Metabolism | Biology

📖 In-Depth Theory

What is Metabolism?

METABOLISM is the sum of ALL the chemical reactions taking place in a cell or organism at any given time.

Every reaction that builds something up or breaks something down is part of metabolism.

Metabolism is what keeps you alive — it powers growth, movement, repair, digestion, nerve signals, and every other process your body performs.

Metabolic reactions fall into two broad categories:

CATABOLISM — breaking large molecules into smaller ones (releasing energy).

ANABOLISM — building large molecules from smaller ones (requires energy).

Building Reactions (Anabolism)

These reactions USE energy (ATP) to build large molecules from small ones:

Protein synthesis:

Amino acids → proteins (e.g. enzymes, haemoglobin, structural proteins).

Requires ATP and takes place at ribosomes.

Cellulose synthesis:

Glucose molecules → cellulose (for plant cell walls).

Starch and glycogen synthesis:

Glucose → starch (storage in plants).

Glucose → glycogen (storage in animal liver and muscles).

Fat synthesis:

Fatty acids + glycerol → triglycerides (for cell membranes, energy storage, insulation).

DNA replication:

Nucleotides → new DNA strands (before cell division).

Breaking-Down Reactions (Catabolism)

These reactions RELEASE energy by breaking large molecules into smaller ones:

Respiration (the most important catabolic reaction):

Glucose + oxygen → carbon dioxide + water (releases ATP).

Digestion:

Starch → maltose → glucose (by amylase).

Proteins → amino acids (by proteases).

Fats → fatty acids + glycerol (by lipase).

Glycolysis:

Glucose broken down in the cytoplasm — the first step of respiration.

Deamination (in the liver):

Excess amino acids → removed amino group → excreted as urea.

The remaining carbon skeleton → used in respiration.

All these reactions are controlled by ENZYMES — which is why temperature and pH are so important for survival.

Why Metabolism Matters

Metabolic rate is the overall speed at which an organism's metabolism runs.

Factors affecting metabolic rate:

BODY SIZE — larger organisms have a higher total metabolic rate (but lower per gram).

MUSCLE MASS — muscle tissue is metabolically very active; more muscle = higher metabolic rate.

EXERCISE — physical activity greatly increases metabolic rate.

TEMPERATURE — ectothermic organisms (e.g. reptiles) have metabolic rates heavily influenced by external temperature.

HORMONES — thyroxine (from the thyroid gland) controls the basal metabolic rate.

UREA PRODUCTION is a key metabolic waste product:

Excess amino acids cannot be stored.

The liver removes the amino group (–NH₂) through DEAMINATION.

Ammonia is produced → converted to UREA (less toxic).

Urea is carried in the blood to the KIDNEYS where it is filtered out and excreted in urine.

⚠️ Common Mistake

Metabolism is the TOTAL of ALL chemical reactions — not just respiration. Students often say 'metabolism = respiration' but metabolism includes digestion, protein synthesis, DNA replication, urea production and many more reactions. Respiration is the most central metabolic process, but not the only one.

📌 Key Note

Metabolism = all chemical reactions in the body. Anabolism = building up (uses ATP). Catabolism = breaking down (releases ATP). Key examples: protein synthesis, respiration, digestion, glycogen synthesis, urea production.

🎯 Matching Activity — Anabolism or Catabolism?

Sort each reaction into building up (anabolism) or breaking down (catabolism). — drag the symbols on the right to match the component names on the left.

Anabolism — building up

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Catabolism — breaking down

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Anabolism — building up

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Catabolism — breaking down

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Catabolism — breaking down

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Anabolism — building up

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Glucose polymerised to make glycogen in liver and muscles

Excess amino acids deaminated in the liver to produce urea

Proteins in food broken down to amino acids by proteases

Amino acids joined to make proteins at ribosomes

Glucose broken down in aerobic respiration to release ATP

Fatty acids and glycerol joined to form triglycerides for fat storage

⭐ Higher Tier Only

Metabolic rate is the rate of all chemical reactions in the body. It is affected by body size, muscle mass (metabolically active), age, temperature (in ectotherms) and hormones (thyroxine from thyroid controls basal metabolic rate). Anabolism = building molecules (protein synthesis, glycogen synthesis, lipid synthesis). Catabolism = breaking molecules down (respiration, digestion, deamination of amino acids in liver → urea).

🎯 Test Yourself

Question 1 of 3

1. What is metabolism?

2. Why is urea produced in the liver?

3. Which of the following is an example of an ANABOLIC (building) reaction?

⭐ How Well Do You Understand This Topic?

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🌿 Metabolism

What is Metabolism?

Building Reactions (Anabolism)

Breaking-Down Reactions (Catabolism)

Why Metabolism Matters

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