Metabolism

Question 1

What is anabolism?

Answer 1

Reactions that synthesise new (larger) molecules from smaller precursors. These need energy.

Question 2

What is catabolism?

Answer 2

Reactions that break down larger molecules into smaller ones, usually to release energy.

Question 3

What is metabolism?

Answer 3

All the chemical reactions in the body. Because of the second law of thermodynamics, this costs energy.

Question 4

What are the 4 phases of metabolism?

Answer 4

Absorptive: dealing with a meal

Post-absorptive: maintaining homeostasis between meals

Fasting: dealing with the challenge of longer periods without food

Intense exercise: responding to dramatic increases in demand

Question 5

What does muscle also contain as a very short term energy store?

Answer 5

phosphocreatine

Question 6

What does adenylate kinase do?

Answer 6

Turns 2 ADPs into an ATP and an AMP

Question 7

What is AMP a marker of?

Answer 7

AMP is a marker of a low-energy state, and regulates a number of metabolic enzymes allosterically.

Question 8

Can acetyl CoA be made from amino acids or fatty acids?

Answer 8

Both, to power the TCA cycle.

Question 9

What can you use TCA intermediates to make?

Answer 9

fatty acids and amino acids.

Question 10

Are carbohydrates an efficient energy store?

Answer 10

Carbohydrates bind a lot of water, making them bulky and relatively inefficient as an energy store.

Question 11

How is glucose stored?

Answer 11

Glucose can be stored as polymers, mainly glycogen, in liver and muscle: this provides a rapid but relatively short-lived energy store.

Question 12

Can glucose be released from muscle?

Answer 12

Liver, but not muscle, can release glucose derived from glycogen into the circulation.

Question 13

Are fats a good energy store?

Answer 13

Fats provide a very dense energy store, because they don’t bind much water, and contain little oxygen.

Question 14

What can the liver do to fatty acids during starvation?

Answer 14

The liver can convert fatty acids into ketone bodies during starvation.

Question 15

Is protein a good store of energy?

Answer 15

Not ideal as a source of energy, as mainly stored as functional proteins, so catabolism impairs cellular function.
But, the liver can convert (most) amino acids to glucose (gluconeogenesis).

Question 16

How do erythrocytes get energy?

Answer 16

Erythrocytes lack mitochondria, so they are unable to carry out aerobic metabolism.
In some ways, that’s quite good, since their main job is to carry oxygen, and not to use it up…
…but it does mean that they have to get all their energy from anaerobic glycolysis, which is pretty inefficient.
Fortunately, they don’t have high metabolic demands,and “waste” pyruvate and lactate can go back to the liver for gluconeogenesis.

Question 17

Plasma glucose concentration below what value can lead to unconsciousness and death?

Answer 17

Under normal conditions, the brain depends on a fairly steady plasma glucose concentration: sudden falls below about 3 mM can lead to unconsciousness and death.

Question 18

Uptake of glucose in adipocytes is sensitive to what?

Answer 18

Insulin-sensitive uptake of glucose, and glucagon-sensitive release of FFA and glycerol.

Question 19

What happens in adipocytes when glucose levels fall?

Answer 19

As glucose levels fall, adipocytes metabolise triglyceride to release FFA and glycerol.

Question 20

Describe metabolism in the liver

Answer 20

First “port of call” for foodstuffs absorbed from the gut. Insulin-sensitive uptake, and glucagon-sensitive release, of glucose.

During periods of high glucose, converts glucose to glycogen.

As glucose levels fall, metabolises glycogen to release glucose.

Gluconeogenesis using lactate, glycerol, amino acids

Ketone body production from Acetyl CoA

Question 21

Describe metabolism in cardiac muscle

Answer 21

Highly aerobic tissue: up to 40% mitochondria, and abundant myoglobin. Fatigue in cardiac muscle is bad news…

Main energy source is fatty acids, but can also use lactate or ketone bodies.

Glucose uptake is insulin-sensitive.

Question 22

Describe type 1 skeletal muscle metabolism

Answer 22

Rather like cardiac muscle: highly aerobic

Adapted to prolonged, relatively modest, activity: for example, postural muscles

Main energy source is fatty acids.

Question 23

Why is skeletal muscle type 2b white?

Answer 23

Few mitochondria, and little or no myoglobin (hence white)

Question 24

What kind of metabolism does skeletal muscle type 2b rely on?

Answer 24

Rely mainly on anaerobic glycolysis, with glucose coming from their glycogen store.

Question 25

Describe metabolism of skeletal muscle type 2a

Answer 25

Intermediate between Type 1 and Type 2b. Contain some mitochondria, and myoglobin. Will maintain aerobic metabolism as much as possible.

Phosphocreatine and glycogen provide a good store of energy.

Adrenalin (or insulin) allow glucose uptake.

At lower exercise levels, fatty acids are the main energy source.

During fasting, release amino acids for gluconeogenesis

Question 26

Why is just keeping plasma glucose high a bad idea?

Answer 26

That’s what happens in diabetes mellitus…

Osmotic effects

Glucose lost in the urine, wasting energy and causing polyuria.

Glycation of proteins, and later cross-linking

- microvascular and macrovascular disease
- peripheral neuropathy
- low-grade inflammatory effects