Digestion Outline

Digestion Outline - Chapter 24 The Digestive System The Digestive System An Overview p 863 All living organisms must obtain nutrients from their

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Chapter 24: The Digestive System The Digestive System: An Overview, p. 863 All living organisms must obtain nutrients from their environment to sustain life. These substances are used as raw materials for synthesizing essential compounds (anabolism ) or are decomposed to provide energy that cells need to continue functioning (catabolism ). The catabolic reactions require two essential ingredients: (1) oxygen and (2) organic molecules (such as carbohydrates, fats, or proteins) that can be broken down by intracellular enzymes. The digestive system consists of a muscular tube, the digestive tract , also called the gastrointestinal (GI) tract or alimentary canal , and various accessory organs. The digestive tract begins at the oral cavity and continues through the pharynx, esophagus, stomach, small intestine, and large intestine, which opens to the exterior at the anus. Functions of the Digestive System Ingestion occurs when materials enter the digestive tract via the mouth. Ingestion is an active process involving conscious choice and decision making. Mechanical processing is crushing and shearing that makes materials easier to propel along the digestive tract. It also increases their surface area, making them more susceptible to enzymatic attack. Digestion refers to the chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium. Simple molecules in food, such as glucose, can be absorbed intact, but epithelial cells have no way to absorb molecules the size and complexity of proteins, polysaccharides, or triglycerides. These molecules must be disassembled by digestive enzymes prior to absorption. Secretion is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by glandular organs. Absorption is the movement of organic substrates, electrolytes (inorganic ions), vitamins, and water across the digestive epithelium and into the interstitial fluid of the digestive tract. Excretion is the removal of waste products from body fluids. The digestive tract and glandular organs discharge waste products in secretions that enter the lumen of the tract. Most of these waste products, after mixing with the indigestible residue of the digestive process, will leave the body. o The lining of the digestive tract also plays a protective role by safeguarding surrounding tissues against (1) the corrosive effects of digestive acids and enzymes; (2) mechanical stresses, such as abrasion; and (3) bacteria that either are swallowed with food or reside in the digestive tract. o The digestive epithelium and its secretions provide a nonspecific defense against these bacteria. When bacteria reach the underlying layer of areolar tissue, the lamina propria , they are attacked by macrophages and other cells of the immune system. The Digestive Organs and the Peritoneum
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This note was uploaded on 12/29/2011 for the course BIOLOGY 24011 taught by Professor Pan during the Fall '11 term at HCCS.

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Digestion Outline - Chapter 24 The Digestive System The Digestive System An Overview p 863 All living organisms must obtain nutrients from their

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