Nutrient digestion and absorption Flashcards
Major sources of carbohydrates
- Sucrose (disacch)
- Lactose (disacch)
- Starches (polysacch)
Starch initial breakdown
Hydrolyzed initially by ptyalin (a-amylase) secreted in saliva
Hydrolysed to maltose and other small glucose polymers
PANCREATIC a-amylase is several times as powerful
Enterocytes lining villi of small intestine
Lactase
Sucrase
Maltase
a-dextrinase
What does lactose split into
Galactose and glucose
What does sucrose split into
Fructose and glucose
What about maltose and other small glucose molecules
Split into multiple small glucose molecules
Are water soluble and absorbed immediately into blood
Pepsin role in protein digestion
Initiates the process
Converts protein to proteoses, peptones and a few polypeptides
Able to digest collagen
Its action terminated in the duodenum
In duodenum and jejenum;
trypsin and chymotrypsin
split proteins into small polypeptides
In duodenum and jejenum;
Carboxypolypeptidase
splits polypeptides into individual amino acids
In duodenum and jejenum;
Proelastase
Converted to elastase
Digests elastin fibres that holds meats together
Enterocytes role in protein digestion
- activates pancreatic proenzymes
- Peptidases aminopolypeptidase and several dipeptides split remaining large polypeptides into tr/dipeptides and aa. These are then transported through membrane into the enterocytes
- Inside there are more peptidases that split all into aa. These pass onto other side of enterocyte and into the blood
What are enterocytes
Line the villi of small intestine (duodenum and jejenum).
Have a cell brush border that contain hundreds of microvilli containing peptidases that protrude through membrane to exterior and in contact w fluids
Types of fat in the diet
Most abundant are Triglycerides; Glycerol nucleus and three fatty acid chains
Also have small quantities phopholipids, cholesterol, and cl esters
Initial digestion of TG
By lingual lipase in the saliva. This is secreted by lingual glands in the mouth (less than 10% of fat digestion)
Digestion of fat
1st step
Emulsification;
Breakdown of fat into very small sizes so that water soluble digestive enzymes can act on globule surface
Most happens in duodenum under influence of bile.
Why is bile needed for emulsification
Doesn’t contain any digestive enzymes but does contain large quantity of bile salts and the phospholipid lechitin.
Polar parts of these are highly soluble in water, and the rest is highly soluble in fat.
SO soluble in fat part dissolves in the fat molecule. The polar parts are projecting out, making the fat particle now soluble and reduces the interfacial tension
What does the reduced interfacial tension mean
That the particle can be broken up into many very small particles, increasing the fat’s total surface area.
This happens by agitation with the water in the small intestine.
Lipase enzymes are water soluble so can only attack on fats cell surface —> good increased SA
Digestion of triglycerides by pancreatic lipase
Most important enzyme for digestion of the TG
Present in enormous quantities in pancreatic juice
The TG is split into fatty acids and 2 monoglycerides.
Bile salts role to form micelles
Hydrolysis of TG is highly reversible so bile salts form micelles (when high enough conc in water). the fat-soluble sterol nucleus encompasses the MG and fatty acids, leaving the polar groups covering. As polar groups negative charge the whole micelle can dissolve in water.
Where does the micelles take the MG and free fatty acids?
To the brush borders of interstinal epithelial cells. There the MG and fatty acids are absorbed into the blood for ABSORPTION.
Bile goes back to chyme to be used again
Digestion of cholesterol esters
Most cholesterol in diet is in form of esters, which are combinations of cholesterol and one molecule fatty acid.
These are hydrolysed by enzyme cholesterol ester hydrolase in pancreatic secretion.
Transported via micelle
Digestion of phospholipids
As these also contain fatty acids, they are hydrolysed by enzyme phospholipidase A2.
Transported via micelle
Why does most absorption take place in the small intestine?
- Contains many lumen-protruding folds called valvulae conniventes, that extend circularly around
- These folds are lined with villi projections
- These villi have microvilli on their surfaces (brush border). These have actinfilaments which cause continuous movement of the microvilli
Therefore the total surface area is very great
- Absorption of Na via active transport
Active transport of Na out of the cell through basolateral walls via Na+/K+?ATPase pump. Into paracellular spaces
This passively drags Cl- out
This reduces Na- conc of brush cell, causing Na+ from the chyme to move in via apical membrane
