How will Absorption and Assimilation of Proteins in the Human body?
Absorption and Assimilation of Proteins: Once the protein is chewed and swallowed, hydrochloric acid and pepsin begin protein digestion in the stomach. HCl helps to kill bacteria in food that could cause infection. It also makes the stomach very acidic with a pH of 1.5. This acidic environment is necessary for HCl to react with pepsinogen to form pepsin so that it can break the central peptide bond in proteins. Rennin is an enzyme that is present in infants to help break down milk protein.
In the duodenum, the first part of the small intestine, the pancreas releases the enzymes, Trypsin, and chymotrypsin that break protein molecules into polypeptides. The polypeptides are further hydrolyzed by the enzymes, aminopeptidase, carboxypeptidase, and dipeptidase into the constituent amino acids. Thus the digestion of proteins is completed in the small intestine.
Absorption of proteins is an active process; it means that it requires some energy source. Adenosine triphosphate (ATP) is the energy source that body utilizes during protein absorption. The maximum absorption of amino acids (obtained after protein digestion) takes place in the duodenum and jejunum part of small intestine, little absorption takes in the ileum, the last part of small intestine. The body uses the transport carrier protein system to absorb amino acids. Each amino acid group has a carrier protein that is responsible for transporting it from the intestines to the mucosa cells. Sodium and potassium are minerals needed for the amino acids to pass from the intestines through the villi and into the bloodstream. Thus, free amino acids are absorbed by co-transport with Na+ into the epithelial cells and secreted into blood capillaries.
Figure showing the transport of amino acids from the small intestine into the blood
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There is virtually no absorption of peptides longer than four amino acids. However, there is the abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed into the small intestinal epithelial cell’s cytoplasm by the action of a single membrane carrier that has recently been characterized. This carrier functions in secondary active transport using an H+ gradient via a transporter called PepT1. Once inside the enterocyte, the vast bulk of absorbed di- and tripeptides are digested into amino acids by cytoplasmic peptidases. These amino acids are eventually transported out of the basolateral side of the intestinal mucosal 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.
Figure showing absorption of di or tripeptides from the small intestine into the blood
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Assimilation of Proteins
Amino acids are not stored but are taken up by the body cells for the synthesis of proteins. Proteins are used for growth, repair, etc. Excess amino acids can be converted into glucose and then fat and are thus stored. This is an irreversible reaction. Amino acids can also be converted to glucose and used as fuel for the cell. During their conversion to glucose the amino acids are de-aminated (removal of amino groups NH2). The liver is the chief site for deamination, i.e., a process by which the amino group is removed from the amino acids resulting in the production of ammonia. The ammonia is soon converted into urea, which is filtered from the blood in the kidney.
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