Flashcards in Physiology - Nutrient Digestion and Absorption 2 Deck (22)
What is the structure of almost all ingested fat?
Almost all ingested fat is in the form of triacylglycerol.
1 glycerol molecule bonded to 3 stearic (fatty) acids
What is triacylglycerol broken down into and what enzyme does this?
Triacylglycerol is broken down into a monoglyceride and 2 fatty acids.
This is done by the enzyme lipase
How do triacylglycerols arrange themselves in water?
Triacylglycerols are insoluble in water and so form large lipid droplets.
Why is it hard for lipase to digest triacylglycerols on its own.
Large lipid droplets dont present a large surface area for lipase to work and so digestion is very slow.
What is emulsification and how does it help the digestion of triacylglycerols?
Emulsification is the dividing of large lipid droplets into smaller droplets of about 1mm diameter.
This creates a larger surface area for lipase to act on and speeds digestion
What two factors does emulsification require?
Mechanical disruption- large droplets are shaken into small droplets by smooth muscle contraction which grinds and mixes the lumen contents.
Emulsifying agent- this prevents the small droplets reforming into large droplets.
The emulsifying agent contains bile salts and phospholipids (both secreted in bile).
these amphiphatic molecules have polar and non-polar portions.
While emulsification speeds up digestion of triacylglycerols what affect does it have on absorption?
None. Absorption is still very slow and is increased by the formation of micelles
Micelles are similar to emulsion droplets but much smaller (4-7um diameter (about a red blood cell))
Micelles = bile salt + monoglycerides + fatty acids + phospholipids.
polar portions of molecules at micelle surface; non-polar portions form the micelle core.
These micelles act as fat taxis carrying tiny amounts of fat to the wall of small intestine to be absorbed
How does the acid microclimate affect micelles
Fatty acids inside the micelles have their charge affected and so the micelle becomes unstable. This makes it much easier for small amounts of free fatty acids (FFA) and monoglycerides to be released and diffuse across the plasma membrane.
Are the micelles themselves absorbed by the small intestine?
What happens to fatty acids and monoglycerides inside small intestine epithelium?
After entering epithelial cells, fatty acids and monoglycerides enter the smooth endoplasmic reticulum (sER) where they are reformed into triacylglycerides by enzymes in the sER.
Triacylglycerol droplets are coated with amphipathic protein (emulsification).
They are then transported through the cell in vesicles formed from the sER membrane- processed through the golgi and exocytosed into extracellular fluid at serosal membrane
What are chylomicrons?
Chylomicrons are extracellular fat droplets.
These also contain phospholipids, cholesterol and fat soluble vitamins
They pass into lacteals between endothelial cells (cannot pass through capillary basement membrane)
What route do triacylglycerols take through the golgi apparatus?
Triacylglycerols take the opposite route to proteins being formed, entering at the trans face and instead exiting at the cis face
Explain the dynamic equilibrium between fatty acids, monoglycerides and micelles
There is a dynamic equilibrium between fatty acids and monoglycerides in solution and in micelles.
As fatty acids and monoglycerides are absorbed into epithelium the micelles are constantly replenished by a large supply of free molecules
What are the two classes of Vitamins?
Fat soluble vitamins and Water soluble vitamins
Give examples of fat-soluble vitamins and explain how they are absorbed
A, D, E and K
They follow the same absorptive path as fat
Give examples of water- soluble vitamins and explain how they are absorbed
B group, C, folic acid and vitamin B12
B group, C and Folic acid are either absorbed by passive diffusion or carrier mediated transport
How is vitamin B12 absorbed?
Vitamin B12 is a large charged molecule.
It binds to intrinsic factor in the stomach to form a complex which is absorbed via a specific transport mechanism in the distal ileum.
Vitamin B12 is needed for maturation of red blood cells and therefore maximum oxygen transport.
A vitamin B12 deficiency can cause pernicious anaemia.
Most of us have enough B12 stored in the liver to last us 3 years.
If you get your stomach removed after this time you will start to develop symptoms of anaemia and require B12 injections every 3 months
Discuss Iron absorption
10% of daily ingested iron is absorbed across the intestine into blood the other 90% is expelled.
Iron is transported across the brush border membrane via DMT1 (divalent metal transporter number 1) into duodenal enterocytes.
Here iron ions are incorporated into ferritin (protein-iron complex -> intracellular iron store)
Ferritin is finite. What happens to the iron which is unbound?
Transported across the serial membrane into blood.
The iron in the blood binds to transferrin and heads to liver
In addition to iron what other metals does DMT1 transport?
Lead and cadmium.
These are poisonous especially in children