Metabolic Stores

Question 1

Where is glucose mainly stored as glycogen ?

Answer 1

In liver and muscle

Question 2

How much moderate exercise does the glycogen supply in the liver and muscle allow for ?

Answer 2

3-5 hours

Question 3

How is glycogen broken down and used as fuel ? Does any of the processed used to break it down use ATP ?

Answer 3

Most (90%) Wis converted directly to glucose-6-P without consuming ATP
This is only possible if the glucose-6-P is to be in the cell in which it is formed (NO ATP, phosphorylation process)
If glucose is required elsewhere in the body, glycogen converted to glucose (through hydrolysis), and then it is phosphorylated when it has been transported to its destination (x1 ATP used)

Question 4

What are the major form in which fuel is stored ?

Answer 4

Lipids

Question 5

How much more energy can lipids produce relative to glycogen, weight-for weight ?

Answer 5

Triglycerides* (= triacylglycerols*) can produce about six times as much energy as glycogen

Question 6

Which reaction produces esters ?

Answer 6

Alcohol with an organic acid

Question 7

What is the difference between a diacyl glycerol and a triacyl glyerol ?

Answer 7

One of the alcohol groups of glycerol is not esterified

Question 8

Describe the process of lipolysis, and state where each step occurs. Which step(s) are responsible for ATP production ?

Answer 8

1. Lipase digests triglycerides into glycerol and 3 FAs

CYTOSOL:

2. FAs combined with CoA, becoming fatty acyl CoA
3. Transporter takes fatty acyl coA into mitochondria
4. Glycerol becomes a glycolysis substrate, forming pyruvate

MITOCHONDRIA

5. Beta oxidation chops 2-carbon acyl units off the FAs
6. Acyl units become acetyl coA and can be used in the citric acid cycle (along with the pyruvate formed from glycerol)

Electrons removed during fatty acid oxidation are passed along the respiratory chain, so indirectly producing ATP

Question 9

Where are the fatty acids and glycerol released by adipose tissue mainly metabolized ?

Answer 9

In the liver

Question 10

When do ketone bodies form ?

Answer 10

Under conditions when other fuels are not available, breakdown of fatty acids leads to the formation of ketone bodies (STARVATION)

Question 11

What are the main ketone bodies ?

Answer 11

Acetoacetate, 3-hydroxybutyrate and acetone

Question 12

Why are ketone bodies produced when other fuels are not available (starvation) ?

Answer 12

Because acetyl-CoA formed in the breakdown of fatty acids cannot enter the citric acid cycle since oxaloacetate becomes depleted because the liver converts it to pyruvate to produce glucose (gluconeogenesis).
As a result, Acetyl CoA is converted to ketone bodies

Question 13

What is the distinctive smell of ketone bodies ? You could smell it in the breath of which type of person ?

Answer 13

Fruity

Someone fasting

Question 14

What are the different ways in which proteins broken down and stored or used as fuel ?

Answer 14

• Dietary proteins broken down into AAs
• Either incorporated into new proteins, converted to carbohydrates (storage) or used as metabolic fuel (used)
• If used, then: some deaminated, yielding NH4 + a keto acid (intermediates of the glycolytic pathway or of the citric acid cycle)
• Others cannot be deaminated so pass their amino group to a keto acid (mainly α-ketoglutarate) = TRANSAMINATION. Resulting keto acid fed into glycolysis or citric acid cycle (as pyruvate, acetyl-CoA, α-ketoglutarate or oxaloacetate), while resulting AA deaminated and used similarly
• NH4+ formed (through direct deamination or transamination followed by deamination) v toxic so rapidly converted to urea (less toxic, passed through the blood to the kidney, then excreted in urine)

Question 15

What is anabolism ?

Answer 15

Synthesis of more complex molecules (requires ATP)

Question 16

What is gluconogenesis ?

Answer 16

Formation of glucose from non-carbohydrate sources

Question 17

Are gluconogenesis and glycolysis the same but in reverse ?

Answer 17

No, similar to glycolysis in reverse, but some enzymes different

Question 18

Where does the bulk of gluconogenesis occur ?

Answer 18

In the liver

Question 19

What examples of molecules can be fed into gluconogenesis ?

