digestion and absorption

location of digestion and absorption

• omnivore/carnvore- most in small intestine

• hindgut fermenters- large inestine

• ruminants- significant uptake in rumen too

what arethe digestive secretions and functions of them?

• salivary glands secreted into mouth and tongue: starts CHO and fat digestion

• stomach: starts protein digestion

• exocrine pancreas secreted into small intestine:digest carbs fat protein

• liver secretes bile into small intestie:aids fat digestion and absorption

• small intestinal glands and brush borer: completes final stages of digestion

α-Amylases

enzyme that catalyses the hydrolysis of starch into sugars. present in saliva

attack alpha 1,4 glycosidic only

attack only in middle of chain- endoglycosidases

salivary alpha amylase

• ph optimum 6.6-6.8

• inactivated in stomach where pH less than 3

pancreatic alpha amylase

• digests most of carbs in small intestine

• secreted from exocrine pancreas

enterocyte

• absorptive functional unit of digestive tract

• enterocytes may use glutamate and glutamines well as fatty acids and glucose as their energy supply

enterocyte life cycle

• lumen toxic

• damaged old enterocytes die by apoptosis instead of repair

• removed from epithelial layer and continuously replaced by new ones

• born from cryps of lieberkhun

• eg parvo kills cells in the crypts before they can mature and surface to the top, giving no chance to absorb nutrients

absorption over the brush border

• passive, facilitated, active transport, endocytosis

• nutrients cross over the baso lateral membrane and enter the submucosa where bloodstream is

• directed to hepatic portal vein to the liver and beyond or indirectly via lymphatic

fructose absorption

absorbed by carrier mediated diffusion down its conc gradient into the cell then out into the bloodstream

transport of glucose and galactose

get into the cell, needs energy

characteristic of glucose transport

• saturable

• comp inhibited by galactose

• sodium dependent, action potential

• inhibited by ATPas inhibitor: ouabain

• requires energy

absorbed through SGLT 1

enter through transport with sodium

in cell glucose leves through GLUT- 2 into bloodstream , carrier mediated diffusion and activation of Na/K pump which creates energy for this process

proteases and groups

• are secreted as inactive precursors called zymogens- inactivated

• endopeptidades- attack and cleavage in middle of amino acid chain. secreted by stomach and pancreas

• exopeptidades- split off amino acids from end of the chain. secreted by pancreas and small intestine gland

pepsin

• endopeptidase

• is a gastric proteases

• activates at low ph less than 3

• all except keratin and mucin

• secreted as pepsinogen by chief cells

• hydrogen ion secreted by parietal cells

• always produced simultaeously with pepsinogen

• both produced in gastric pits

pancreatic endopeptidases

• 3 endopepidases are produced by the pancreas and secreted as zymogens into small intestine

• trypsinogen becomes trypsin

• chymotrypsinogen becomes chymotrypsin

• pro elastase becomes elastase

• alkaline optimum

exopeptidases

• after pancreatic endopeptidase digestion peptides are attacked by exopeptidases

• carboxypeptidases split AAs fromm the carboxy terminal. are secreted by the pancreas as zymogens

• amino peptidases split AAs from the N terminal. are made in SI glands. are found on the intestinal cell membrane

fat digesting enzymes

• lingual lipase is active in the mouth and stomach

• pancreatic lipase is active in the small intestine and require phospholipid and bile acids from bile for activation

bile fucntion and release

• aid in digestion and absorption of fat

• bile also excretes excess cholesterol and toxic breakdown products of haem (bile fragments)

• are synthesised from cholesterol in the liver

• stored in gall bladder

• released in repsonse to hormone cholecystokinin

• act in the small intestine aiding digestion and absorption of fat

• some excreted in faeces

• most retuns to liver via HP vein

emulsification

1. forcing the stomachs content through the pyloric sphincter into duodenum

2. bile salts stabilise/coat the resulting lipid droplets and prevent them from coalescing

3. prestalsis helps furhter stabilise the emulsion droplets and keep them in suspension

what are the products of fat digestion?

free fatty acids

2- monoacylglycerol

1- monoacylglycerol

glycerol

absorption of fats

• large fat droplets need to be solubilised for absorption to occur

• bile acids and phospholipids from bile break the lipid droplets down to smaller particles called micelles

• micelles are small and increase the SA for absorption

• micelles carry the fat digestion products to the brush border for absorption

• TAG are re synthesised inside the cells for secretion into lymph

what are chylomicrons

• secreted into lymph and enter the blood via the thoracic duct

• some of the TAG is broken down by lipoprotein lipase on the surface of capillary endothelial cells forming chylomicron reminants

• fatty acids are released to the tisues

• chylomicron remnants are released from the endothelium and taken up by the liver and further metabolised to other lipid particles eg HDL LDL VLDL

absorption of water and electrolytes

• vitamins and minerals are absorbed mainly in the small intestine by mechanisms eg passive diffusion, carrier mediated transport and active transport

• large vol of water are secreted into the GIT in digestive secretions and this must be re absorbed

• absorption of water follows electrolyts and is mainly linked to sodium movement. occurs in small and large intestines