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Amino acids, not proteins, are absorbed; proteins rely on prior digestion to amino acids. Most absorption of amino acids occurs in the jejunum; there is a lesser contribution from the ileum.
Amino acids are absorbed by a co-transport mechanism with sodium ions. Both sodium ion and amino acid combine with a cell surface protein receptor. There are different receptors for the groups:
In addition, certain amino acids may have there own specific transporter e.g. proline. The receptor then conveys both molecules to the inside of the cell. The energy for this transport is derived from the concentration gradient for sodium across the cell membrane. Na-K ATPase transporters actively and continuously pump sodium ions outwards to maintain the gradient.
After absorption into an enterocyte, amino acids can have one of two fates:
The movement of sodium into the cell is accompanied by chloride and water movement into the intercellular space and eventually the bloodstream.
There may be a small amount of absorption of amino acids as di- and tripeptides. These are probably digested within the cell to amino acids.
Defects of the amino acid protein carrier can lead to conditions like Hartnup's disease and cystinuria.