Nucleic acid digestion
The nucleic acids DNA and RNA are found in most of the foods you eat. Two types of pancreatic nuclease are responsible for their digestion. The nucleotides produced by this digestion are further broken down by two intestinal brush border enzymes so they be through the alimentary canal wall. The large food molecules that must be broken down into subunits are summarized [link]
| Absorbable Food Substances | |
|---|---|
| Source | Substance |
| Carbohydrates | Monosaccharides: glucose, galactose, and fructose |
| Proteins | Single amino acids, dipeptides, and tripeptides |
| Triglycerides | Monoacylglycerides, glycerol, and free fatty acids |
| Nucleic acids | Pentose sugars, phosphates, and nitrogenous bases |
Absorption
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. 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 ( [link] ).
Digestive secretions and absorption of water
Absorption can occur through several mechanisms: active transport to the movement of a substance across a cell membrane using cellular energy (ATP) to move the substance refers to an area of lower concentration to an area of higher concentration using cellular energy (ATP) to move the substance. Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration.
Carbohydrate absorption
All carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of 120 grams per hour. All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the feces.
Protein absorption
Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids. Almost all (95 to 98 percent) protein is digested and absorbed in the small intestine via diffusion.
Lipid absorption
About 95 percent of lipids are absorbed in the small intestine. Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion. The lipid subunits are too big to pass through the basement membranes of blood capillaries, instead they enter the large pores of lacteals . The lacteals come together to form the lymphatic vessels. the lymphatic vessels and are part of the lymphatic system..
Lipid absorption
Nucleic acid absorption
The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport. These products then enter the bloodstream.
Water absorption
Each day, about nine liters of fluid enter the small intestine. About 2.3 liters are ingested in foods and beverages, and the rest is from GI secretions. About 90 percent of this water is absorbed in the small intestine. Water absorption is driven by the concentration gradient of the water: The concentration of water is higher in chyme than it is in epithelial cells. Thus, water moves down its concentration gradient from the chyme into cells. As noted earlier, much of the remaining water is then absorbed in the colon.
Chapter review
The small intestine is the site of most chemical digestion and almost all absorption. Chemical digestion breaks large food molecules down into their chemical building blocks, which can then be absorbed through the intestinal wall and into the general circulation. Intestinal brush border enzymes and pancreatic enzymes are responsible for the majority of chemical digestion. The breakdown of fat also requires bile.
Most nutrients are absorbed by transport mechanisms at the apical surface of enterocytes. Exceptions include lipids, fat-soluble vitamins, and most water-soluble vitamins. With the help of bile salts and lecithin, the dietary fats are emulsified to form micelles, which can carry the fat particles to the surface of the enterocytes. There, the micelles release their fats to diffuse across the cell membrane. The fats are then reassembled into triglycerides and mixed with other lipids and proteins into chylomicrons that can pass into lacteals. Other absorbed monomers travel from blood capillaries in the villus to the hepatic portal vein and then to the liver.