Active transport, bulk transport and cell signalling

Last time

Before we start: last time, in Immobilised enzymes and where enzymes work, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Going uphill

Diffusion only moves things downhill. But cells often need to pull in a nutrient that is already scarce, or move large particles that no channel could fit. That costs energy, and it needs special machinery. Once inside, a cell also has to listen to messages from outside, which is cell signalling.

Active transport

Active transport moves molecules or ions across a membrane against their concentration gradient, from low to high concentration, using energy from A T P. It uses carrier proteins in the membrane. The substance binds to the carrier, A T P is hydrolysed, and the carrier changes shape, releasing the substance on the other side. Cells that do a lot of active transport have many mitochondria, to make the A T P. It stops if respiration is inhibited.

The sodium-potassium pump

The pump is a carrier protein that is also an A T P ase. In each cycle, it moves three sodium ions out and two potassium ions in. The result is a net movement of one positive charge out each cycle. So the inside of the cell becomes negative, and a concentration gradient of sodium is set up. In 30 cycles, 90 sodium ions go out and 60 potassium ions come in. The sodium gradient is then used to drive other transport.

Co-transport

Co transport uses the sodium gradient made by the pump. Sodium ions flow back into the cell down their gradient, through a co transporter protein. The co transporter carries a second molecule, such as glucose or an amino acid, in with the sodium, against its own gradient. The A T P is used by the pump, not by the co transporter, so it is called secondary active transport. This is how the gut and kidney tubules absorb glucose.