Fat Digestion & Absorption Flashcards
Which pathway is deployed to liberate energy from fatty acids?
beta oxidation liberates acetyl CoA from fatty acids in tissues which can then be used in the krebs cycle
steps of beta oxidation
- fatty acyl CoA is oxidised to FADH2 (energy source)
- Hydration
- Oxidation to NADH (energy source)
- 2 carbons are cleaved from the carbon chain to give acetyl CoA which enters the krebs cycle and generates energy; rest of the chain recycles into beta oxidation
Fat soluble vitamins which need fat to be absorbed are
ADEK
Essential fatty acids are ‘essential’ as they can only be gained from an endogenous source. Name the major 3 essential fatty acids?
Linoleic acid
Linolenic acid
Arachidonic acid
% of fat digestion that occurs in the stomach
15%
Ingested lipids are
triglycerides (90%)
cholesterol esters (5%)
phospholipids & lipovitamins (5%)
Which enzymes break down lipids in the oral cavity and stomach?
Lingual lipase and gastric lipase (gastric more of an effect) convert triglyceride to diglyceride
What kinds of fatty acids are absorbed in the stomach?
medium and short chain fatty acids are ionised at gastric pH and passively absorbed through stomach mucosa into portal blood
long chain insoluble, remain in fat droplet
Fatty acids generated in the stomach stimulate release of which hormone? What does it do?
Cholecystokinin - CCK
stimulates flow of bile and pancreatic enzymes into the duodenum
What is the major lipolytic enzyme?
pancreatic lipase
What does pancreatic lipase require to work?
Alkaline pH
Phospholipids
Bile salts
Fatty acids
What does co-lipase
acts as an anchor for pancreatic lipase binding the lipid droplet
What does pancreatic lipase do?
degrades triglycerides to monoacylglyceride (MAG) and free fatty acids
What other enzymes are also present in pancreatic juice and what do they act on in the fat droplet?
phospholipase A2 (PLA2) breaks down phospholipase cholesterol ester hydrolase breaks down cholesterol esters so a fatty acid is liberated
What are the components of an emulsified fat droplet
triglycerides
phospholipids
cholesterol esters
fat soluble vitamins
Fat droplets are amipathic - they have a hydrophobic side and a hydrophillic side. This stops them re-combining. What substances do they 2 sides interact with?
Hydrophyllic - chyme
Hydrophobic - phospholipids
Bile salts dissociate from the micelle just before it enters enterocytes - what happens to them?
they go to the liver via enterohepatic circulation/the portal vein and are recycled
What happens to the cholesterol, phosphlipids, long chain fatty acids, MAG, and vitamins when they enter the enterocytes?
they enter the smooth ER:
MAG and FFA are re-esterified to triglycerides, some cholesterol is kept and some is turned back to cholesterol esters, phospolipids and vitamins kept
Protein coat added in RER, buds off and APO-B48 added to outside (or in other words a CHYLOMICRON is formed)
Exocytosed
Chylomicrons are exocytosed from enterocytes into the interstitial fluid. What happens to them next?
enter lymphatic system via lacteals (highly specialised lymphatic capillaries) > circulatory system via thoracic duct
Which enzyme is responsible for removing the triglyceride from chylomicrons and lipoproteins (HDL, LDL, VLDL) so triglyceride leaves circulation and move into muscle and adipose tissue?
lipoprotein lipase on endothelium - hydrolyses triglycerides to free fatty acids and glycerol which it takes up and uses to make ATP (muscle) or turnes back to triglycerides and stores (adipose)
How do short and medium chain fatty acids cross enterocytes and where do they go afterwards?
Cross the enterocyte un-modified and are absorbed straight into the blood
Once lipoprotein lipase has acted on chylomicrons and lipoproteins HDL, LDL etc, where does the remnant chylomicron/lipoprotein go?
The remnant chylomicron/lipoprotein is now mainly cholesterol and its goes back to the liver
What is cholesterol used for in the body
to make cell membranes
to make bile salts
to make steroid hormones
Describe reverse cholesterol transport
Cells die and release lipid which is absorbed into HDLs.
Lipid in walls of vessels can be absorbed into HDL aswell – so HDL can reduces atheromatus plaques.
In the blood, cholesterol in the HDL gets transferred into LDL via a transfer protein CETP. During this process, the profile of the apoprotein contained in the LDL is changed it now promotes removal of cholesterol by the liver when they pass through - “HDL promotes LDL removal”
