A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids.
In the duodenum, other enzymes— trypsin , elastase , and chymotrypsin —act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases those that break down peptides. Specifically, carboxypeptidase , dipeptidase , and aminopeptidase play important roles in reducing the peptides to free amino acids.
The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in Figure 2 and Table 2. Figure 2. Protein digestion is a multistep process that begins in the stomach and continues through the intestines. Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase.
However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates.
Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts.
Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.
Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases enzymes that break down lipids. If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete.
By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in Figure 3. Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining.
The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme.
The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water.
Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein. Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids.
The LCT gene mutations change single protein building blocks amino acids in the lactase enzyme or result in an enzyme that is abnormally short. The mutations are believed to interfere with the function of the lactase enzyme, leading to undigested lactose in the small intestine and causing severe diarrhea.
Lactose intolerance in adulthood is caused by gradually decreasing activity expression of the LCT gene after infancy, which occurs in most humans. Genetics Home Reference has merged with MedlinePlus. Learn more. The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health. LCT gene lactase. In a majority of humans, this non-persistence of lactase activity in adulthood is associated with "lactose intolerance".
The difference between lactase persistence and non-persistence is genetically determined. There are a large number of brush border peptidases, which collectively can hydrolyze the diverse amino acid sequence diversity present in dietary proteins.
Major classes of peptidases include: Exopeptidases that hydrolyze terminal amino acids from peptides. Some of these enzymes have activity against C-terminal residues and others work on N-terminal amino acids. Their activity yields free amino acids or dipeptides. As an example of enzyme specificity and diversity, aminopeptidase P hydrolyzes N-terminal amino acid from di-, tri- and oligopeptides but only if they are linked to proline, whereas aminopeptidase A catalyzes the hydrolysis of terminal acidic amino acids such as glutamate and aspartate.
Endopeptidases cleave peptide chains internally, yielding smaller peptides of varying chain length.
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