Fat Digestion & Absorption

Question 1

Which pathway is deployed to liberate energy from fatty acids?

Answer 1

beta oxidation liberates acetyl CoA from fatty acids in tissues which can then be used in the krebs cycle

Question 2

steps of beta oxidation

Answer 2

1. fatty acyl CoA is oxidised to FADH2 (energy source)
2. Hydration
3. Oxidation to NADH (energy source)
4. 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

Question 3

Fat soluble vitamins which need fat to be absorbed are

Answer 3

ADEK

Question 4

Essential fatty acids are ‘essential’ as they can only be gained from an endogenous source. Name the major 3 essential fatty acids?

Answer 4

Linoleic acid
Linolenic acid
Arachidonic acid

Question 5

% of fat digestion that occurs in the stomach

Answer 5

15%

Question 6

Ingested lipids are

Answer 6

triglycerides (90%)
cholesterol esters (5%)
phospholipids & lipovitamins (5%)

Question 7

Which enzymes break down lipids in the oral cavity and stomach?

Answer 7

Lingual lipase and gastric lipase (gastric more of an effect) convert triglyceride to diglyceride

Question 8

What kinds of fatty acids are absorbed in the stomach?

Answer 8

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

Question 9

Fatty acids generated in the stomach stimulate release of which hormone? What does it do?

Answer 9

Cholecystokinin - CCK

stimulates flow of bile and pancreatic enzymes into the duodenum

Question 10

What is the major lipolytic enzyme?

Answer 10

pancreatic lipase

Question 11

What does pancreatic lipase require to work?

Answer 11

Alkaline pH
Phospholipids
Bile salts
Fatty acids

Question 12

What does co-lipase

Answer 12

acts as an anchor for pancreatic lipase binding the lipid droplet

Question 13

What does pancreatic lipase do?

Answer 13

degrades triglycerides to monoacylglyceride (MAG) and free fatty acids

Question 14

What other enzymes are also present in pancreatic juice and what do they act on in the fat droplet?

Answer 14

phospholipase A2 (PLA2) breaks down phospholipase
cholesterol ester hydrolase breaks down cholesterol esters so a fatty acid is liberated

Question 15

What are the components of an emulsified fat droplet

Answer 15

triglycerides
phospholipids
cholesterol esters
fat soluble vitamins

Question 16

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?

Answer 16

Hydrophyllic - chyme

Hydrophobic - phospholipids

Question 17

Bile salts dissociate from the micelle just before it enters enterocytes - what happens to them?

Answer 17

they go to the liver via enterohepatic circulation/the portal vein and are recycled

Question 18

What happens to the cholesterol, phosphlipids, long chain fatty acids, MAG, and vitamins when they enter the enterocytes?

Answer 18

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

Question 19

Chylomicrons are exocytosed from enterocytes into the interstitial fluid. What happens to them next?

Answer 19

enter lymphatic system via lacteals (highly specialised lymphatic capillaries) > circulatory system via thoracic duct

Question 20

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?

Answer 20

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)

Question 21

How do short and medium chain fatty acids cross enterocytes and where do they go afterwards?

Answer 21

Cross the enterocyte un-modified and are absorbed straight into the blood

Question 22

Once lipoprotein lipase has acted on chylomicrons and lipoproteins HDL, LDL etc, where does the remnant chylomicron/lipoprotein go?

Answer 22

The remnant chylomicron/lipoprotein is now mainly cholesterol and its goes back to the liver

Question 23

What is cholesterol used for in the body

Answer 23

to make cell membranes
to make bile salts
to make steroid hormones

Question 24

Describe reverse cholesterol transport

Answer 24

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”

Question 25

We absorb exogenous cholesterol but we also make it in an endogenous pathway in the liver. Which enzyme in the liver is involved in synthesis of cholesterol?

Answer 25

HMG CoA Reductase

Question 26

How do statins work?

Answer 26

HMG CoA Reductase Inhibitors

However they partially increase cholesterol synthesis (feedback response in cell) and they increase the no. of LDL receptors (imp) meaning more LDL cholesterol is removed from circulation

Question 27

Side effects of statins

Answer 27

5/1000
myosititis (weak, aching muscles)
hepatitis
contraindicated in pregnant women