Lecture 22: Digestion, absorption and transport of lipids

Question 1

What are the major classes of lipoproteins?

Answer 1

There are 4 main classes of lipoproteins

• Chylomicrons
• Very low density (VLD)
• Low density (LDL)
• High density (HDL)

Question 2

What are bile salts? What do they do?

Answer 2

Bile salts acs as biological detergents which emulsify fats = have hydrophilic and hydrophobic surfaces

• Made from cholesterol
• Synthesised in liver and stored in gall bladder as bile
• Secreted into the small intestine in response to cholecystokinin
• Form micelles with triacylglyecerols ti increase surface area

Question 3

What does bile contain ?

Answer 3

• Water
• Bile acids
• Electrolytes
• Phospholipids
• Cholesterol - increased amount = gall stones
• Bile pigments e.g bilrubin

Question 4

How are micelles formed?

What is their structure?

Answer 4

Bile salts associate with lipid molecules in the intestine to form micelles
- within the micelle, the hydrophobic face of bile salt associated with lipid molecules in the centre while the hydrophillic face associates with the aqeuosus environment

Question 5

What enzymes is responsible for hydrolysis lipids?

Answer 5

Pancreatic lipase cleaves lipids from positions 1 and 3 of the glycerol backbone.
Also contained in the pancreatic juice are cholesterolesterase and phospholipase enemies hydrolyse cholesterol esters and phospholipids

Question 6

How are lipids digested?

Answer 6

1. Lipids are emulsified by bile salts to form micelles which increases the surface area of the lipid
2. Lipids are cleaved by pancreatic lipase - from positions 1 and 3 of the glycerol backbone

Question 7

How are lipids absorbed across the intestinal cells?

What happens after cross the brush border?

Answer 7

3. After lipids are cleaved by pancreatic lipase forming monocylglycerols they are incorporated into smaller micelles and then absorbed across the brush border
4. Absorbed lipids are resynthesises and assembled into lipoproteins inside epithelial cells
   - –> absorbed —-> transported to ER to be resynthesised into triacylglycerol and cholesterol esters

Question 8

How does Xenical inhibit pancreatic lipase?

Answer 8

Xenical acts by mimicking a triacylglycerol molecule which is the natural substrate

• Once Xenical is covalently bound to pancreatic lipase, the enzyme is inactivated and can no longer hydrolyse fatty acids
• —-> fat remains undigested and is excreted in the faeces

Question 9

What is the role of apoproteins in the metabolism of lipoproteins?

Answer 9

Lipoprotien lipase is activated by apoCII which hydrolyses the triacylglycerol core of the chylomicron particle to release fatty acids

Question 10

How do defects in the lipid transport pathway lead to disease?

What is an example of disease?

Answer 10

LIPID malabsorption causes an excess of fat and fat-soluble vitamins in the faeces = is caused by conditions that intervene with bile or pancreatic lipase secretions
Familial hypercholesterolaemia FH = is caused by defects in LCL receptor —> leads to build up of LDL - cholesterol in blood

Question 11

What is a lipoprotein?

• Function
• Structure

Answer 11

Lipoproteins consist of a phospholipid monoilayer(contains apolipoproteins) surrounding a triacylglyecerol core
Function: Lipoproteins transport lipids in blood
—-> delivery system to get lipids IN and OUT of the cell

Question 12

Describe the main classes of dietary lipids?

Answer 12

= Triacylglyecerol: body derives its energy from
- 90% of fat is in this form
= Phospholipids
= Cholesterol: Is an important precursor for bile acids, vitamin D and steroid hormones

Question 13

What are the two major lipid transport pathways?

Answer 13

1. Exogenous chylomicron pathway = transports dietary fat

2. Endogenous VLDL pathway = transports endogenously synthesised fat