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

2. Anabolic Reactions (Building Up)

Anabolic reactions join small molecules together to form larger ones, usually requiring energy.

Anabolic Reaction	What Happens
Photosynthesis	CO₂ and H₂O joined to form glucose (requires light energy)
Protein synthesis	Amino acids joined to form proteins
Glucose → Starch	Glucose molecules joined to form starch for storage
Glucose → Cellulose	Glucose molecules joined to form cellulose for cell walls
Lipid synthesis	Glycerol and fatty acids joined to form lipids
Glucose + Nitrates → Amino acids	Small molecules combined to form amino acids

graph TD
    A["METABOLISM"] --> B["Catabolic reactions"]
    A --> C["Anabolic reactions"]
    B --> B1["Break down large molecules"]
    B --> B2["Release energy"]
    B --> B3["e.g. respiration, digestion"]
    C --> C1["Build up large molecules"]
    C --> C2["Require energy"]
    C --> C3["e.g. photosynthesis, protein synthesis"]

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Key Metabolic Reactions You Must Know

The AQA 8464 specification lists several metabolic reactions you should be able to describe:

1. Respiration (Catabolic)

Breaks down glucose to release energy — covered in detail in earlier lessons.

2. Photosynthesis (Anabolic)

Builds glucose from CO₂ and H₂O using light energy — covered in earlier lessons.

3. Synthesis of Proteins

• Amino acids are assembled into proteins on ribosomes.
• The order of amino acids is determined by DNA.
• Proteins include enzymes, hormones (e.g. insulin), antibodies and structural proteins (e.g. collagen).

4. Conversion of Glucose to Starch, Glycogen or Cellulose

• Starch — storage in plants.
• Glycogen — storage in animals (liver and muscles).
• Cellulose — structural component of plant cell walls.

All three are polysaccharides made by joining many glucose molecules together.

5. Formation of Lipids

• Lipids are formed from glycerol and fatty acids.
• They are used for energy storage, insulation and cell membranes.

6. Synthesis of Amino Acids

• Formed from glucose and nitrate ions (absorbed from the soil in plants).
• In animals, excess amino acids are broken down in the liver by a process called deamination — the amino group is removed and converted to urea, which is excreted by the kidneys.

graph LR
    A["Glucose"] --> B["Starch (plants)"]
    A --> C["Glycogen (animals)"]
    A --> D["Cellulose (cell walls)"]
    A --> E["Amino acids (+ nitrate ions)"]
    E --> F["Proteins"]
    A --> G["Lipids (fats and oils)"]
    A --> H["Respiration → Energy"]

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Why Metabolism Matters

Understanding metabolism helps you see how all the reactions in the body are linked:

• Photosynthesis produces glucose.
• Respiration breaks down glucose to release energy.
• The energy from respiration drives anabolic reactions (building molecules).
• Digestion breaks down food into small molecules that enter metabolic pathways.
• Waste products from metabolism (CO₂, urea) must be removed from the body (excretion).

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Common Mistakes

Mistake	Correction
Saying metabolism is just "one reaction"	Metabolism is the sum of all chemical reactions in an organism
Confusing anabolic and catabolic	Anabolic = building up; Catabolic = breaking down. Think "anabolic = adding"
Forgetting that enzymes control metabolic reactions	All metabolic reactions are enzyme-controlled
Not linking metabolism to energy transfer	Catabolic reactions release energy; anabolic reactions use energy
Forgetting deamination	Excess amino acids are deaminated in the liver — amino group removed and converted to urea

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Worked Example

Question: Give two examples of anabolic reactions and two examples of catabolic reactions in the human body.

Answer:

Type	Example 1	Example 2
Anabolic	Protein synthesis (amino acids → proteins)	Glycogen synthesis (glucose → glycogen)
Catabolic	Aerobic respiration (glucose → CO₂ + H₂O)	Digestion of proteins (proteins → amino acids)

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Summary

• Metabolism is the sum of all chemical reactions in a living organism.
• Catabolic reactions break down large molecules into smaller ones and release energy (e.g. respiration, digestion).
• Anabolic reactions build small molecules into larger ones and require energy (e.g. photosynthesis, protein synthesis).
• Key metabolic reactions include: respiration, photosynthesis, protein synthesis, lipid formation, starch/glycogen/cellulose synthesis, and amino acid synthesis.
• Excess amino acids are broken down by deamination in the liver, producing urea.
• All metabolic reactions are controlled by enzymes.

Exam Tip (AQA 8464): When asked to define metabolism, always write: "Metabolism is the sum of all the chemical reactions in a cell or organism." Then give at least one example of a catabolic reaction and one anabolic reaction.

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Deeper Exploration: Metabolic Rate and Energy Requirements

Metabolic rate is the rate at which chemical reactions take place in a cell or organism. It is measured as the rate at which energy is released (e.g. in kJ/day, or equivalently by O$_2$2​ consumption).

Basal Metabolic Rate (BMR)