Digestion and absorption of lipids- the handling of iron in the body Flashcards
What are the sites of lipid digestion?
- The mouth, stomach, small intestine.
Discuss lipid digestion in the mouth.
- Mechanical digestion: mastication emulsifies lipids into tiny droplets.
- Chemical digestion: saliva contains lingual lipase which hydrolyses medium and long chain triglycerides.
- Products are: diglycerides, monoglycerides + fatty acids. ‘Serine proteases’
Discuss lipid digestion in the stomach.
- 2 enzymes act in stomach: lingual + gastric lipase.
- Both hydrolyse ester bonds in medium and long chain triglycerides.
- Products: diglycerides + fatty acids.
Discuss lipid digestion in the small intestines.
- Emulsification continues, to increase surface area for the enzymes to work on.
- This is achieved via peristalsis and bile salts secreted by gallbladder: detergent properties.
Discuss chemical digestion is small intestine.
- Pancreas secretes pancreatic lipase + pancreatic co-lipase.
- Act together to work on triglycerides. they form a complex that spreads over the surface of fat droplet.
- Continue to hydrolyse triglycerides to monoacylglycerol and fatty acids.
What are the pancreatic enzymes involved in lipid degradation?
- Cholesterol esterase: hydrolyses cholesterol esters to cholesterol and free fatty acids.
- Phospholipase A2: digests phospholipid into Lyso-phospholipid by removing 1 fatty acid. Activated by trypsin.
- Lysophospholipase: removes the remaining fatty acid, leaving glycerylphosphoryl.
*Bile salts required to encourage all.
What are products of lipid digestion?
Mixed micelle:
- Bile salts
- Free fatty acids
- Free cholesterol
- 2-monoacylglycerol
- Phospholipid
Discuss absorption of lipids into enterocytes.
- Mixed micelle gets to brush borders.
- Layer of water adjacent to brush border known as unstirred water layer.
- As hydrophilic layer of micelles interacts with this layer.
- It facilitates transport of the hydrophobic lipids across the water layer to the brush border membrane.
- Short medium FAs don’t need micelle for transport.
Discuss chylomicron formation.
-FAs and monoglycerides are transported to ER
-then built back up into triglycerides.
- The triglycerides are grouped with other digestion products-phospholipids, cholesterol ester, free cholesterol.
- These products are packaged into a transporter known as chylomicrons and goes to Golgi apparatus.
Discuss the chylomicron life cycle.
- Chylomicrons are extruded from Golgi apparatus into exocytotic vesicles.
- Transported into the basolateral membrane.
- These vesicles then fuse with the plasma membrane + undergo exocytosis, then dumped into extracellular space where there enter lymph.
Discuss iron level regulation?
- Very carefully regulated.
- Body has 2-4g of iron.
- Average intake is 20mg daily.
Discuss Iron digestion.
- Iron comes into body attached to haem(red meat) or non-haem iron (freely).
- In the stomach + SI: haem iron hydrolysed by proteases to remove iron from haem group.
- HCl also aids in releasing iron from food.
Discuss iron absorption.
- For iron to enter enterocyte, needs to be in Fe2+.
- Haem iron: absorbed into enterocyte via haem carrier protein 1.
- Once inside, haem oxidase acts and we get Fe2+ and haem.
- Non-haem iron: Fe3+ is reduced by reductase duodenal cytochrome B (DcytB) to Fe2+
- Then transported via divalent metal transporter 1(DMT1).
Discuss the storage of iron.
- Stored in enterocyte.
- Iron is stored as Fe3+ as ferritin.
- Fe2+ in enterocyte is converted back to Fe3+ by hephaestin.
- Fe3+ is moved around body by transferrin.
What are the things that interfere with the absorption of iron?
- Tannins in tea
- Oxalate, Phytate from plants
- Antacids that contain phosphate
