; down gradient; channel or carrier proteins; no ATP.
• Osmosis: water; down a water potential gradient; partially permeable membrane; no ATP.
• Active transport: ions and molecules; against the gradient; carrier proteins; ATP.
• Endocytosis and exocytosis: large molecules or cells; vesicles; ATP.