Active Transport

Active transport is the movement of molecules or ions across a membrane from a region of lower concentration to a region of higher concentration, against their concentration gradient, using energy from ATP and specific carrier proteins.The ATP is made by respiration, so cells that do a lot of active transport have many mitochondria.How a carrier protein (pump) works:
1. The molecule or ion binds to a specific binding site on the carrier protein, on one side of the membrane.
2. ATP binds to the carrier and is hydrolysed to ADP and inorganic phosphate (P_i). The phosphate attaches to the carrier.
3. This causes the carrier protein to change shape, so it opens to the other side of the membrane.
4. The molecule or ion is released on the other side.
5. The phosphate is released and the carrier returns to its original shape, ready to be used again.Because the binding site fits only particular molecules or ions, active transport is selective.The sodium-potassium pump is found in the membranes of most animal cells. For each ATP hydrolysed, it pumps 3 sodium ions out of the cell and 2 potassium ions in. This keeps the sodium concentration low inside the cell and helps set up the resting potential of neurones.Worked example: to pump 300 sodium ions out of a cell, the pump must work 300 ÷ 3 = 100 times, so it uses 100 molecules of ATP.Other examples include uptake of mineral ions such as nitrate by root hair cells, where the concentration in the soil is lower than in the cell, and reabsorption of glucose and ions in the kidney.