active transport

• active transport used energy to move molecules and ions across plasma membranes, usually against a conc. gradient. Carrier proteins and co-transporters are involved in active transport
• carrier proteins:

  • process is pretty similar to facilitated diffusion - a molecule attached to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side
  • 2 main differences between active transport and facilitated diffusion
• active transport usually moves solutes from a low to high conc. whereas in FD they always move from a high to a low conc.
• active transport requires energy - FD does not
• ATP (a molecule produced by resp) is a common source of energy in the cell, so it is important for AT. ATP undergoes a hydrolysis reaction, splitting into ADP and Pi. This releases energy so that solutes can be transported
• Co-transporters - a type of carrier protein

  • bind two molecules at a time
  • the conc. gradient of one of the molecules is used to move the other molecule against its own conc. gradient
  • e.g. sodium ions move across a membrane down their conc. gradient. This moves glucose across the membrane too, against its conc. gradient
• Glucose is absorbed into the bloodstream in the small int. In the mammalian ileum (the final part of a mammal's small int) the conc. of glucose is too low for glucose to diffuse out into the blood. So glucose is absorbed from the lumen (middle)of the ileum by co-transport
• step 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 conc. gradient - there is now a higher conc. of sodium ions in the lumen of the ileum than inside the cell
• step 2:

  • this causes sodium ions to diffuse from the lumen of 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
• step 3:

  • glucose diffuses out of the cell, into the blood
  • down its conc. gradient
  • through a protein channel by facilitated diffusion
• factors affecting the rate of active transport:

  • the speed of individual carrier proteins - the faster they work, the faster the rate of active transport
  • the number of carrier proteins present - the more proteins there are, the faster the rate of active transport
  • the rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport can't take place
• when active transport moves molecules and ions against their concentration gradient, a decreasing conc. gradient doesn't affect the rate of AT