Digestive System for AP Biology (page 2)

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By — McGraw-Hill Professional
Updated on Oct 24, 2011

Those of you on the left side of the bus have a good view of the pancreatic duct as it expels lipase, amylase, trypsin, and chymotrypsin. Lipase is the major fat-digesting enzyme of the body. It receives some help in the handling of the fat from a product made in the liver called bile. Bile contains bile salts, phospholipids, cholesterol, and bile pigments such as bilirubin. The bile is stored in the gallbladder and is dumped into the small intestine upon the arrival of food. The bile salts help digest the fat by emulsifying it into small droplets contained in water. (Emulsification is a physical change—bile does not contain any enzymes.) Amylase continues the breakdown of carbohydrates into simpler sugars. Maltase, lactase, and sucrase break maltose, lactose, and sucrose, respectively, into monosaccharides. Trypsin and chymotrypsin work together to handle the digestion of the peptides in our diet. Trypsin cuts peptide bonds next to arginine and lysine; chymotrypsin cuts bonds by phenyalanine, tryptophan, and tyrosine. Like pepsin, these two proteolytic enzymes are secreted as inactive forms: trypsinogen and chymotrypsinogen. Trypsinogen is activated first to become trypsin, which, in turn, activates chymo trypsin. Some of you might ask "If the proteolytic enzymes only cut at certain sites, how do we finish digesting the proteins?" Trypsin and chymotrypsin are examples of enteropeptidases. It is the exopeptidases that complete the digestion of proteins by hydrolyzing all the amino acids of the remaining fragments.

After the small intestine comes the large intestine (colon). The two meet up in the lower right corner of the abdomen. The colon has three main parts: the ascending, transverse, and descending colon. There are two major functions for this part of the system—the first is to reabsorb water used in the digestive process. A failure to reabsorb enough water in this process will lead to diarrhea. A second function of the colon is the excretion of salts when their concentration in the blood is too high. The food enters the large intestine, travels up the ascending colon, across the transverse colon, down the descending colon into the rectum, where it is stored until it gets eliminated … but we don't need to go there. We've seen enough for now.

Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology

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