BIOL 2P97 Short Answer Question Assignment #1 – Digestive System 1. In the mouth, chemical digestion begins when food mixes up with saliva. Saliva contains the enzyme amylase to aid in the chemical digestion of the food in the mouth. Salivary amylase is the first enzyme to access food that is ingested. It acts upon starch and breaks it down into maltose, a smaller carbohydrate. Amylase is produced and secreted by the salivary glands, specifically by the submandibular gland within the oral cavity. As food moves down the esophagus and reaches the stomach, it starts to secrete gastric juices. Gastric juices contain enzymes, hydrochloric acid, water, mucus, and electrolytes. Enzymes in the gastric juices include pepsinogen and gastric lipase which are both secreted by the chief cells in the stomach. Pepsinogen is activated by the presence of hydrochloric acid from the parietal cells, in which then transforms into pepsin that helps break down protein to peptides or polypeptides. Gastric lipase digest lipids into glycerol and fatty acids. The pancreas is an accessory organ that secrete digestive enzymes such as lipase, amylase, colipase, trypsin, carboxypeptidase and chymotrypsin. These enzymes are produced by the Acinar cells and are secreted via the ampulla that enters the duodenum. Bile salts makes it easier for ingested fats to be dissolved. They’re made by the liver and stored in the gallbladder. When digested food enters the duodenum from the pyloric sphincter, cholecystokinin (CCK) is released by the enteroendocrine cells. The CCK then stimulated the secretion of bile that was stored in gallbladder into the gastrointestinal tract. 2. Smooth muscles are located within the wall of the small intestine. They’re found in the muscularis layer of the small intestine wall. There are two different orientation of the smooth muscle in the gastrointestinal tract which are circular and longitudinal. The three main types of motility that occur within the gastrointestinal tract are peristalsis, segmentation and tonic contractions (Leung, 2014). Peristalsis is the process of moving food by the means of wave-like contractions of the longitudinal muscles. Segmentation Contractions are used for chopping up and mixing up the chyme in the guts. Tonic contractions help divide the stomach into functional segments by the sphincters (Leung, 2014). After carbohydrates are fully digested into monosaccharides, they are then attached to specific transporters. Sodium ions are transported to the outside of the cell, which in terms, results in a lower concentration of sodium concentration in the inner part of the cell. The sodium gradient helps the glucose and galactose to be transferred to the epithelial cells using Sodium Glucose co- Transporters (SGLT) which are also symporters. SGLT uses the gradient of the sodium to cotransport glucose and galactose with sodium against its concentration gradient. Being allowed by the glucose cotransporters (GLUT), fructose is now able to move down its concentration gradient. 3. Villi and microvilli are the main cells that are responsible for nutrient absorption from the gastrointestinal tract. The majority of nutrients absorption occurs at the small intestine. Increased surface area has a great effect on the absorption, increased surface area means increased area for nutrient absorption to occur in the gastrointestinal tract has mucosal fold that help increases its surface area. Circular folds significantly increase the surface area of the small intestine and the numerous elongated microscopic projections, villi and microvilli. Lipids are emulsified by the bile salts that are excreted through bile ducts from the gallbladder. They’re absorbed after being ingested by pancreatic lipase in the form of monoglycerides and fatty acids through passive diffusion.