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Transcript
Digestion & Absorption • Water and ions • Carbohydrate (CHO) • Protein • Fat Vocabulary: trypsin/ogen, enterokinase, proteolytic, chylomicron, chyme, micelle SF Biology II 1 Digestion & Absorption GENERAL PRINCIPLES • Breakdown of complex molecules – Enzymes (pH) • Absorption into gut cells – Mechanism of absorption – Active [energy] – Passive [no direct energy ] – carriers SF Biology II 2 Bulk flow of liquid in gut • Input – Ingestion ~ 2 litres per day – Secretion (gut) ~ 7 litres/day • Output – Faeces ~100 ml/day • Conclude ~ 9 litres/day absorbed SF Biology II 3 Absorption of Water • Passive • Osmotic gradient – Due to Na+ [active] • ‘Leaky’ tight junctions (paracellular) – Regulated • ~95% absorption in small intestine SF Biology II 4 Carbohydrate (CHO) Digestion SF Biology II 5 Carbohydrate (CHO) Digestion SF Biology II 6 CHO Digestion — Summary • Polysaccharides to disaccharides (gut amylases) • Disaccharides to monosaccharides (brush border) – Glucose, galactose enter cells by energy-dependent secondary active transport • i.e. ‘piggy-back on Na+ gradient (active) SF Biology II 7 Carbohydrate Absorption • Glucose, galactose enter capillaries down concentration gradient • Fructose enters capillaries via passive carrier mediated transport SF Biology II 8 Protein Digestion Essentially similar to CHO • Proteins to peptides – Gastric pepsinogen – Activated by HCl AND pepsin – Pancreatic proteases (trypsin) SF Biology II 9 Protein Digestion contd Essentially similar to CHO • Peptides to amino acids (brush border) • Absorbed by secondary active transport – Depends on Na+ transport SF Biology II 10 Pancreas • Endocrine – Insulin,glucagon • Exocrine – Enzymes (acini) – Bicarbonate (ducts) SF Biology II 11 Digestion and Absorption of Fat • Emulsification by bile salts • Pancreatic lipase • Micelles are watersoluble SF Biology II 12 Digestion and Absorption of Fat • Monoglycerides and FFA enter cells by diffusion • Triglyceride synthesis • Add protein • Chylomicrons • To lacteal (lymph) SF Biology II 13 Fat Digestion & Absorption • Fat to triglycerides (pancreatic lipase) • Bile salts emulsify (surface area) • Bile salts — micelles containing monoglycerides and free fatty acids (FFA) • Enter passively • Triglyceride synthesis — chylomicrons • Exocytosis and thence to lacteals SF Biology II 14 3. Regulation of Digestive Function • Neural mechanisms • Humoral [hormonal] mechanisms • Control of movement • Control of secretions • Acts to maintain contents of gut SF Biology II 15