Lipid Metabolism

Question 1

How is lipid transported from the GI tract into the body?

Answer 1

• they go into the lymphatics as chylomicron –> drains into the thoracic duct –> venous blood –> removed from plasma
• adipose and liver membranes contain lipoprotin lipase –> hydrolyzes the chylomicron into triglyceride and fatty acids
• the triglycerides immediately diffuse into cells –> then is resynthesized to triglyceride

Question 2

How is the fat stored in adipose tissue transported to another site?

Answer 2

• released from the cells as free fatty acids
• the free fatty acids are bound to albumin in the plasma

Question 3

What are the 3 major classes of lipoprotein?

Answer 3

1. VLDL - very low density lipoproteins: mainly triglycerides
2. IDL - intermediate density lipoproteins: a bit less triglycerides than VLDL
3. LDL - low density lipoproteins: removal of almost all triglycerides
4. HDL: high density lipoproteins: 50% protein with smaller concentration of lipids

Question 4

Where are the lipoproteins formed?

Answer 4

almost all are formed in the liver

Question 5

How are lipoproteins formed and transported?

Answer 5

they are formed from carbohydrates, and transported to other parts of the body as VLDL

Question 6

What is the major function of adipose tissue?

Answer 6

to store fat

Question 7

What type of cells are fats?

Answer 7

modified fibroblasts, can store large amounts of pure triglycerides

Question 8

What are the 3 major roles the liver has in lipid metabolism?

Answer 8

1. degrade fatty acids into smaller compound to be used for energy
2. to synthesize triglycerides, mainly from carbohydrates and proteins
3. to synthesize other lipids, especially from cholesterol and phospholipids

Question 9

What are the 3 main steps to use triglycerides for energy?

Answer 9

1. hydrolysis of triglycerides to fatty acids and glycerol
2. transport to active tissue
3. oxidized to release energy
   Almost all tissues can use fat, except brain and red blood cells

Question 10

Where does fatty acid oxidation occur?

Answer 10

mitochondria, carrier mediated (by carnitine)
once inside, it dissociates from the carnitine

Question 11

What are the end production of fatty acid oxidation?

Answer 11

Beta oxidation
- releases 1 carbon segments to form acetyl Co-A –> citric acid cycle –> CO2 and H+
- the H+ oxidized to form ATP

Question 12

What’s the function of acetoacetic acid?

Answer 12

The liver is the site for a large amount of fatty acid degradation to acetyl Co-A, but it doesn’t need that much for itself
- the acetyl Co-A pairs and condenses to form acetoacetic acid, which is then converted to beta hydroxybutyric acid
- beta hydroxybutyric acid freely diffuses through the liver cell membrane –> goes into bloodstream –> diffuses into target cells, and reversed back to acetyl Co-A –> goes to citric acid cycle

Question 13

Where does most of the triglyceride synthesis occur?

Answer 13

liver

Question 14

Where is the triglyceride stored?

Answer 14

once made in the liver, they are transported by lipoproteins to adipose tissues for storage until needed for energy

Question 15

What is the first step of making lipids from carbohydrates?

Answer 15

conversion of carbohydrates into acetyl co-A
- this happens normally in degradation of glucose by glycolysis

Question 16

How does acetyl co-A become triglycerides??

Answer 16

• fatty acids bind with glycerol to form triglycerides

Question 17

What’s the importance of alpha-glycerophosphate?

Answer 17

the glycerol portion of triglyceride is furnished with alpha-glycerophosphate, a breakdown product of glycolysis
- therefore, triglyceride formation is dependent on glucose availability

Question 18

Why is fatty acids derived from carbohydrate important?

Answer 18

• not all the cells in the body can store carbohydrates in the form of glycogen
• fat is also more energy dense

Question 19

How is triglycerides synthesized from protein?

Answer 19

many amino acids can be converted to acetyl Co-A, which can be convereted to triglyceride

Question 20

How does the amount of carbohydrate influence energy use and fat metabolis?

Answer 20

excess carbohydrates = fat sparing
- they are used preferentially for energy
- they can also make triglycerides
- therefore, they also increase fat stores

Question 21

What are the hormonal (4) regulation of fat utilization?

Answer 21

1. Epinephrine/ norepinephrine
   - release from adrenal glands stimulated during heavy exercise
   - activates hormone sensitive triglyceride lipase –> rapid breakdown of triglycerides and mobilization of fatty acids
2. ACTH
   - in response to stress, release cortisol, which also activates hormone sensitive triglyceride lipase
3. Growth hormone
   - similar to ACTH, but less potent, less activation of hormone sensitive lipase
   A LACK of insulin will active hormone sensitive lipase
4. Thyroid hormone
   - rapid mobilization of fat
   - probably indirectly from increased metabolism

Question 22

What are the 3 major types of phospholipids?

Answer 22

1. lecithins
2. cephalins
   - large numbers of cephalins form thromboplastine
3. sphingomyelins
   - act as insulators in myelin

Question 23

Where is the endogenous cholesterol formed?

Answer 23

liver

Question 24

How does cholesterol go round the body?

Answer 24

circulates in lipoproteins

Question 25

What’s the function of cholic acid?

Answer 25

• forms bile salt with other substances
• used by the adrenal glands to from adrenocortical hormone
• used by the ovaries for progesterone and estrogen
• used by the testes for testosterone