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06102026–2028 syllabus

BIOLOGY · CHAPTER 12

Respiration

How cells release energy with or without oxygen.

Core + Supplement3 connected sectionsSyllabus-aligned guide

LEARNING OBJECTIVES

What you will be able to do

  • state energy uses
  • compare aerobic and anaerobic respiration
  • write equations
  • explain oxygen debt

AT A GLANCE

Syllabus0610Coverage2026–2028Sections3LevelCore + Supplement

INTRODUCTION · THE BIG IDEA

How cells release energy with or without oxygen.

Respiration transfers energy from nutrient molecules into forms cells can use. Oxygen allows complete breakdown of glucose, while anaerobic pathways release less energy.

Do not confuse the cellular reactions with breathing: ventilation supplies oxygen and removes carbon dioxide, but respiration occurs inside cells.

01

SECTION 01

Aerobic respiration

Core concept

Aerobic respiration uses oxygen to break down glucose and releases energy for contraction, synthesis, division, active transport, growth, impulses and temperature control.

DETAILED EXPLANATION

  • Yeast respiration rises with temperature to an enzyme optimum, then falls with denaturation.
RULE 1
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
02

SECTION 02

Anaerobic pathways

Core concept

Without oxygen, glucose is incompletely broken down and releases much less energy.

DETAILED EXPLANATION

  • Muscle: glucose → lactic acid during vigorous activity.
ORIGINAL STUDY DIAGRAMAnaerobic outcomes
1yeast → ethanol + CO₂
2muscle → lactic acid
03

SECTION 03

Oxygen debt

Core concept

Lactic acid accumulation creates an oxygen debt removed during recovery.

DETAILED EXPLANATION

  • Breathing remains faster and deeper to supply oxygen.
  • Liver aerobically respires lactic acid.

STEP-BY-STEP EXAM EXAMPLE

Original worked example

Comparing aerobic and anaerobic exercise

  1. Both pathways break down glucose and release energy.
  2. Aerobic respiration uses oxygen and completely breaks glucose into carbon dioxide and water.
  3. In muscle without enough oxygen, anaerobic respiration forms lactic acid.
  4. Incomplete breakdown releases less energy per glucose molecule.

Answer: Anaerobic respiration supplies energy quickly but yields less and creates an oxygen debt.

QUICK CHAPTER SUMMARY

The ideas to carry forward

  • Respiration releases usable energy.
  • Aerobic respiration yields more energy.
  • Anaerobic products differ.
  • Recovery removes lactic acid.

QUICK REVISION CHECKLIST

Can you do each of these without your notes?

  • state energy uses
  • compare aerobic and anaerobic respiration
  • write equations
  • explain oxygen debt
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