Role and mechanism of dietary fats EXTRA

Question 1

How are lipids transported in the circulation?

Answer 1

Packaged in lipoproteins

Question 2

What is the clinical relevance of blood lipid levels?

Answer 2

Abnormal levels of lipids in certain lipoproteins are linked to an increase risk of atherosclerosis

Question 3

How is the lipid involved in cholesterol levels?

Answer 3

It is central to the regulation of cholesterol levels in the body.
It synthesizes cholesterol for export to other cells, and also removes cholesterol from the body by converting it to bile salts and putting it into the bile where it can be eliminated in the feces. Also, the liver synthesizes the various lipoproteins involved in transporting cholesterol and other lipids throughout the body.

Question 4

How is cholesterol synthesis in the liver carried out?

Answer 4

Under negative feedback regulation

Question 5

What does increased cholesterol in a hepatocyte cause?

Answer 5

Decreased activity of HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis

Question 6

What are lipoproteins?

Answer 6

Particles that contain triacylglycerol (TAG), cholesterol, phospholipids and amphipathic proteins called apolipoproteins

Question 7

What are the four major types of lipoprotein?

Answer 7

• chylomicrons
• very low-density lipoprotein (VLDL)
• low-density lipoprotein (LDL)
• High-density lipoprotein (HDL)

Question 8

What do chylomicrons and VLDL do?

Answer 8

deliver TAG to cells in the body as they are triglyceride rich

Question 9

Where is VLDL and chylomicrons synthesised?

Answer 9

Chylomicrons are synthesized by enterocytes from lipids absorbed in the small intestine.
VLDL is synthesized in the liver.

Question 10

How is TAG removed from VLDL and chylomicrons?

Answer 10

TAG is stripped from chylomicrons and VLDL through the action of lipoprotein lipase, an enzyme that is found on the surface of endothelial cells

Question 11

What happens after TAG is stripped from VLDL and chylomicrons?

Answer 11

lipoprotein lipase digests the TAG to fatty acids and monoglycerides, which can then diffuse into the cell to be oxidized, or in the case of an adipose cell, to be re-synthesized into TAG and stored in the cell.

Question 12

What does LDL do in the body?

Answer 12

Delivers cholesterol to cells, where it is used in membranes or for the synthesis of steroid hormones

Question 13

What happens to VLDL particles when they are stripped of TAG?

Answer 13

They become more dense, and are remodelled at the liver and transform into LDL

Question 14

How do cells take up cholesterol?

Answer 14

By receptor-mediated endocytosis

Question 15

What is the mechanism behind cells taking up cholesterol?

Answer 15

LDL binds to a specific LDL receptor and is internalized in an endocytic vesicle. Receptors are recycled to the cell surface, while hydrolysis in an endolysosome releases cholesterol for use in the cell.

Question 16

What does HDL do in the body?

Answer 16

Involved in reverse cholesterol transport

Question 17

Where is excess cholesterol eliminated?

Answer 17

Via the liver, which secretes cholesterol in bile or converts it to bile salts.

Question 18

What does the liver remove?

Answer 18

LDL and other lipoproteins from the circulation by receptor-mediated endocytosis

Question 19

What happens in reverse cholesterol transport?

Answer 19

Excess cholesterol from cells is brought back to the liver by HDL in a process known as reverse cholesterol transport. HDL is synthesized and secreted by the liver and small intestine. It travels in the circulation where it gathers cholesterol to form mature HDL, which then returns the cholesterol to the liver via various pathways.