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Mechanical Digestion

Mechanical Digestion. Overview of Digestion Composition of Food Mechanical Digestion Movement of Food through the GI tract Chemical Digestion. What is Digestion?. Digestion : the breakdown of food into smaller parts, which can be absorbed and utilized by the body

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Mechanical Digestion

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  1. Mechanical Digestion Overview of Digestion Composition of Food Mechanical Digestion Movement of Food through the GI tract Chemical Digestion

  2. What is Digestion? • Digestion: the breakdown of food into smaller parts, which can be absorbed and utilized by the body • carbohydrates are digested into monosaccharides • proteins are digested into amino acids • fats are digested into fatty acids and glycerol

  3. Composition of Food • Some components of food are not digested, but are absorbed directly (water, salts, monosaccharides, most vitamins) • Some components of food require digestion prior to absorption (carbohydrates, proteins, fats, nucleic acids) • Some components of food are not digested, and are never resorbed (cellulose, from plants)

  4. Mechanical Digestion • The majority of mechanical digestion occurs in the oral cavity • Grinding and tearing of food decreases its size, increasing its surface area • Mastication is controlled by the mastication reflex, which is stimulated by the presence of a bolus of food

  5. Deglutition (swallowing) There arethree phases to deglutition: • voluntary phase: the tongue pushes a bolus of food toward oropharynx

  6. Deglutition (swallowing) • pharyngeal phase: bolus in the oropharynx stimulates a reflex response: - soft palate elevates - pharynx elevates - pharyngeal constrictor muscles constrict - upper esophageal sphincter relaxes - vocal cords close - epiglottis closes laryngeal opening

  7. Deglutition (swallowing) • esophageal phase: bolus moved toward stomach by peristaltic waves - peristaltic waves cause relaxation of the lower esophageal sphincter - peristaltic waves stimulated by presence of food in the esophagus - food and liquid can be moved against the force of gravity by peristalsis in the esophagus

  8. Movement of Food through the GI tract • In the stomach, food is mixed by tonic contractions (last phase of mechanical digestion) - mixing waves (every 20 seconds) - peristalic waves (more powerful) • Chyme: mixture of food in the stomach • Gastric emptying: pyloric contractions move food through the pyloric sphincter - rate depends on component of food - influenced by the autonomic system

  9. Movement of Food (cont.) • In the small intestine, mixing and propulsion of contents continues • Segmental contractions mix the intestinal contents • Peristaltic contractions move the contents along the digestive tract • Contractions increase in response to filling of intestinal lumen • Also regulated by parasympathetic system, pH, tonicity of contents • Peristaltic waves continue to move material through colon to rectum

  10. Chemical Digestion Digestion of food components takes place in specific locations of the digestive tract, by specific enzymes • Carbohydrate digestion begins in the oral cavity: - Salivary amylase: digests complex CHOs to dextrans, maltose, simple sugars - optimum pH: 6.9 - starches are not digested in the oral cavity • In the stomach, the pH drops to < 3.0 - salivary amylase stays active for about 20 minutes - acidic environment inactivates amylase after that

  11. Carbohydrate Digestion (cont.) • In the small intestine, digestion of carbohydrates continues: - pH rises - pancreatic amylase released into the duodenum - large CHOs further digested - dissacharides cleaved by dissacharidases on microvilli

  12. Digestion of Protein • No digestion of protein occurs in the oral cavity • In the stomach, pepsin begins to break down peptide bonds - pepsin formed from pepsinogen (zymogen) - pH optimum for pepsin = 1 to 3 - pepsin is inactivated at higher pH - only about 15% of protein digestion occurs in the stomach

  13. Digestion of Proteins (cont.) • In the small intestine, protein digestion is completed - pH = 7 to 9 - pancreas secretes trypsinogen, chymotrypsinogen, and carboxypetidase • Trypsinogen => trypsin (activated by enterokinase); digests at arginine or lysine residues • chymotrypsinogen => chymotrypsin; digests at carboxy linkages of hydrophobic amino acids • carboxypeptidase: digests from carboxy terminus • aminopeptidase (from small intestine): digests from amino terminus

  14. Digestion of Lipids • The first step in lipid digestion is emulsification with bile - bile salts secreted by liver, stored in the gall bladder - digestive enzymes are water soluble, act at surface of lipid droplets - bile is secreted into the duodenum (small intestine) • Pancreatic lipase converts lipids to free fatty acids and glycerol • Cholesterol esterase converts cholesterol esters into cholesterol and free fatty acids

  15. NEXT LECTURE..... Accessory Digestive Organs

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