Lipid metabolism

Question 1

Does lipid digestion occur in the mouth?

Answer 1

The water-soluble enzymes do not break down fat and oil in the mouth.

However, the mouth contains a small amount of lingual lipase that does breakdown some of the ester connections within the triacylglycerol (TAG).

Question 2

What role(s) does the stomach play in lipid digestion?

Answer 2

Once the fat/oil enters the stomach, it separates into smaller droplets, making it easier to digest.

The stomach also contains gastric lipase enzymes, which can break some of the ester bonds within TAG.

However, most of the breakdown of TAG occurs in the small intestine.

Question 3

How do the pancreatic lipases convert triacylglycerol (TAG) into MAG and fatty acids in the small intestine?

Answer 3

Enzymes break down TAG into MAG + two fatty acids

The MAG and fatty acids then form micelle structures with a polar outer shell and a non-polar centre

Question 4

What are chylomicrons?

Answer 4

Deliver TAGs to the bloodstream (via lymphatic system)

Many TAGs combine with protein, cholesterol and phospholipids

Question 5

When would TAGs be stored or metabolised?

Answer 5

When TAG is not required to generate ATP (energy needs of the body have been met), the TAG will be
transported to an adipocyte (within the adipose tissue) to be stored as fat

When TAG is required to generate ATP (to meet the body’s energy needs), the TAG is broken down
into fatty acids and glycerol to be metabolised

Question 6

What are the 3 steps for fatty acid oxidation?

Answer 6

1. FA activation
2. Transport (into mitochondrial matrix)
3. B-oxidation

Question 7

Input and output for beta-oxidation?

Answer 7

INPUT: Fatty acids (specifically acyl CoA)

OUTPUT: acetyl CoA and the reduced coenzymes FADH2 and NADH

Question 8

Before a fatty acid can undergo Beta-oxidation, it must be activated and then shuttled across the inner
mitochondrial membrane. The activating agent and shuttle molecule are…?

Answer 8

CoA and carnitine

Question 9

The starting material for the process of ketogenesis is..?

Answer 9

Acetyl CoA

Question 10

Under what circumstances is TAG used as the main source of energy?

Answer 10

TAGs are the main source of energy in the fasting state, which occurs after food restriction

Question 11

What must happen to a fatty acid before it can be used in beta-oxidation?

Answer 11

The fatty acid must attach to a CoA unit to become an acyl CoA, before the acyl CoA moves from
the cytosol into the mitochondria via the carnitine shuttle

Question 12

Purpose, input and output for ketogenesis?

Answer 12

Input: multiple acetyl CoA
Output: ketone bodies

Purpose: produce the ketone bodies which can be used as a backup energy source in some organs

Question 13

What is acetyl CoA used for when there is no oxaloacetate available to begin the CAC?

Answer 13

Used to begin ketogenesis

Question 14

What is lipogenesis and where does it occur?

Answer 14

Fatty acids are synthesised from Acetyl CoA

Cytosol

Question 15

What is the starting input (reactant) for lipogenesis and what is the final output (end products)?

Answer 15

INPUT: Many units of acetyl CoA

OUTPUT: a fatty acid

Once three fatty acids have been made by lipogenesis they can each bond to a glycerol (via ester bonds) to form a triacylglycerol (TAG) unit, which can be stored as fat in the adipose tissue

Question 16

What’s the rule to work out how many lipogenesis cycles are required to create a fatty acid?

Answer 16

(No. of carbons in fatty acid/2) – 1

Question 17

What’s the reducing agent needed for lipgenesis?

Answer 17

NADPH

Question 18

Under what circumstance is lipogenesis most likely to occur inside the cell?

Answer 18

After the cells’ energy needs have been met, carbohydrate derived acetyl CoA can be used as the input for lipogenesis