Explain the Transport of Amino Acids?

More than one transport or carrier system functions in the absorption of amino acids. The active carrier system for neutral amino acids shares a common membrane carrier. Neutral amino acids and those with the short or polar side chains (serine, threonine and alanine) are transported by the shared carrier system. The basic amino acids, that is, lysine, arginine and histidine share a carrier system with cystine. The mechanism of transport and uptake are depicted in Figure.

Figure: Carrier-mediated sodium-dependent amino add transport

In Figure, you may have noticed that the amino acid transport is dependent on the Na⁺ ion. The dependence of amino acid transport on Na⁺ ion suggests a direct interaction between the carrier and Na⁺ ion. This is similar to that observed in the absorption of glucose, as you may recall studying in the last unit. The amino acid associates with the carrier and Na⁺ ion in the microvilli and the complex travels to the inner side of the membrane where it dissociates, releasing the amino acid and Na ion into the cytoplasm as illustrated in Figure. Thus the amino acid leaves the absorptive cell with sodium. The carrier, Na ion is re-circulated back to the lumen for reuse. As sodium enters the cell, potassium is pumped out via a Na⁺ K⁺ ATPase system. As sodium leaves the cell, potassium flows back in and the electrolyte balance is maintained. The Na⁺ ion is then actively transported out of the cell.