Answer 19

Glycerol (from triglycerides) and the carbon backbone of amino acids can fed into this pathway

Question 20

Briefly identify the main steps in lipid synthesis.

Answer 20

1 .Glycerol made from glucose through glycolysis

2. 2-C acyl units from Acetyl CoA linked together by FA synthase to form FAs
3. One glycerol + 3 FAs = 1 triglyceride

Question 21

Where are FAs incorporated into triglycerides ?

Answer 21

In adipose tissue

Question 22

Where is the bulk of triglycerides stored ? Where else is it stored ?

Answer 22

In adipose tissue

In liver and muscles

Question 23

Why is it that our energy production cannot be as high when we are using lipids as fuel rather than carbohydrate ?

Answer 23

Because fats are not used as effectively as fuel as carbohydrate.

Question 24

How are the AA not in the diet formed ?

Answer 24

Made by transferring amino groups to keto acids

Question 25

What is the largest source of protein that can be broken down for fuel in times of need ?

Answer 25

Muscle

Question 26

Explain the metabolic characteristics of brain (sources of energy).

Answer 26

• Normally: Uses only glucose. CANNOT store glucose or glycogen so requires constant supply in blood. Also cannot convert glucose-6-P to to glucose, so cannot export glucose.
• STARVATION: May use ketone bodies (mainly acetoacetate and 3-hydroxybutyrate) formed in other tissues

Question 27

How much of the body consumption of glucose does the brain account for ? How many g of glucose is that per day ?

Answer 27

60%

120 g per day

Question 28

Why can’t the brain use FAs as fuel ?

Answer 28

Because of BBB, protecting the brain from other potentially dangerous chemicals

Question 29

Explain the metabolic characteristics of skeletal muscle (sources of energy).

Answer 29

• Can use glucose, FAs, ketone bodies
• Resting state: Mainly uses FAs but can also use glucose (has glycogen reserves so converts glycogen to glucose-6-P, cannot convert this to glucose, so cannot export glucose)
• Active state: rate of glycolysis&raquo_space; citric acid cycle, so pyruvate must be converted to lactate. Lactate then passes into the blood and to the liver, where it is converted back to glucose

Question 30

How much of body glycogen is stored in muscle ?

Answer 30

3⁄4 of body glycogen

Question 31

How does the liver takes some of the metabolic burden away from muscle ?

Answer 31

• Under conditions when muscle protein is being broken down, many amino acids pass their amino group to pyruvate (transamination) to form alanine. Alanine enters the blood and in the liver can be converted to back to pyruvate, which is used to synthesize glucose
• Pyruvate also converted to lactate which passes into the blood and to the liver, where it is converted back to Pyruvate then into glucose.

Question 32

What are fat cells called ? What is the main function of these cells ?

Answer 32

Adipocytes

Synthesize and store triglycerides (=triacylglycerols) and release fatty acids and glycerol in times of need

Question 33

Which factor determines the rate at which fatty acids are esterified ?
Which factor influences the activity of the lipase that hydrolyses triglycerides into FAs and glycerol ?

Answer 33

Availability of glucose

Hormones (the lipase is hormone-sensitive)

Question 34

Explain the metabolic characteristics of the liver.

Answer 34

-Normal: Can readily store and release GLUCOSE and can make new GLUCOSE (gluconeogenesis) from LACTATE, ALANINE and GLYCEROL in the diet or released by other tissues
When available, synthesizes FAs, esterifies them and then secretes them into the blood as very low density lipoprotein (VLDL)

-Fasting: liver generates ketone bodies for use in other tissues

Question 35

Which attribute of the liver ensures that the liver has ready access to nutrients absorbed by the gut ?

Answer 35

Hepatic Portal System

Question 36

How are tryglcerides mobilised and utilised when body is in fasting state ?

Answer 36

Acetyl-CoA formed in the breakdown of fatty acids cannot enter the citric acid cycle since oxaloacetate becomes depleted because the liver converts it to pyruvate to produce glucose (gluconeogenesis).
As a result, Acetyl CoA (produced as a result of lipolysis) is converted to ketone bodies (instead of going through citric acid cycle, because it cannot do the latter).