It is the main system for amino acid absorption. It is an active process that needs energy. The energy needed id derived from ATP. Absorption of one amino acid molecule needs one ATP molecule. There are 7 carrier proteins, one for each group of amino acids. Each carrier protein has to sites one for amino acid and one for Na+. It co-transports amino acid and Na+ from intestinal lumen to cytosol of intestinal mucosa cells. The absorbed amino acid passes to the portal circulation, while Na+ is extruded out of the cell in exchange with K+ by sodium pump
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