What is active transport?
Active transport is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration — that is, against a concentration gradient — using energy released by respiration. Because it works against the natural direction of diffusion, energy is essential.
Key idea
Active transport = movement against the concentration gradient, using energy from respiration and carrier proteins in the cell membrane.
How carrier proteins work
Specific carrier (transport) proteins span the membrane. A particle (such as a mineral ion or glucose) binds to the carrier protein. Using energy from respiration, the protein changes shape and moves the particle across the membrane, then returns to its original shape to carry another particle. Cells that carry out a lot of active transport, such as root hair cells and cells lining the small intestine, contain many mitochondria to supply this energy.
Examples in living organisms
- Root hair cells absorb mineral ions (e.g. nitrate) from the soil, where the ion concentration is lower than inside the cell.
- Small intestine: glucose is absorbed into the blood even when blood glucose is already higher.
- Kidney: useful glucose is reabsorbed from the filtrate back into the blood.
Remember
- Active transport needs energy; diffusion and osmosis do not.
- A respiratory poison (e.g. cyanide) or lack of oxygen stops active transport.
- Carrier proteins are specific to the particles they transport.