Digestion of carbohydrates, proteins & fats - Dietary substrates, enzymes, end-products of digestion

Digestion of carbohydrates Dietary substrates – starch, sucrose, lactose Salivary & pancreatic alpha-amylase* (starch dextrins, maltotriose, maltose) 2

Digestion of CHO contd S.I. microvilli membrane enzymes -dextrinase* (dextrins maltotriose & maltose - maltase* (maltotriose glucose) (maltose glucose) - lactase* (lactose glucose, galactose) - sucrase* (sucrose glucose, fructose) 3

Digestion of Proteins Stomach pepsin* (protein proteoses, peptones, polypeptides) Pancreatic trypsin*,chymotrypsin* (proteoses, peptones, polypeptides Polypeptides, amino acids) Pancreatic carboxypeptidase* (polypeptides amino acids) 4

Digestion of proteins contd Brush border amino-peptidase* (polypeptides amino acids) Cytoplasm di & tri-peptidases* (di & tri-peptides amino acids) *enzyme activity highest in upper S.I. 5

Digestion of Fats Dietary substrates – triglycerides (neutral fats), sterol esters, sterols(e.g cholesterol) Lingual lipase*, gastric lipase*(?) (triglycerides FFA + MG), 30% Pancreatic lipase* (emulsified triglycerides FFA + MG) Pancreatic cholesterol esterase* (cholesterol esters FFA + cholesterol) 6

Absorption of food Route of absorption Mechanism(s) of absorption 7

ROUTE OF ABSORPTION OF CARBOHYDRATES Int. lumen enterocyte capillaries (ISF) Gen. circuln. Liver portal vein 8

MECHANISMS OF CHO ABSORPTION Glucose & Galactose -2 o active transport (lumen enterocyte) with Na + (SGLT1) -Facilitated diffusion (enterocyte ISF) (GLUT2) *Glucose & Na + share same cotransporter or symport, sodium-dependent glucose transporter, SGLT. 9

CHO ABSORPTION CONTD Fructose - facilitated diffusion independent of Na+ lumen enterocyte; (GLUT5) enterocyte ISF; (GLUT2) *highest capacity for absorption is the duodenum & upper jejunum 10

Carbohydrate absorption 11

ABSORPTION OF PROTEINS Route - same as for CHO Mechanisms of absorption S.I. lumen enterocyte amino acids – 2 o active transport Di & tri-peptides - with Na+ (rapid) –D-amino acids - passive diffusion (slow) Enterocyte ISF Passive diffusion/facilitated diffusion 12

Protein absorption 13

END PRODUCTS OF FAT DIGESTION Short chain FFA (< 12 C atoms) Long chain FFA (>12C atoms) Monoglycerides cholesterol 14

SHORT CHAIN FFA (route & mechanism of absorption) Route - same as for CHO & proteins mechanism s.i. lumen enterocyte (passive diffn) enterocyte ISF (passive diffn) 15

LONG CHAIN FFA, Monoglyerides, Cholesterol (route/mechanism) S.I. lumen enterocyte (chylomicrons) passive diffn + protein carriers Enterocyte ISF (lymphatic system) exocytosis Lymphatic system thoracic duct venous circulation *absorption mainly in duodenum & jejunum 16

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MICELLES Spherical water-soluble complexes formed by interaction of bile salts & lipids in s.i Polar hydroxy portion of bile salts faces the outside, non-polar sterol portion faces the inside i.e hydrophobic centre and hydrophilic exterior Products of fat digestion are contained in the hydrophobic centre 18

FUNCTIONS OF MICELLES Keep lipids in solution (solubilization) Provide mechanism for lipid transportation to mucosal cells (for absorption), thus preventing reesterification of lipids Provide a saturated solution of the end- products of fat digestion (at the mucosal membrane) thereby building up a concn. gradient for passive diffusion. 19

CHYLOMICRONS Large lipoprotein complexes formed in the enterocytes. They enter the circulation via the lymphatic ducts Formed from re-esterified fatty acids and cholesterol coated with a layer of protein, cholesterol & phospholipids Circulatory chylomicrons are hydrolysed by lipoprotein lipase to FFA & glycerol. The chylomicron remnants are taken up by the liver. 20

ABSORPTION OF VITAMINS *mostly in upper small intestine, vit B12 and vit C in ileum Water soluble vitamins - most can be absorbed by simple diffn (especially in high doses) - specific transport mechanisms for some e.g vit B12 – active with intrinsic factor Fat soluble vitamins - passive diffusion through micelles 21

ABSORPTION OF WATER About 9 litres present in G.I daily About 98% is reabsorbed mostly in S.I. Daily water loss in stool is about 200ml Water moves by osmosis across the intestinal mucosa in response to osmotic gradients. 22

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