Digestive System Study Guide PDF

STUDY GUIDE FOR THE DIGESTIVE SYSTEM The principle functions of the digestive system are the mechanical and enzymatic breakdown of ingested food into molecules that can be absorbed and used by the body, and the absorption of these molecules. The typical American diet consists of t...

STUDY GUIDE FOR THE DIGESTIVE SYSTEM The principle functions of the digestive system are the mechanical and enzymatic breakdown of ingested food into molecules that can be absorbed and used by the body, and the absorption of these molecules. The typical American diet consists of three main food molecules: carbohydrates (poly- and disaccharides), proteins (chains of amino acids), and fats (triglycerides, cholesterol, and long chain fatty acids). The mechanical and enzymatic breakdown (i.e. digestion) of food begins in the oral cavity where food is introduced into the digestive tract, chewed, and digestive enzymes for carbohydrates and fats are secreted in the saliva from the salivary glands. After this initial processing, the food is swallowed and passes down the esophagus into the stomach. In the stomach enzymatic digestion of proteins begins, and fat digestion continues. The acid secreted into the lumen of the stomach helps in digestion of proteins; however, it denatures the enzyme that began digestion of the carbohydrates in the oral cavity, so digestion of carbohydrates in the ingested food stops when food passes into the stomach. While initial digestion of food begins in the oral cavity and continues in the stomach, most digestion and absorption of the nutrient molecules (i.e. glucose) occurs in the small intestine. Most of the digestive enzymes of the small intestine are secreted from the pancreas. Many of these enzymes are secreted as inactive proenzymes, which are converted into their active form after they are released into the lumen of the digestive tract. The synthesis of proenzymes prevents these enzymes from digesting the cells that synthesize them. An example of a pancreatic proenzyme is trypsinogen. Trypsinogen is converted into trypsin (its active form) by an enzyme called enteropeptidase. Enteropeptidase is synthesized by epithelial cells that line the small intestine. Rather than being secreted into the lumen of the digestive tract, this enzyme remains attached to the cell membrane on the lumen side of the epithelial cell. These types of enzymes that are expressed on the cell surface of the epithelial cells of the digestive tract are called brush- border enzymes. You should know what trypsin digests, the other proenzymes that it is responsible for activating, and what each of these trypsin-activated enzymes digests. In addition to the proenzymes, the pancreas also secretes an enzyme called pancreatic amylase, which resumes digestion of carbohydrates in the ingested food. The epithelial cells of the small intestine also synthesize brush-border digestive enzymes. These brush- border digestive enzymes include sucrase, maltase, and lactase. You should know the disaccharides that each of these digestive enzymes digests, and the products of that digestion. The monosaccharides (glucose, fructose, and galactose) that results from the digestion of carbohydrates are absorbed across the epithelial cells of the small intestine and pass into the blood in the capillaries of the small intestine for distribution to the cells of the body. The proteins in the ingested food are broken down into single amino acids and di- and tripeptides, which are absorbed across the epithelial cells of the small intestine and pass into the blood in the capillaries of the small intestine for distribution to the cells of the body. Some small polypeptides are also absorbed across the epithelial cells of the small intestine by transcytosis. The triglycerides and long chain fatty acids that are ingested are digested into monoglycerides and free fatty acids that are able to diffuse across the cell membrane of the epithelial cells of the small intestine. The cholesterol that is ingested is transported from the lumen of the small intestine by a transport molecule. Once they are inside the epithelial cells of the small intestine the monoglycerides and free fatty acids are assembled by the smooth endoplasmic reticulum back into triglycerides and they are the combined with cholesterol and proteins to form chylomicrons. The chylomicrons are then exocytosed into the interstitial fluid of the wall of the small intestine and pass into the lymph capillaries (lacteals). The chylomicrons pass through the lymphatic system and eventually enter the venous circulation through the thoracic ducts. The fate of these absorbed nutrient molecules depends on the metabolic state the cells of the body are in. Suggested end of chapter questions: 8th ed.: 1a-g, 3,4,5,6,7,8,9a-h,11,12,13,14,15 map 1,16b,c,d,e,g,17b,19,20, 23, 25