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