Salam Academy
Sign inCreate account
Learn9700Paper 4 · A Level Structured QuestionsEnergy and respirationATP, glycolysis and anaerobic respiration
Energy and respiration · Subtopic 1 of 2

ATP, Glycolysis and Anaerobic Respiration

ATP (adenine + ribose + 3 phosphates) is the immediate energy source: hydrolysis to ADP + Pᵢ by ATP hydrolase releases a small, usable amount of energy; it is resynthesised by ATP synthase. Glycolysis (cytoplasm): glucose is phosphorylated (2 ATP used), split into 2 triose phosphate, then oxidised to 2 pyruvate, producing 4 ATP (net 2) and 2 reduced NAD. Without oxygen, NAD must be regenerated: in mammals pyruvate → lactate (lactate dehydrogenase); in yeast/plants pyruvate → ethanol + CO₂. Anaerobic respiration yields only 2 ATP per glucose. Respiratory quotient RQ = CO₂ produced ÷ O₂ used: carbohydrate 1.0, protein ~0.9, lipid ~0.7. Lipids release the most energy per gram (more H atoms).

Cambridge 9700 Paper 4Edexcel YBI11 WBI15AQA 7402 Paper 2≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe ATP structure and roles.
  • Describe glycolysis and its products.
  • Compare anaerobic pathways.
  • Calculate and interpret RQ.
Pause and think

Why do cells use ATP instead of releasing energy straight from glucose?

Section 2 of 14

Core idea

Start hereThe energy currency

Every muscle twitch, every pump and every new protein is paid for in ATP.

The big idea

Glycolysis splits glucose into pyruvate in the cytoplasm, making a little ATP; without oxygen, NAD must be recycled by forming lactate or ethanol.

Glycolysis and what happens next

Oxygen decides the route.

Worked exampleRespiratory quotient

Seeds use 25 cm³ O₂ and release 20 cm³ CO₂. RQ and likely substrate?

Goal
RQ.
Strategy
CO₂ ÷ O₂.
  1. RQ=2025=0.80\text{RQ}=\frac{20}{25}=0.80
    Definition.
  2. 0.7<0.80<1.00.7<0.80<1.0
    Between lipid and carbohydrate: mixture, or protein (~0.9).
Answer0.80: a mix of substrates, likely including lipid.
✓ CheckOily seeds have low RQ ✓.

Summary

  • ATP hydrolysis.
  • Glycolysis net 2 ATP.
  • Lactate / ethanol regenerate NAD.
  • RQ.
Key words
Glycolysis
Splitting of glucose to pyruvate in the cytoplasm.
Respiratory quotient
CO₂ produced ÷ O₂ used.
HistoryLohmann, 1929. Karl Lohmann discovered ATP in muscle extracts; Fritz Lipmann later called it the cell's "energy currency".
Fun factYour ATP turnover. You recycle roughly your own body mass in ATP every day, though only about 250 g exists at any moment.
Keep reading for free

Create a free account to read the rest of this lesson: key formulae, worked examples, explore the graph, common mistakes, exam tip, guided practice and more. Your answers are checked and your progress is saved.

Create free accountSign in