1. The trachea and the esophagus are located side by side. What mechanisms prevent food from entering the trachea and the lungs when swallowing? 2. Explain the mechanism that moves food through the gastrointestinal tract. 3. What is the role of the stomach in digestion? Describe how the structure of the stomach relates to its function. 4. Strong acids can dissolve or chemically burn living matter. Why is the stomach tissue not damaged by stomach acid? Describe the mechanism by which the stomach lining is protected from the very acidic gastric juice.A 5. Pepsinogen is sometimes referred to as a pre-enzyme because it is the inactive form of the enzyme pepsin. Why is it necessary to have a pre-enzyme?A. A 6. A cheaper but tougher cut of steak can be made more enjoyable by the process of "tenderizing." Use the Internet and other sources to find methods of tenderizing meat. (a) Explain the process of meat tenderizing. (b) What are the natural sources of chemical tenderizers? (c) How is the process of meat tenderizing similar to digestion?

1. The primary mechanism preventing food from entering the trachea during swallowing is the **epiglottis**. This leaf-shaped flap of cartilage acts like a lid, folding down over the opening of the trachea (the glottis) when you swallow. This directs food into the esophagus, the tube leading to the stomach. Additionally, the **vocal cords** within the larynx close during swallowing, further sealing off the airway. Finally, the swallowing reflex temporarily inhibits breathing, minimizing the risk of aspiration.<br /><br />2. Food is moved through the gastrointestinal tract by a process called **peristalsis**. This involves coordinated, rhythmic contractions of the smooth muscles lining the walls of the digestive organs. These wave-like contractions propel food forward, from the esophagus to the stomach, through the small and large intestines, and finally to the rectum. Other movements, like segmentation in the small intestine, aid in mixing food with digestive juices but don't directly propel it forward.<br /><br />3. The stomach plays several key roles in digestion:<br /> * **Storage:** It acts as a temporary reservoir for ingested food, allowing us to consume larger meals less frequently.<br /> * **Mechanical Digestion:** The stomach's muscular walls churn and mix food with gastric juices, breaking it down into smaller particles.<br /> * **Chemical Digestion:** Gastric glands in the stomach lining secrete gastric juice, which contains hydrochloric acid (HCl) and pepsinogen. HCl activates pepsinogen into pepsin, an enzyme that begins the digestion of proteins. HCl also helps kill bacteria ingested with food.<br /> The stomach's structure supports these functions. Its muscular walls are thick and capable of powerful contractions for churning. The rugae, folds in the stomach lining, allow the stomach to expand to accommodate large volumes of food. Gastric glands within these folds produce the necessary digestive secretions.<br /><br />4. The stomach lining is protected from the corrosive effects of gastric acid by several mechanisms:<br /> * **Mucus Layer:** The stomach lining is coated with a thick layer of mucus, which acts as a physical barrier against the acid. This mucus is alkaline, neutralizing the acid at the surface of the epithelial cells.<br /> * **Tight Junctions:** The epithelial cells lining the stomach are tightly joined together by tight junctions, preventing acid from leaking between them and damaging underlying tissues.<br /> * **Bicarbonate Secretion:** Bicarbonate ions are secreted into the mucus layer, further neutralizing acid and maintaining a pH near neutral at the cell surface.<br /> * **Rapid Cell Turnover:** Stomach lining cells are constantly being shed and replaced, ensuring that damaged cells are quickly removed before significant damage can occur.<br /><br />5. Pepsinogen exists as a pre-enzyme to prevent it from digesting the proteins within the cells that produce it. If pepsin were active within these cells, it would cause significant damage. Only when pepsinogen is secreted into the stomach lumen and comes into contact with the acidic environment is it converted into its active form, pepsin. This ensures that protein digestion occurs only where and when it is needed.<br /><br />6. (a) **Meat tenderizing** involves breaking down the tough connective tissues in meat, primarily collagen, to make it more palatable and easier to chew. This can be achieved through mechanical methods like pounding or piercing, or through chemical methods using enzymes or acids.<br /><br /> (b) Natural sources of chemical tenderizers include:<br /> * **Papain:** From papaya<br /> * **Bromelain:** From pineapple<br /> * **Ficin:** From figs<br /> * **Actinidin:** From kiwi fruit<br /><br /> (c) Meat tenderizing is similar to digestion in that both processes involve breaking down proteins. In digestion, enzymes like pepsin in the stomach and trypsin and chymotrypsin in the small intestine break down proteins into smaller peptides and amino acids. Meat tenderizers, particularly those derived from fruits like papaya and pineapple, also contain proteolytic enzymes that break down collagen and other proteins in meat, making it more tender. Both processes rely on enzymatic action to break down proteins, although the specific enzymes and target proteins may differ.<br />