Metabolism

Metabolism is a term that encompasses all the chemical reactions that take place in an organism. These reactions are catalysed by enzymes and are essential for life. This lesson brings together many of the concepts from earlier in the Bioenergetics topic and extends them to cover the full range of metabolic reactions in the body. This is an important topic for AQA GCSE Biology, particularly at Higher tier.

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What Is Metabolism?

Metabolism is the sum of all the chemical reactions in a cell or the body. These reactions are organised into metabolic pathways — sequences of reactions where the product of one reaction becomes the substrate for the next.

Metabolic reactions can be broadly divided into two types:

Type	Description	Energy Change	Examples
Anabolic reactions	Building up larger molecules from smaller ones	Energy required (endothermic)	Photosynthesis, protein synthesis, glycogen synthesis
Catabolic reactions	Breaking down larger molecules into smaller ones	Energy released (exothermic)	Respiration, digestion, glycogen breakdown

graph TD
    A[Metabolism] --> B[Anabolic Reactions]
    A --> C[Catabolic Reactions]
    B --> B1[Small molecules joined to make large molecules]
    B --> B2[Energy required]
    C --> C1[Large molecules broken down into small molecules]
    C --> C2[Energy released]
    B1 --> D[Protein synthesis]
    B1 --> E[Glycogen synthesis]
    B1 --> F[Lipid synthesis]
    C1 --> G[Respiration]
    C1 --> H[Digestion]
    C1 --> I[Glycogen breakdown]

Exam Tip: The AQA specification defines metabolism as "the sum of all the reactions in a cell or the body." You must be able to give this definition and provide examples of both anabolic and catabolic reactions. Think of anabolic as "building up" and catabolic as "breaking down."

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Key Metabolic Reactions

1. Respiration (Catabolic)

Respiration is the most fundamental metabolic reaction. It breaks down glucose to release energy, which drives all other metabolic reactions.

• Aerobic: glucose + oxygen ---> carbon dioxide + water (+ energy)
• Anaerobic (animals): glucose ---> lactic acid (+ less energy)
• Anaerobic (plants/yeast): glucose ---> ethanol + carbon dioxide (+ less energy)

The energy released by respiration is used to power all the anabolic reactions listed below.

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2. Photosynthesis (Anabolic)

Photosynthesis is an anabolic reaction that builds glucose from carbon dioxide and water, using light energy.

• carbon dioxide + water ---(light energy)---> glucose + oxygen

This is the ultimate source of almost all the glucose (and therefore energy) in living systems.

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3. Synthesis of Starch, Glycogen and Cellulose (Anabolic)

Glucose molecules can be joined together to form larger carbohydrate molecules:

Molecule	What It Is	Where It Is Found	Function
Starch	Polymer of glucose	Plants (leaves, tubers, seeds)	Energy storage in plants
Glycogen	Polymer of glucose	Animals (liver, muscles)	Energy storage in animals
Cellulose	Polymer of glucose	Plant cell walls	Structural support

All three are polysaccharides — long chains of glucose monomers. The synthesis of each requires energy from respiration.

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4. Synthesis of Amino Acids and Proteins (Anabolic)

Amino acids are made by combining glucose with nitrate ions absorbed from the soil (in plants) or obtained from the diet (in animals).

• Plants: glucose + nitrate ions ---> amino acids ---> proteins
• Animals: amino acids from digested food ---> proteins

Proteins are polymers of amino acids, joined by peptide bonds. They are needed for:

• Enzymes — biological catalysts
• Structural proteins — collagen, keratin
• Hormones — insulin, growth hormone
• Antibodies — immune defence
• Transport proteins — haemoglobin

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5. Synthesis of Lipids (Anabolic)

Lipids (fats and oils) are synthesised from glycerol and fatty acids, which are themselves derived from glucose.

Lipids are used for:

• Energy storage — lipids store more energy per gram than carbohydrates
• Cell membranes — phospholipids form the bilayer of cell membranes
• Insulation — subcutaneous fat in animals
• Protection — fat around organs acts as a shock absorber
• Waterproofing — waxy cuticles on leaves

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6. Breakdown of Excess Proteins (Catabolic) [H]

In the body, excess amino acids cannot be stored. They are broken down in the liver in a process called deamination:

• The amino group (-NH2) is removed from the amino acid
• This produces ammonia (NH3), which is toxic
• The ammonia is converted to urea in the liver
• Urea is transported in the blood to the kidneys, where it is filtered out and excreted in urine

Step	What Happens	Where
1. Deamination	Amino group removed from excess amino acids	Liver
2. Ammonia produced	Toxic waste product	Liver
3. Ammonia converted to urea	Less toxic form	Liver
4. Urea transported to kidneys	Via the blood	Blood
5. Urea excreted	Filtered out and removed in urine	Kidneys

Exam Tip: Deamination is a Higher tier topic. The key point is: excess amino acids are broken down in the liver, the amino group becomes ammonia, which is converted to urea, and urea is excreted by the kidneys. This links Bioenergetics to the Homeostasis topic.

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7. Breakdown of Glucose in Respiration (Catabolic)

Glucose is broken down step by step through a series of enzyme-controlled reactions. In aerobic respiration, the complete breakdown of glucose releases a large amount of energy. In anaerobic respiration, the breakdown is incomplete and less energy is released.

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8. Formation of Urea from Amino Acids [H]

As described above, the formation of urea is a metabolic pathway involving deamination in the liver. This is an important link between the digestive system, the liver and the excretory system.

graph LR
    A[Excess Amino Acids] --> B[Deamination in Liver]
    B --> C[Ammonia NH3]
    B --> D[Remaining carbon compound used for energy]
    C --> E[Converted to Urea]
    E --> F[Transported to Kidneys in blood]
    F --> G[Excreted in Urine]

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The Role of Enzymes in Metabolism

All metabolic reactions are catalysed by enzymes. Enzymes are biological catalysts — they speed up chemical reactions without being used up.

Key enzyme facts relevant to metabolism:

• Each enzyme has a specific active site that fits a specific substrate (the lock and key model)
• Enzymes work best at an optimum temperature and optimum pH
• If the temperature is too high, enzymes denature — their active site changes shape
• The rate of enzyme-controlled reactions increases with temperature up to the optimum, then drops sharply

Factor	Effect on Enzyme Activity
Temperature	Increasing temperature increases rate up to the optimum; above this, enzymes denature
pH	Each enzyme has an optimum pH; deviation from this reduces activity
Substrate concentration	Increasing substrate concentration increases rate until all active sites are occupied