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Main stages of Food Processing (Digestive System) 4 main stages of food processing: Ingestion, Digestion, Absorption, and Elimination Stage 1: Ingestion: the act of eating Stage 2: Digestion: the process of breaking down food into molecules small enough for the body to absorb Why? Because we cannot use macromolecules. We must break down the polymers into smaller, usable components, and then rebuild them in our own bodies Polysaccharides simple sugars Fatsglycerol and fatty acids Proteinsamino acids Nucleic acids nucleotides Where does digestion start? the mouth! Chewing food increases the surface area exposed to digestive enzymes (salivary amylase) Stage 3: Absorption: cells take up (absorb) the small molecules from the digestive tract Stage 4: Elimination: undigested material passes out of the digestive tract Digestive System Organs Overview • Each organ has specialized food-processing functions • After chewing and swallowing, it takes 5-10 seconds for food to pass down esophagus into the stomach • Food is broken down for 2-6 hours in stomach • Final digestion and nutrient absorption in the small intestine takes 5-6 hours • In 12-24 hours, any undigested material passes through the large intestine, and feces is expelled through the anus Physical and chemical digestion begins in the mouth. Saliva contains salivary amylase, an enzyme that hydrolyzes starch and glycogen into smaller pieces. • The tongue tastes food, and helps shape it into a ball called a bolus. • The pharynx (throat) is a junction that opens to both esophagus and trachea • Peristalsis = waves of contractions from smooth muscles in the walls of the digestive tract, pushes food along Stomach has a very elastic wall so it can stretch if you eat a lot of food. It secretes very acidic gastric juice (digestive fluid, pH2). Also pepsin, enzyme that hydrolyzes proteins Stomach Stomach has a coating of mucus, secreted by epithelial cells, to protect it from acidic gastric juice • It doesn’t completely protect it so the epithelium is continually damaged and completely replaced by mitosis every 3 days • Pyloric sphincter-at end of stomach before small intestine Duodenum • the first 25 cm or so of the small intestine, most digestion occurs here • Where food molecules from stomach (acid chyme) mixes with digestive juices from the pancreas, liver, gall bladder, and gland cells of the intestinal wall • Liver has many functions, including production of bile • Gall bladder stores the bile until needed • Pancreas enzymes include proteases (protein-digesting enzymes) Small intestine: duodenum, jejunum, ileum • Most absorption takes place in the small intestine • Has a huge surface area (300 m2) – roughly the size of a tennis court • Folds and tiny projections called villi and microvilli increase surface area for absorption Large intestine (colon) • Main function: reclaiming water • 90% of water in digestive tract is reabsorbed in small intestine and large intestine • Feces (waste) becomes more solid as it moves down the large intestine. – Diarrhea? Not enough water was reabsorbed (usually because lining of colon is irritated by a bacterial infection) – Constipation? Feces moved too slowly and too much water was reabsorbed Rectum and anus • Got gas? As a byproduct of their metabolism, many colon bacteria generate gases, including methane and hydrogen sulfide • Feces are stored in the rectum until they can be eliminated through the anus