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Absorption of Proteins in Small Intestine
Proteins begin chemical digestion in the stomach, where the digestive enzyme pepsin cleaves peptide bonds and transforms proteins into smaller polypeptides. The polypeptides eventually end up in the small intestine, where the pancreatic peptidases such as trypsin, chymotrypsin and carboxypeptidase cleave these polypeptides into smaller peptides. At the brush border of the enterocytes, membrane-bound digestive enzymes break down these small peptides even further into amino acids, dipeptides and tripeptides. Amino acids are absorbed by the enterocytes via a sodium dependent co-transport system. This is a secondary active transport system, which means the cell must utilize ATP to create an electrochemical gradient for sodium and use that electrochemical gradient to bring the amino acids into the cell. The dipeptides and tripeptides however use a hydrogen-ion dependent co-transport system. This means that they use a proton electrochemical gradient to bring the dipeptides and tripeptides into the cell. Once these dipeptides and tripeptides are in the cell, they are usually broken down into their constituent amino acids. These amino acids are eventually transported out of the basolateral side of the cell and into the blood system that takes the amino acids to the different cells of the body (especially liver cells). These cells utilize the amino acids to synthesize proteins.
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