What form does most ingested fat take? And what are hte products of its digestion?
Triacylglcyerol, A union of glycerol and 3 fatty acids
Its broken down to Monoglyceride and 2 FA
What enzymes digests TAG? what is the major issue with this enzyme?
Its water soluble vs TAG which is fat soluble and so forms lipid droplets.
This decreases the surface area for lipase to act making it very slow
How do we increase hte surface area for fat digestion?
produces droplets around ~1mm
Explain the process of fat emulsification
1) Smooth muscle contraction grinds and mixes the luminal contents, breaking up the fatty droplets
2) emulsifing agents bile salt and phospholipids prevent the droplets reforming into big ones
How do emulsifying agents work?
Phospholipids and bile salts are anphipathic (i.e. they have a polar and non-polar end).
The polar end faces outward interfacing with the water and repelling other droplets
How do we speed up fat absorption in the gut?
By forming the products into micelles, same idea as emulsified droplets but smaller (~4-7um).
What are the constituents of a micelle?
- Products of fat digestion (monoglyceride + FA)
- Bile salt
How do the products of Fat digestion get from micelles to the intestinal epithelium?
1) They reach the brush border by being squelched around (brownian motion)
2) In the acid microclimate aorund the villi the fat products take on H+ ions, becoming uncharged
3) The products can then diffuse across the plasma membrane
The micelle itself is not absorbed
Explain the equilibrium of Fatty Acids & monoglyceride in solution vs in micelles
A dynamic equibilibrium where products are reduced by H+ ions and pass from micelles into solution in the brush border whilst more are taken up by micelles in the gut lumen.
What happens to the products of fat digestion once theyre in the enterocytes?
1) Taken up by Smooth ER (because not water soluble so cant sit in cytoplasm)
2) Reformed into TAG
How are the products of fat digestion passes form the enterocyte into the blood?
1) TAG formed into vesicles from sER membrane
2) Processed through golgi apparatus into chylomicron
3) Exocytosed through basolateral membrane into ECM
4) Chylomicrons are still too big to pass through capillary basement membrane so enters lacteals -> Intestinal duct -> Thoracic Duct -> Blood
What is a chylomicron?
Made of Phospholipids, cholesterol, TAG & proteins/vitamins
What is a lacteal?
A lymphatic vessel of the small intestine whose function is to absorb fats
What are the important vitamins we absorb from our gut and what are their classes?
- B Group
- Folic Acid
How do vitamins get absorbed through the gut wall?
Fat soluble ones follow the same path as fat.
= Micelle -> solution -> diffusion -> chylomicron -> Lacteal
Water soluble ones are absorbed by passive diffusion or carrier mediated transport
How does vitamin B12 get absorbed?
Its large and charged so passive diffusion ain’t an option.
Binds to intrinsic factor (glycoprotein) forming a complex which is absorbed by a specific transport mechanism in the ileum.
Where is vitamin B12 stored?
In the liver, around 3 years worth at any time
What happens if you have a B12 deficiency?
Red blood cells fail to mature and you get pernicious anaemia
How much iron reaches our blood?
~10% of what we ingest daily
A cell of intestinal lining
How is iron transported from the gut lumen into enterocytes?
By DMT1 (divalent metal transporter 1) in the duodenum. This is H+ coupled transport just like that of peptides
What happens to the bulk of iron in enterocytes?
- Its incorporated into a protein-iron complex called ferritin (an intracellular iron store)
- Its stored there until the enterocyte dies (5 day life) and is shat out
- Hence more iron in your diet -> More ferritin -> Darker faeces. Isnt science awesome
How does iron get from the enterocyte to your blood? and what happens to it then
Any that doesn’t form ferritin is transported across the serosal membrane,
From there its bound to the protein transferrin and taken to the liver for use.
How is iron intake regulated?
Ferritin expression is regulated based on your body iron level.
Too little iron (anaemia) -> Less ferritin -> More iron absorbed
and vice versa for hyperaemia