Download 2.BIIDigestion1

Digestion & Absorption
•
Water and ions
•
Carbohydrate (CHO)
•
Protein
•
Fat
Vocabulary: trypsin/ogen, enterokinase, proteolytic, chylomicron,
chyme, micelle
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Digestion & Absorption
GENERAL PRINCIPLES
• Breakdown of complex molecules
– Enzymes (pH)
• Absorption into gut cells
– Mechanism of absorption
– Active [energy]
– Passive [no direct energy ]
– carriers
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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
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Absorption of Water
• Passive
• Osmotic gradient
– Due to Na+ [active]
• ‘Leaky’ tight junctions (paracellular)
– Regulated
• ~95% absorption in small intestine
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Carbohydrate (CHO) Digestion
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Carbohydrate (CHO) Digestion
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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)
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Carbohydrate Absorption
• Glucose, galactose enter capillaries
down concentration gradient
• Fructose enters capillaries via passive
carrier mediated transport
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Protein Digestion
Essentially similar to CHO
• Proteins to peptides
– Gastric pepsinogen
– Activated by HCl AND pepsin
– Pancreatic proteases (trypsin)
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Protein Digestion contd
Essentially similar to CHO
• Peptides to amino acids (brush border)
• Absorbed by secondary active transport
– Depends on Na+ transport
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Pancreas
• Endocrine
– Insulin,glucagon
• Exocrine
– Enzymes
(acini)
– Bicarbonate
(ducts)
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Digestion and Absorption of Fat
• Emulsification by
bile salts
• Pancreatic lipase
• Micelles are watersoluble
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Digestion and Absorption of Fat
• Monoglycerides and FFA
enter cells by diffusion
• Triglyceride synthesis
• Add protein
• Chylomicrons
• To lacteal (lymph)
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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
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3. Regulation of Digestive
Function
•
Neural mechanisms
•
Humoral [hormonal] mechanisms
•
Control of movement
•
Control of secretions
•
Acts to maintain contents of gut
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