Metabolism — The Sum of All Reactions

Metabolism is a concept that ties together everything you have learned in Bioenergetics. The AQA GCSE Combined Science Trilogy specification (8464) requires you to understand what metabolism means and be able to give examples of metabolic reactions in the body.

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What Is Metabolism?

Metabolism is the sum of all the chemical reactions that take place in an organism. Every reaction that occurs in a living cell — from breaking down glucose in respiration to building proteins from amino acids — is a metabolic reaction.

$$\text{Metabolism} = \sum(\text{all chemical reactions in an organism})$$Metabolism=∑(all chemical reactions in an organism)

Metabolic reactions are controlled by enzymes — biological catalysts that speed up reactions without being used up.

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Types of Metabolic Reactions

Metabolic reactions fall into two main categories:

1. Catabolic Reactions (Breaking Down)

Catabolic reactions break large molecules into smaller ones, usually releasing energy.

Catabolic Reaction	What Happens
Respiration	Glucose broken down to CO₂ and H₂O (releases energy)
Digestion	Large food molecules broken down into smaller, soluble molecules
Glycogen → Glucose	Glycogen broken down to release glucose for respiration