Digestion reduces them to their constituent amino acids. You usually consume about 15 to 20 percent of your total calorie intake as protein. The digestion of protein starts in the stomach, where HCl and pepsin break proteins into smaller polypeptides, which then travel to the small intestine Figure.
Chemical digestion in the small intestine is continued by pancreatic enzymes, including chymotrypsin and trypsin, each of which act on specific bonds in amino acid sequences.
At the same time, the cells of the brush border secrete enzymes such as aminopeptidase and dipeptidase , which further break down peptide chains. This results in molecules small enough to enter the bloodstream Figure. Digestion of Protein Flow Chart Proteins are successively broken down into their amino acid components. Lipid Digestion A healthy diet limits lipid intake to 35 percent of total calorie intake. The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains.
Small amounts of dietary cholesterol and phospholipids are also consumed. The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase. However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine.
Pancreatic lipase breaks down each triglyceride into two free fatty acids and a monoglyceride. The fatty acids include both short-chain less than 10 to 12 carbons and long-chain fatty acids.
Two types of pancreatic nuclease are responsible for their digestion: deoxyribonuclease , which digests DNA, and ribonuclease , which digests RNA. The nucleotides produced by this digestion are further broken down by two intestinal brush border enzymes nucleosidase and phosphatase into pentoses, phosphates, and nitrogenous bases, which can be absorbed through the alimentary canal wall.
The large food molecules that must be broken down into subunits are summarized Figure. The mechanical and digestive processes have one goal: to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi. The absorptive capacity of the alimentary canal is almost endless. Each day, the alimentary canal processes up to 10 liters of food, liquids, and GI secretions, yet less than one liter enters the large intestine. Almost all ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small intestine.
Although the entire small intestine is involved in the absorption of water and lipids, most absorption of carbohydrates and proteins occurs in the jejunum. Notably, bile salts and vitamin B 12 are absorbed in the terminal ileum. By the time chyme passes from the ileum into the large intestine, it is essentially indigestible food residue mainly plant fibers like cellulose , some water, and millions of bacteria Figure.
Absorption can occur through five mechanisms: 1 active transport, 2 passive diffusion, 3 facilitated diffusion, 4 co-transport or secondary active transport , and 5 endocytosis. As you will recall from Chapter 3, active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration up the concentration gradient.
Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane. Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher.
Finally, endocytosis is a transportation process in which the cell membrane engulfs material. It requires energy, generally in the form of ATP. Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions. Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid. Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein.
In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane.
Once inside the cell, they are packaged for transport via the base of the cell and then enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via the thoracic duct.
The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP. The routes of absorption for each food category are summarized in Figure.
All carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of grams per hour. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver , gallbladder , and pancreas are major accessory organs that have a role in digestion.
These organs secrete fluids into the digestive tract. Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by the body cells in metabolism. The digestive system prepares nutrients for utilization by body cells through six activities, or functions.
Any remaining nutrients and some water are absorbed as peristaltic waves move the chyme into the ascending and transverse colons. This dehydration, combined with peristaltic waves, helps compact the chyme.
The solid waste formed is called feces. It continues to move through the descending and sigmoid colons. The large intestine temporarily stores the feces prior to elimination.
The body expels waste products from digestion through the rectum and anus. This process, called defecation, involves contraction of rectal muscles, relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external anal sphincter. The defecation reflex is mostly involuntary, under the command of the autonomic nervous system.
But the somatic nervous system also plays a role to control the timing of elimination. This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and carbohydrates. Aside from the electrochemical gradient of sodium just discussed, several other concepts are required to understand absorption in the small intestine. Also, dietary sources of protein, carbohydrate and fat must all undergo the final stages of chemical digestion just prior to absorption of, for example, amino acids, glucose and fatty acids.
At this point, its easiest to talk separately about absorption of each of the major food groups, recognizing that all of these processes take place simultaneously.
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