Co-Transport

• Co-transport is illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum in the small intestine.
• 𝙏𝙃𝙀 𝙉𝘼⁺/ 𝙆⁺ 𝙋𝙐𝙈𝙋:

  • a complete cycle takes about 10 milliseconds​
  • ATPase is the enzyme involved in making the Na⁺/K⁺ pump work
  • ATPase is responsible for about 1/5 of the cell's energy expenditure
  • digoxin, a common plant-based drug from foxgloves, acts as an inhibitor of ATPase in the cardiac membranes
• Sodium ions diffuse into the epithelial cells down the concentration gradient through a co-transporter protein in the cell-surface membrane. As the sodium ions diffuse in through this carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them.

  The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.
• 𝘾𝙊-𝙏𝙍𝘼𝙉𝙎𝙋𝙊𝙍𝙏 𝙊𝙁 𝙂𝙇𝙐𝘾𝙊𝙎𝙀:

  1. Sodium ions are actively transported into the blood from the epithelial cells via the Na+/K+ pump.
  2. This creates a concentration gradient (higher concentration of Na+ ions in the lumen)
  3. Sodium ions diffuse from the lumen into epithelial cells (down the concentration gradient) via the sodium-glucose co-transporter proteins.
  4. The co-transporter carries glucose into the cell with sodium.
  5. Glucose concentration inside the cell increases.
  6. Glucose diffuses into the blood (down its concentration gradient) via facilitated diffusion.
• 𝙏𝙃𝙀 𝙎𝙊𝘿𝙄𝙐𝙈-𝙋𝙊𝙏𝘼𝙎𝙎𝙄𝙐𝙈 𝙋𝙐𝙈𝙋:

  1. cytoplasmic Na+ binds to the sodium-potassium pump
  2. Na+ binding stimulates phosphorylation by ATP
  3. phosphorylation causes the protein to change its conformation, expelling Na+ to the outside
  4. extracellular K+ binds to the protein, triggering the release of the phosphate group
  5. loss of the phosphate restores the protein's original conformation
  6. K+ is released and Na+ sites are receptive again; the cycle repeats.
• Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some of these cells have membranes with a carrier protein called NHE3. NHE3 actively transports one sodium ion into the cell in exchange for one proton (hydrogen ion) out of the cell.

  Use your knowledge of transport across cell membranes to suggest how NHE3 does this.

  1. Co-transport;
  2. Uses hydrolysis of ATP;
  3. Sodium ion and proton bind to the protein;
  4. Protein changes shape to move sodium ion and/or proton across the membrane;