INTEGRATION OF METABOLISM and ORGAN SPECIALISATION

INTEGRATION OF METABOLISM AND ORGAN SPECIALISATION

Use these links to navigate to selected parts of this section of the Introductory Metabolism module.

Introduction
Brain
Muscle
Adipose tissue
Liver

Introduction

The pathways and reactions which collectively make up metabolism do not occur in isolation. Each of the pathways is inter-related and inter-dependent on the other pathways. Similarly, each organ has a metabolic relationship with the rest of the body, although each has its own role in the overall metabolism of biological fuels.

Brain

The brain is characterised by a very high respiration rate. It makes up only about 2% of the total body mass but uses 20% of the total body consumption. The rate at which the brain undergoes aerobic metabolism is independent of neural activity.

Most of the ATP powers an active transport system known as Na⁺/K⁺ ATP’ase. The role of this system is to pump K⁺ into cells and Na⁺ out of cells maintaining membrane potentials required for nerve transmission.

Glucose is the major fuel used by the brain and this is supplied by the liver via the bloodstream under all conditions. Only after prolonged starvation does the brain change to using ketone bodies as a fuel. The brain does not store any fuel reserves.

Muscle

Muscle is a major energy consumer. It uses glucose from its own supply of stored glycogen and other fuels supplied to it from other tissues.

These include :

glucose
fatty acids
ketone bodies

Glycolysis is a preferred ATP production pathway in fast twitch (white) muscle fibres This results in lactate production; the lactate is then used by other tissues, for example slow twitch muscle, brain and cardiac muscle, or transported to the liver where it is reconverted to glucose to supply the muscle.

Slow twitch muscle, which includes cardiac muscle, utilises aerobic metabolism for ATP synthesis i.e. complete fuel oxidation to CO₂.

Adipose Tissue

Adipose tissue is widely distributed throughout the body with an average 70 kg human possessing about 15 kg of fat tissue. The mass of adipose tissue contains about 600,000 kilojoules of energy and is sufficient to sustain life up to three months without any food intake.

Adipose tissue stores fatty acids from the diet or synthesised in the liver as triglycerides.

Liver

The liver is described as the body's central metabolic clearing house. The liver is the site of entry to the body of all foods following digestion. It then maintains circulating nutrient levels to supply other tissues.

The liver has a role in glucose metabolism :

synthesis of glycogen when glucose is plentiful - insulin stimulates
release of glucose from glycogen when needed - glucagon stimulates
synthesis of glucose 6-phosphate from other dietary sugars
synthesis of glucose from lactate and amino acids when needed

It has a role in fat metabolism :

when demand for energy is high :
when demand for fuels is low :

It has a role in amino acid metabolism :

de-amination of amino acids --------> NH4⁺ ---------> urea
carbon skeleton ----------> glucose or ketones

The liver also has a variety of other metabolic functions :

plasma protein synthesis
nucleic acid base degradation
iron storage
detoxification
bilirubin conjugation

INTRODUCTORY METABOLISM MODULE