Active transport

• Active transport uses energy to move molecules and ions across plasma membranes usually against a concentration gradient. Carrier proteins and co-transporters are involved in active transport
• Carrier proteins

  The process is pretty similar to facilitated diffusion - a molecule attaches to the carrier protein, the protein changes shape and moves the molecule across the membrane releasing it to the other side - however there's two main differences:

  • Active transport usually moves solutes from a low to a high concentration - in facilitated diffusion they always move from a high to low concentration
  • Active transport requires energy - facilitated diffusion does not
• Co-transporters

  • Co-transporters are a type of carrier protein. They bind two molecules at a time
  • The concentration gradient of one of the molecules is used to move glucose across the membrane too against its concentration gradient
• Glucose is absorbed into the bloodstream in the small intestine. In the mammalian ileum the concentration of glucose is too low for glucose to diffuse out into the blood.

  So glucose is absorbed from the lumen of the ileum by co-transport

  1. Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood, by the sodium-potassium pump. This creates a concentration gradient - there's now a higher conc of sodium ions in the lumen of the ileum than inside the cell
  2. This causes sodium ions to diffuse from the lumen into the ileum into the epithelial cell, down their conc gradient. They do this via the sodium-glucose co-transporter proteins. The co-transporter carries glucose into the cell with the sodium. As a result the conc of glucose inside the cell increases
  3. Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion
• Factors affecting the rate of facilitated diffusion:

  • The speed of individual carrier proteins — the faster they work, the faster the rate of active transport.
  • ƒ  The number of carrier proteins present — the more proteins there are, the faster the rate of active transport.
  • ƒ  The rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport can’t take place.