Introduction to lipid metabolism

Question 1

What are the most common lipid molecules

Answer 1

1. Fatty acids

Question 2

Where do fatty acids come from

Answer 2

1. Diet – about 30-40% of calories are from fatty acids
2. Adipose tissue
3. De novo synthesis- Made from carbohydrates and some amino acids

Question 3

Describe adipose tissue role

Answer 3

1. fat storage cells can release fat when needed
2. Found under skin
3. Has function to store more triglycerol than average cell
4. Wales and other animals have large layer- blubber

Question 4

Where does the digestion of fats begin and what is required

Answer 4

1. Begins in small intestine

2. Requires input from liver and pancreas

Question 5

Describe the role of the liver in fat digestion

Answer 5

1. Liver produces and releases BILE via the gall bladder
2. Bile contains a number of slightly different compounds- bile acids and bile salts derived from cholesterol
3. These act as detergents emulsifying the lipids and forming small droplets of fat
4. Opening into duodenum where pancreas and liver have common point where their materials enter- develops from single bud of cells in small intestine which develops into pancreas and liver during development
5. Gall bladder not actually needed

Question 6

Describe the role of the pancreas in fat digestion

Answer 6

1. Produces digestive enzymes and bicarbonate solution

2. Enzymes involved in digestion are called lipases

Question 7

What happens in the intestine

Answer 7

1. Large lipid droplets mixed and biles salts churn in the intestine producing smaller droplets- micelles
2. Micelles mixed with pancreatic lipase which forms monoglyceride and fatty acids
3. These go into small intestine and into endoplasmic reticulum
4. Forms chylomicrons which are exocytosed into lymph

Question 8

Describe the action of triacylglycerol lipase

Answer 8

1. Catalyses hydrolysis of triacylglcyerols at their 1 and 3 positions- ester linkage between backbone and fatty acid
2. Forms 1,2 diacylglycerol + fatty acid
3. Then 2-acylglycerol + fatty acid
4. Act the water-lipid interface so surface area for lipase to attach to is important
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Question 9

What happens once triacylglycerol lipase has acted on the triacyl glycerols

Answer 9

1. Fatty acids and 2-acylglycerol are then able to diffuse into intestinal epithelial cells-cells lining the intestine
2. Once inside the cells these components are reconstituted into triacylglycerols
3. Automatically begin to enter membrane of cells that line intestine-hydrophobic
4. But can’t stay as gradient
5. Simple diffusion- no active process

Question 10

Describe the role of phospholipase A2

Answer 10

1. Acts upon phospholipids to aid their digestion by removal of the fatty acid residue from position C2
2. Forms lysophospholipid + fatty acid
3. This can diffuse across membranes of epithelial cells of small intestine to lymphatic system

Question 11

Name two pancreatic lipases

Answer 11

1. Triacylglycerol lipase

2. Phospholipase A2

Question 12

How are lipids transported

Answer 12

1. Lipoprotein complexes

Question 13

Name 4 lipoprotein complexes and compare their densities

Answer 13

1. Chylomicrons- lowest density
2. VLDL - very low density
3. LDL- low density
4. HDL- high density

Question 14

Describe the properties of chylomicrons

Answer 14

1. Lowest density
2. Highest proportion of fats- float on water
3. Small amount of proteins- allow them to be generated by epithelial cells and move into the lymph

Question 15

Describe how proportion of fats and proteins change in lipoproteins

Answer 15

1. VLDL- Significantly more proteins and phospholipids than chylomicrons
2. HDL- most amount of proteins

Question 16

Describe the structure of lipoproteins

Answer 16

1. Lipid droplets surrounded by proteins and phospholipids that make them soluble in blood
2. Some free fatty acids can be transported in blood, solubilised by binding to serum albumin

Question 17

Describe the role of VLDL and LDLs

Answer 17

1. Transport triacylglycerols and cholesterol from the liver to the tissues

Question 18

Describe the role of HDL

Answer 18

1. Transport cholesterol from the tissues to the liver

Question 19

Describe what takes place in the capillary

Answer 19

1. Contain lipoprotein lipase which interact with chylomicrons to release free fatty acids
2. This forms chylomicron remnants
3. These go back to liver
4. Proteins component causes them to interact with cells on surface
5. Converted into VLDLS which then go into capillary and the process is repeated
6. LDLs can move into extrahepatic tissues and are processed
7. HDL- reverse cholesterol transport

Question 20

Describe lipoprotein lipases

Answer 20

1. Present on capillary surfaces of the tissues that absorb lipid from the blood
2. e.g. Mammary, muscle and adipose tissues
3. The enzyme is activated by Apo-CII component of chylomicrons and VLDL components
4. Fatty acids not particularly soluble but diffuse out of capillary because of gradient