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
Transporters which facilitate Na+ movement into cell
- Na+/glucose co-transporter
- Na+/amino acid co-tranporter
- Na+/H+ cotransporter
- Osmosis of water
Next step is movement of water into paracellular spaces as the increased concentration of Na+ ions in paracellular space has increased the osmotic gradient.
Osmosis between the tight junctions and through the cells
Finally moves into the circulating blood of the villus
Aldosterone effect on Na+ absorption
Causes increased activation of the enzyme and transport mechanisms for all aspects of Na+ absorption.
Conserves NaCl and water in the body when a person becomes dehydrated
Absorption of Cl- ions
In duodenum and jejenum the Cl- absop is quick as it follows the Na+
In ileum and parts of large intestine it is absorbed by Cl-/HCO3- exchanger
Exits on basolateral side via chloride channels
Absorption of HCO3- in duodenum and jejenum
- When Na+ absorbed in brush cells from chyme, exchanged for H+
- These combine with the HCO3- to form carbonic acid, which then dissociates into water and CO2.
- Water remains in chyme, CO2 is readily absorbed into blood and expired through lungs
What are carbohydrates absorbed as?
80% of all carbohydrates are absorbed as glucose
Remaining 20% are galactose and fructose
Glucose and galactose absorption
The movement of Na+ out of basolateral membrane makes Na+ conc in cell very low.
Therefore Na+ from chyme wants to move into brush cell, however only carrier SGLT1 does so by facilitated diffusion
Therefore binds to glucose or galactose and pull them into cell
Fructose absorption
transported by facilitated diffusion via GLUT5 all the way through epithelium
On entering the cell, most will be converted into glucose and transported with rest of glucose into the blood
How are proteins absorbed
Most require co-transport mechanism (transporters on apical membrane)
Few transported by facilitated diffusion
How are anionic amino acids absorbed
via SLC1A1 transporter
Requires cotransporter of Na+
H+ out and K+ out
How are neutral and cationic aminoacids absorbed
via SLC5A14 transporeer
Also requires Na+ contransporter
Absorption of fats after micelles
The micelles that hold MG and fatty acids penetrate into recesses of microvilli and let MG and fatty acids diffuseinto the interior of the brush cells (as lipids are soluble in the membrane).
IF local acid pH then this can happen
Absorption of the fats if it is not a local acid pH
Then can be transported using transport proteins FAT(P1) plus CD36
What happens to fats in the cell
- move to endoplasmic reticulum where they bind with lipoproteins
- then packaged into chylomicrons (vesicles)
- These are exocitosed and they travel to liver via the lymphatics system
Iron absorption
Most of diet iron is Fe3+, but it is the Fe2+ form that is absorbed.
Fe3+ reductase activity occur near brush borders. Gastric secretions dissolve the iron
Iron absorption occurs in the duodenum via DMT1.
What happens after iron is in the cell
Some is stored as ferritin
Remainder is transported out into blood through basolateral transporter ferroportin 1. Hp facilitates
In the blood it is converted back to Fe3+ and bound to transferrin
Composition of foeces
three quarters water one quarter solid matter the solid matter is; - 30% dead bacteria - 10-20% fat - 10-20% inorganic matter - 2-3% protein - 30% undigested roughages
What makes the feces brown
Stercobilin
Urobilin
(derivatives from bilirubin)
