Digestion of Fats, minerals and vitamins Flashcards
What are the parts of a triacylglycerol molecule?
1 Glycerol
3 stearic acids
Where does all fat digestion take place and what enzyme is responsible?
Digested in small intestine by pancreatic lipase
Why would digestion of lipid droplets be very slow if only lipase was involved?
Triacylglycerol molecules stick together and form large lipid droplets:
- because it is not water soluble
Lipase is water soluble so can not penetrate/enter the lipid droplets - so could only act on the outer molecules
When lipase acts on triacylglycerol molecules, what is produced?
1 monoglyceride & 2 fatty acids
What process is needed to allow faster digestion of lipid droplets?
Emulsification
Then formation of Micelles
Why does emulsifacation/micelle formation make lipid digestion faster?
Increases the total surface area of lipid droplets and thus increases the area accessible to lipase
What is a micelle?
Very small droplet made up of:
- Bile salt
- Monoglycerides
- fatty acids
- phospholipids
They are similar to emulsion droplets but much smaller, so to further increase the exposed area of lipids
Describe the structure of micelles and how this keeps them very small
Phospholipids and bile salts form the surface of the micelle. They are anphipathic which means the have a polar and non polar end.
The polar (hydrophilic) end sticks outwards and the non-polar (hydrophobic) end sticks in.
The polar end interfaces with the water of the lumen and repels other droplets so they dont coagulate together
The digested products of fats are contained within micelles.
How are these absorbed out of the small intestine?
1) Micelles must be broken down at the epithelium, releasing their contents into solution
2) The acid microclimate around villi donates the H+ to the fat products making them uncharged/unpolarised.
3) This makes them lipid soluble so they can simply diffuse across the plasma membranes, out of the lumen
Explain how the equilbirum between fatty acids and monoglycerides works between the solution and inside micelles
Dynamic equilibrium between fatty acids and monoglycerides in solution and in micelles - retains most of fat digestion products in solution while constantly replenishing supply of free molecules for absorption
(odd question but just learn that)
What happens to micelles after they are broken down?
They are not absorbed so their non-digestive components just float about and then form new micelles
What happens to fat digestion products after leaving the intestinal lumen?
(answer up to the point they leave the sER
After entering the epithelial cells:
- Fatty acids and monoglycerides enter Smooth endoplasmic reticulum (sER)
- Enzymes within the sER reform the FAs & monoGs back into triacylglycerols (TAGs)
- TAGs coated in anphiphatic protein => emulsification
- They then leave sER in vesicles (more on this in next few slides)
Explain what happens to TAG droplets in the epithelial cells when they leave the sER
- TAG droplets are carried through the cell in vesicles formed from sER membrane
- Vesicles travel to Golgi apparatus - where they processed (stuffs added and theyre folded and stuff) into Chylomicrons
- Chylomicrons are exocytosed into extracellular fluid at serosal membrane
What are chylomicrons?
Extracellular fat droplets
They also contain phospholipids, cholesterol & fat-soluble vitamins
In many ways they are similar to micelles - however - they are much smaller (~1 µm diameter)
What is the fate of chylomicrons after leaving enterocytes?
Chylomicrons pass into lacteals between endothelial cells (cannot pass through capillary basement membrane)
What are the two classes of vitamins?
Fat soluble (aka lipid derived)
Water soluble
What are the fat soluble vitamins?
How are they absorbed?
A, D, E, K
To remember, think of asking a random abdn person:
Ah Dont Even Know
They follow the same absorptive path as fats (why theyre found in chylomicrons)
What are the water soluble vitamins and how are they absorbed?
B group, C and folic acid
Either absorbed by passive diffusion or carrier-mediated transport. (kinda similar to Glucose etc) with exceptions
How is vitamin B12 absorbed?
B12 is a large, charged molecule so cant follow the routes of others
Instead, it binds to ‘Intrinsic factor’ - which is secreted in the stomach
This forms a complex which is absrobed by a specific transport mechanism in distal ileum
What does vitamin B12 deficiency cause?
Pernicious anaemia
(failure of RBC maturation)
What role do the intestines play in iron absorption?
10% of daily ingested iron absorbed across intestine into blood
How is iron absorbed?
Transported across membrane via DMT1 into duodenal enterocytes
Here, some iron ions are joined to a specific protein to form ferritin for storage in the cell
Other iron ions pass through the serosal membrane into the blood and bind to Transferrin
What regulates the expression of ferritin in the body?
Ferritin expression regulated depending on body’s iron status
Hyperaemia => increased ferritin levels => more iron is then stored in enterocytes
Anaemia => decreased ferrtin levels => more iron released from enterocytes into the blood
