Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 37

A digestive system breaks food molecules into their components and absorbs them. Mechanical digestion breaks food into smaller pieces to increase the surface area exposed to chemical digestion; in chemical digestion, hydrolytic enzymes further break food down into component molecules.
Digestion breaks proteins down to amino acids, fats (triglycerides) to fatty acids and glycerol, and starches to monosaccharides. The circulatory system distributes the products to cells, where they are oxidized to provide energy for cellular activities, used to make new cellular materials, or stored.
Animals have a variety of mechanisms for obtaining food. Heterotrophy allowed organisms to obtain nutrients from the environment.
Simple organisms have intracellular digestion; more complex organisms have extracellular digestion in a special cavity outside the cells. Digestive cavities can have one opening or two. In a system with two openings, food enters through the mouth and is digested and absorbed; undigested material leaves through the second opening, the anus.
Nutrients promote growth, maintenance, and repair of body tissues. Macronutrients, or energy nutrients, include carbohydrates, proteins, and fats. Micronutrients include vitamins and minerals. Water is also vital. Kilocalories measure the energy food provides. One gram of carbohydrate or protein yields 4 kilocalories, and 1 gram of fat yields 9. An inadequate diet causes a primary nutrient deficiency. A secondary nutrient deficiency reflects a metabolic defect.
In humans, digestion begins in the mouth, where teeth break food into smaller pieces. Salivary glands produce saliva, which moistens food and begins starch digestion. Swallowed food moves through the esophagus to the stomach. Waves of contraction called peristalsis move food along the digestive tract.
The stomach stores food, churns it until it liquefies, and mixes it with gastric juice. Hydrochloric acid in the gastric juice activates pepsinogen, forming the protein-splitting enzyme pepsin.
In the small intestine, chemical digestion continues. Enzymes produced by the small intestine digest proteins and carbohydrates. The small intestine absorbs digested nutrients through its tremendous surface area, provided by circular folds, villi, and microvilli. In each villus, amino acids and monosaccharides enter capillaries, and digested fats enter lacteals.
Material remaining after absorption in the small intestine passes to the colon, which absorbs water, minerals, and salts, leaving feces. Many bacteria digest remaining nutrients and produce useful vitamins that are then absorbed. Feces exit the body through the anus.
The pancreas, liver, and gallbladder aid digestion. The pancreas supplies pancreatic amylase, trypsin, chymotrypsin, lipases, and nucleases. The liver supplies bile, which emulsifies fat to prepare it for lipase action, and the gallbladder stores bile. The nervous and endocrine systems regulate digestive secretion.