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SF Biology II 1
Digestion & AbsorptionDigestion & Absorption
• Water and ions
• Carbohydrate (CHO)
• Protein
• Fat
Vocabulary: trypsin/ogen, enterokinase, proteolytic, chylomicron,
chyme, micelle
SF Biology II 2
Digestion & AbsorptionDigestion & 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 3
Bulk flow of liquid in gutBulk 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 4
Absorption of WaterAbsorption of Water
• Passive• Osmotic gradient
– Due to Na+ [active]
• ‘Leaky’ tight junctions (paracellular)– Regulated
• ~95% absorption in small intestine
SF Biology II 5
Carbohydrate (CHO) Digestion Carbohydrate (CHO) Digestion
SF Biology II 6
Carbohydrate (CHO) Digestion Carbohydrate (CHO) Digestion
SF Biology II 7
CHO Digestion — SummaryCHO 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 8
Carbohydrate AbsorptionCarbohydrate Absorption
• Glucose, galactose enter capillaries down concentration gradient
• Fructose enters capillaries via passive carrier mediated transport
SF Biology II 9
Protein DigestionProtein Digestion
Essentially similar to CHO
• Proteins to peptides– Gastric pepsinogen– Activated by HCl AND pepsin– Pancreatic proteases (trypsin)
SF Biology II 10
Protein Digestion Protein Digestion contdcontd
Essentially similar to CHO
• Peptides to amino acids (brush border)
• Absorbed by secondary active transport– Depends on Na+ transport
SF Biology II 11
PancreasPancreas
• Endocrine– Insulin,glucagon
• Exocrine– Enzymes
(acini)
– Bicarbonate (ducts)
SF Biology II 12
Digestion and Absorption of FatDigestion and Absorption of Fat
• Emulsification
by bile salts
• Pancreatic
lipase
• Micelles are
water-soluble
SF Biology II 13
Digestion and Absorption of FatDigestion and Absorption of Fat
• Monoglycerides and FFA enter cells by diffusion
• Triglyceride synthesis
• Add protein
• Chylomicrons
• To lacteal (lymph)
SF Biology II 14
Fat Digestion & AbsorptionFat 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 15
3. Regulation of Digestive 3. Regulation of Digestive FunctionFunction
• Neural mechanisms
• Humoral [hormonal] mechanisms
• Control of movement
• Control of secretions
• Acts to maintain contents of gut