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C1.1.2—Role in metabolism

Metabolism is the total network of enzyme-controlled chemical reactions that build, break down and transform molecules inside living cells and organisms.

Syllabus
First assessment 2025
Objective
C1.1.2
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2016
Most common paperPaper2
Typical marks1–4

Common command terms

  • Define
  • Explain

Recent exam appearances

November 2016Paper2 ["SL"] · TZ03(a)[ 1 ]C1.1.2—Role in metabolism
May 2013Paper3 ["SL"] · TZ2C2(b)[ 4 ]C1.1.2—Role in metabolism
Practice this objective

Coverage 2013–2016 · Updated 15 Jul 2026

Enzymes Organize Metabolic Reactions

Metabolism is the complex network of interdependent and interacting chemical reactions occurring in a living organism; each step is catalysed by a specific enzyme.

Enzyme specificity requires many different enzymes, because each active site catalyses only a limited reaction. Linking enzyme-controlled steps lets the products of one reaction become substrates for another.

Cells control metabolic flux by changing enzyme synthesis, activity, location or access to substrate. Regulation at one key step can alter the output of an entire pathway while other pathways continue independently.

In a pathway A → B → C, one enzyme catalyses A → B and a different enzyme catalyses B → C; inhibiting the first step reduces both B formation and downstream C production.

Metabolism is not one reaction or only energy release: it includes all interacting anabolic and catabolic reactions and the controls acting through their enzymes.

Role in metabolism

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, commonly using Define / Explain.

Command terms

Define / Explain

What earns marks

Build the answer around this relationship: Metabolism is the total set of chemical reactions in cells or organisms.

Representative question

Question 1

[Maximum number: 4]

Explain the role of enzymes in metabolic pathways.

Enzyme Catalysis and Rate

Enzymes are biological catalysts that lower activation energy and remain unchanged. Their globular protein shape creates active-site specificity; induced fit aligns substrates; molecular motion and collisions affect rate; temperature, pH, and substrate concentration change activity; assays measure substrate loss or product formation over time.

  • Define enzyme as biological catalyst, effective in small amounts and unchanged.
  • Use active site, specificity, induced fit, ES complex, and activation energy in mechanism answers.
  • Use curve shapes: temperature optimum/denaturation, pH optimum, and substrate saturation plateau.
  • For practicals, state what is measured per unit time and use initial rate, controls, and replicates.

Concept essentials

  • Metabolism is the total set of chemical reactions in cells or organisms.
  • Metabolic pathways are controlled by specific enzymes.
  • Each pathway step may depend on a different enzyme.
  • Inhibiting or lacking an enzyme can block a pathway.