Co-transport and absorption

• What is co-transport?

  The coupled movement of two substances across a cell membrane via a carrier protein. It is a combination of facilitated diffusion and active transport
• Increasing rate of movement across membranes:

  • the epithelial cells lining the ileum have microvili which provide a large surface area for insertion of carrier proteins
  • increase the amount of protien channels and carrier proteins in the membrane (increase its density)
• Role of diffusion in absorption:

  • in the ileum there is a concentration gradient, which glucose moves down via facilitated diffusion from the ileum into the blood
  • blood is constantly pumped so glucose is constantly removed
  • this maintains the concentration gradient
• Role of active transport in absorption:

  • Some glucose and amino acids are absorbed by active transport, via co-transport
  • Sodium ions are actively transported out of epithelial cells by the sodium-potsssium pump via a carrier protien into the blood
  • So there is a higher conc of sodium ions in the small intestine than in the epithelial cells
  • Sodium ions diffuse out down the concentration gradient into the epithelial cells
  • As the sodium ions diffuse they take glucose or amino acids with them
  • Glucose or amino acids pass into the blood via facilitated diffusion