Lipid Transport

Question 1

What role do bile salts play in fat digestion in the small intestine?

Answer 1

Bile salts emulsify fats into micelles, increasing the surface area of lipids and making them more accessible to lipases.

Question 2

Why are bile acids effective emulsifiers in the small intestine?

Answer 2

Bile acids are amphipathic, which allows them to form micelles that orient ester bonds in TAGs at the micelle surface, improving lipase accessibility.

Question 3

What is the function of intestinal lipases in lipid digestion?

Answer 3

Intestinal lipases hydrolyze triacylglycerols (TAGs) into free fatty acids (FFAs) and monoacylglycerols by breaking ester bonds using water.

Question 4

How do lipases act on triacylglycerols (TAGs) to facilitate fat digestion?

Answer 4

Lipases catalyze the hydrolysis of TAGs, releasing free fatty acids and monoacylglycerols, which can then be absorbed by intestinal cells.

Question 5

What happens to free fatty acids and monoacylglycerols after hydrolysis in the intestine?

Answer 5

They are absorbed by intestinal epithelial cells (enterocytes), where TAGs are resynthesized and packaged into chylomicrons.

Question 6

What are chylomicrons, and what is their function in lipid transport?

Answer 6

Chylomicrons are lipoprotein complexes that transport dietary TAGs to peripheral tissues. They contain a hydrophobic core of TAGs and a hydrophilic surface of phospholipids and apolipoproteins.

Question 7

How do chylomicrons travel to target tissues in the body?

Answer 7

Chylomicrons are transported through the bloodstream to peripheral tissues, where they release TAGs for uptake.

Question 8

What happens to triacylglycerols (TAGs) in chylomicrons once they reach target tissues?

Answer 8

TAGs are hydrolyzed to free fatty acids and monoacylglycerols, which are then absorbed by the cells of target tissues.

Question 9

After uptake into target tissues, how are free fatty acids used?

Answer 9

Free fatty acids are either stored in adipose tissue as TAGs or oxidized in muscle cells to produce energy.

Question 10

Summarize the journey of dietary lipids from ingestion to storage or energy production.

Answer 10

Dietary lipids are emulsified by bile salts, hydrolyzed by lipases, absorbed into enterocytes, packaged into chylomicrons, transported to tissues, then either stored as TAGs in adipose tissue or oxidized for energy in muscle.

Question 11

Where are lipids stored within adipocytes?

Answer 11

Lipids are stored in lipid droplets within adipocytes.

Question 12

Describe the structure of a lipid droplet.

Answer 12

Lipid droplets have a hydrophobic core of triacylglycerols (TAGs) and sterol esters, surrounded by a monolayer of phospholipids.

Question 13

What proteins coat the surface of lipid droplets, and what is their function?

Answer 13

The surface of lipid droplets is coated with perilipins, which restrict lipid mobilization by regulating access to fatty acids.

Question 14

How do perilipins contribute to lipid mobilization?

Answer 14

Perilipins control when and under which conditions lipids are mobilized or exported from the lipid droplet.

Question 15

What is the first step in the fat storage pathway?

Answer 15

The pathway begins when glucagon or another signaling molecule binds to a receptor on the adipocyte.

Question 16

What is the role of adenylate cyclase (AC) in the fat storage pathway?

Answer 16

AC produces cAMP, which activates protein kinase A (PKA), initiating the phosphorylation cascade.

Question 17

How does PKA affect hormone-sensitive lipase (HSL) and perilipin?

Answer 17

PKA phosphorylates both HSL and perilipin, leading to perilipin dissociating from the lipid droplet.

Question 18

What occurs after perilipin dissociates from the lipid droplet?

Answer 18

CGI-58 dissociates from perilipin, increasing access to fatty acids within the droplet.

Question 19

What enzyme metabolizes triacylglycerols (TAGs) in lipid droplets?

Answer 19

Lipases metabolize TAGs into glycerol and free fatty acids.

