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C1.1.3—Anabolic and catabolic reactions

Anabolic reactions build larger molecules, while catabolic reactions break molecules down and release usable materials or energy for cellular processes.

Syllabus
First assessment 2025
Objective
C1.1.3
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper1
Typical marks1–3

Common command terms

  • Identify
  • Distinguish

Recent exam appearances

May 2023Paper1 ["HL"] · TZ15[ 1 ]C1.1.3—Anabolic and catabolic reactions
May 2022Paper1 ["HL"] · TZ28[ 1 ]C1.1.3—Anabolic and catabolic reactions
May 2017Paper2 ["HL"] · TZ15(c)[ 3 ]C1.1.3—Anabolic and catabolic reactions
November 2016Paper2 ["HL"] · TZ03(a)[ 2 ]C1.1.3—Anabolic and catabolic reactions
Practice this objective

Coverage 2016–2023 · Updated 15 Jul 2026

Anabolism Builds; Catabolism Breaks Down

Anabolic reactions build larger molecules from smaller units and require energy; catabolic reactions break down or oxidize molecules and often release usable energy.

Anabolism commonly joins monomers by condensation, forming covalent bonds and releasing water. Catabolism includes hydrolysis of macromolecules in digestion and oxidation of respiratory substrates.

Anabolism Catabolism
Amino acids → proteins by condensation Proteins → amino acids by hydrolysis in digestion
Glucose → glycogen Glycogen or other macromolecules → smaller units
Carbon dioxide → organic molecules in photosynthesis Glucose/fatty acids oxidized during respiration

Catabolic respiration can supply ATP and reducing power that drive anabolic protein or glycogen synthesis, linking the two parts of metabolism.

Energy input or release is a typical consequence, not the sole classification rule: identify whether the cellular pathway constructs or breaks/oxidizes material.

Anabolic and catabolic reactions

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Distinguish.

Command terms

Identify / Distinguish

What earns marks

Build the answer around this relationship: Anabolism builds larger or more complex molecules from smaller units.

Representative question

Question 1

[Maximum number: 2]

Identify the following processes as either anabolism or catabolism by placing a tick ( ✓ ) in the correct box.

ProcessAnabolismCatabolism
Light-independent reactions of photosynthesis□\square□\square
Glycolysis□\square□\square

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

  • Anabolism builds larger or more complex molecules from smaller units.
  • Catabolism breaks molecules down into smaller products.
  • Condensation is commonly anabolic, while hydrolysis is commonly catabolic.
  • Metabolism includes both anabolic and catabolic reactions.