Active transport and co-transportAQA A-Level Biology: Revision notes

Active transport is the movement of molecules or ions across a membrane against a concentration gradient (from a lower to a higher concentration), using energy from the hydrolysis of ATP and specific carrier proteins.

Compare this with diffusion and facilitated diffusion, which are passive: they move substances down a gradient and need no metabolic energy.

Carrier proteins in active transport work like this:

• A specific molecule or ion binds to a complementary binding site on the carrier, on one side of the membrane.
• ATP is hydrolysed to ADP + Pi (catalysed by ATP hydrolase). The phosphate group binds to the carrier.
• The carrier changes shape (a conformational change), releasing the substance on the other side of the membrane.
• The phosphate leaves, and the carrier returns to its original shape.

Because each carrier has a specific binding site, active transport is selective, and the rate levels off when all carriers are occupied.

ATP + H₂O → ADP + Pi releases energy in a small, usable amount at the site where it is needed. Moving a substance against its gradient needs energy, so the rate of active transport depends on:

• the supply of ATP, which relies on respiration (so respiratory inhibitors such as cyanide, or a lack of oxygen, reduce it)
• the number of carrier proteins in the membrane
• the concentration of the substance, up to the point where all carriers are saturated

Cells that carry out a lot of active transport, such as ileum epithelial cells and root hair cells, have many mitochondria.

Co-transport is the movement of two different substances across a membrane by the same protein. It is how glucose (and amino acids) are absorbed by the epithelial cells of the ileum:

1. The sodium-potassium pump actively transports Na⁺ out of the epithelial cell into the blood (and K⁺ in), using ATP.
2. This keeps the Na⁺ concentration inside the cell low, so there is a concentration gradient from the lumen into the cell.
3. Na⁺ diffuses into the cell through a co-transport protein, which carries glucose in at the same time, even against its own gradient.
4. Glucose builds up in the cell and moves into the blood by facilitated diffusion through a carrier protein.

Typical A Level questions give you data or a scenario and ask you to explain the effect of a change:

• Respiratory inhibitor or no oxygen: less ATP, so less active transport.
• Plateau in uptake: all carrier proteins saturated.
• Sodium-potassium pump inhibited: sodium gradient lost, so less co-transport of glucose.

Key differences: diffusion is passive and down a gradient; facilitated diffusion is passive and uses a channel or carrier; active transport is against a gradient and uses ATP and a carrier; co-transport uses the gradient of one substance to move another.