Digestion
The two major and interrelated processes of digestion-that is, the mechanical and the chemical-proceed simultaneously. In the first category are the muscular contractions of the walls of the gastrointestinal tract, which move the food in solution (chyme), making contact possible between the food and the digestive enzymes. The second-chemical digestion-is the process of hydrolysis by which carbohydrates, fats, and proteins are divided into simpler units, which can be absorbed through the walls ofthe small intestine. Summarizes briefly the chemical digestion of carbohydrates, fats, and proteins. They are discussed in more detail later. Each enzyme involved in the process is specific for the substratefor e.xample, pepsin (gastric protease) acts only on protein and is capable of breaking them down only as far as polypeptides.
In addition to the enzymes listed, there are other chemical substances that affect digestion. The stomach secretes hydrochloric acid (HCl), which activates the gastric protease pepsinogen to pepsin, creates the proper acidity for the digestion of protein, acts as a bactericidal agent, and increases the solubility of certain minerals, such as iron and calcium. Gastric secretions also contain mucin, which protects the lining of the stomach from the HCl both by neutralizing the strongly acid contents and by forming a protective covering on the gastric epithelium.
Bile is produced by the liver and stored in the gallbladder, which releases it into the duodenum through the common bile duct, as a result of hormonal action ini tia ted by the presence offat. Bile plays an important role in the digestion and absorption of fat through emulsification, which provides a larger surface for lipase action, and through solubilization of digestion products, which allows them easier access to the absorptive surface at the epithelial brushborder. These processes are performed mainly by bile sal ts, but the phospholipids present in bile also participate. The solubilization function of bile salts is critical for the absorption of water-insoluble nutrients (fat-soluble vitamins, cholesterol) and digestion products (fatty acids and monoglycerides). The absorptive surface of the microvilli is surrounded by a thin layer of water (0.01 mm-1.0 mm), which is not mixed by peristalsis and muscular contractions. In order to be absorbed, nutrients must diffuse through this unstirred water layer. Whereas the small, water-soluble molecules cross this barrier rapidly, the rate of diffusion is very slow for large and water-insoluble molecules. By providing them transport vehicles through micelle formation, bile facilitates the diffusion of fat-soluble substances through the unstirred water layer. Micelles are aggregates of molecules that are hydrophilic on the outside (due to polar regions of bile salt and phospholipid molecules) and hold their passengers in the hydrophobic center. Even though the rate of diffusion of the micelles is slow, their slowness is overcome by the large load of the nutrients delivered to the absorptive surface by each micelle.
Bile acids (cholic and chenodem.)'cholic acid) are synthesized from cholesterol and combine with the amino acids glycine or taurine in the liver to form conjugated bile acids before being secreted into the gallbladder. At the pH of the bile and intestines, they form highly soluble salts with Na+ or K+ and participate in the digestive process mainly as conjugated bile salts. Some deconjugation by intestinal bacteria may take place, but normally this occurs in the lower part of the intestines after they have already participated in the digestion and absorption of fat.