Question 20

How are free fatty acids transported in the bloodstream after release from adipocytes?

Answer 20

Free fatty acids are transported by serum albumin in the blood.

Question 21

What happens to free fatty acids once they reach target cells?

Answer 21

Free fatty acids dissociate from serum albumin, enter target cells via plasma membrane transporters, and are oxidized for fuel.

Question 22

What are the 4 primary classes of lipoproteins, and what are their main functions?

Answer 22

Chylomicrons: Transport dietary lipids from the intestine to peripheral tissues.
VLDL (Very-Low Density Lipoprotein): Transport endogenous lipids from the liver to peripheral tissues.
LDL (Low-Density Lipoprotein): Transport cholesterol and cholesterol esters between the liver and peripheral tissues.
HDL (High-Density Lipoprotein): Transport cholesterol and cholesterol esters from extrahepatic tissues back to the liver.

Question 23

What factors determine each lipoprotein’s specific function?

Answer 23

Each lipoprotein’s function is determined by its synthesis site, lipid composition, and apolipoprotein content.

Question 24

Describe the exogenous pathway of lipid transport. in 3 steps

Answer 24

Dietary fats are packaged into chylomicrons in enterocytes and enter the bloodstream.
Lipoprotein lipases hydrolyze TAGs within capillaries, releasing free fatty acids (FAs) to tissues.
Remaining chylomicron remnants are endocytosed by the liver and degraded in lysosomes.

Question 25

Describe the endogenous pathway of lipid transport in 3 steps

Answer 25

Excess lipids are converted to TAGs and cholesterol esters in the liver, packaged into VLDL, and released into the blood.
Lipases release free FAs from VLDL, which are taken up by peripheral tissues.
TAG loss transforms VLDL into LDL, which is taken up by peripheral tissues or returned to the liver if not endocytosed.

Question 26

What is the role of LDL receptors in lipid transport?

Answer 26

LDL receptors mediate endocytosis of LDL in peripheral tissues, allowing cholesterol and cholesterol esters to enter the cells.

Question 27

Explain the process of reverse cholesterol transport in 3 steps

Answer 27

HDL, produced by the liver, gathers cholesterol esters from chylomicron/VLDL remnants and extrahepatic cells.
HDL then returns these cholesterol esters to the liver.
Cholesterol esters may also be transferred to LDL via cholesterol ester transfer protein.

Question 28

What is enterohepatic circulation, and what is its role in cholesterol management?

Answer 28

Enterohepatic circulation involves bile salt synthesis, secretion, and reabsorption by the liver and gallbladder.
It helps remove cholesterol, as cholesterol itself cannot be catabolized or used for energy.

Question 29

How can dietary fiber impact cholesterol levels?

Answer 29

Fiber binds to bile salts, promoting the excretion of cholesterol and helping to lower cholesterol levels in the body.

Question 30

What enzyme catalyzes cholesterol ester formation, and what are its products?

Answer 30

LCAT (lecithin-cholesterol acyltransferase) catalyzes the formation of cholesterol esters from cholesterol and lecithin, resulting in cholesterol ester and lysophosphocholine.

Question 31

Why is cholesterol ester more suitable for storage than free cholesterol?

Answer 31

Cholesterol ester is more hydrophobic than free cholesterol, making it less likely to interact with biological membranes and thus more suitable for storage.

Question 32

What is atherosclerosis, and how is it related to cholesterol levels?

Answer 32

Atherosclerosis is the buildup of fats, cholesterol, and other substances on artery walls, forming plaques that restrict blood flow or can burst to cause clots. High levels of LDL can aggravate it, while HDL acts protectively.

Question 33

Name some drugs available in Canada for treating obesity and describe their function.

Answer 33

Contrave: Manages dependencies, including alcohol, opioid, and nicotine.
Wegovy: Prescribed for weight management.
Saxenda: A GLP-1 agonist used for obesity treatment.
Xenical: An intestinal lipase inhibitor that blocks fat absorption.