Respiration

Revise Respiration for Biology 5 (IAL) WBI15 (A2 Level) — revision notes and instant AI marking.

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Edexcel IAL Biology · Unit Topic

Respiration

Cells break glucose apart gradually, in stages, so that the energy released can be safely packaged into ATP instead of destroying the cell as heat.

Summary Overview Glycolysis Link & Krebs Electron Transport Chain Anaerobic Respiration Respiratory Quotient Core Practicals Memorise Checklist Exam Tips

📋 Summary — The Whole Chapter in One Glance

Aerobic respiration breaks down glucose using oxygen to release energy, trapped as ATP, in four stages: glycolysis → link reaction → Krebs cycle → oxidative phosphorylation.
Each stage happens in a specific place in the cell — cytoplasm, mitochondrial matrix, or inner mitochondrial membrane — and you MUST know exactly where.
Coenzymes NAD and FAD shuttle hydrogen atoms around; coenzyme A shuttles acetate.
Glycolysis splits glucose into 2 pyruvate, nets 2 ATP, and makes 2 reduced NAD — and it needs NO oxygen.
The link reaction converts pyruvate → acetyl CoA, releasing CO₂ and making reduced NAD (no ATP made here).
The Krebs cycle regenerates oxaloacetate, releasing CO₂ and loading up NAD/FAD with hydrogens, plus a little ATP by substrate-level phosphorylation.
Oxidative phosphorylation uses the chemiosmotic theory: electrons flow down the ETC, pump protons into the intermembrane space, and protons flooding back through ATP synthase make the bulk of the ATP (up to 38 total per glucose).
Oxygen is the final electron acceptor — no oxygen, no ETC, no Krebs cycle, no link reaction. Everything backs up.
Anaerobic respiration keeps glycolysis running by regenerating NAD via lactate fermentation (animals/some microbes) or ethanol fermentation (yeast/plants).
The Respiratory Quotient (RQ = CO₂ ÷ O₂) tells you which substrate is being respired: 1.0 = carbohydrate, 0.9 = protein, 0.7 = lipid.
Two core practicals: DCPIP/methylene blue to track dehydrogenase activity in yeast, and a respirometer to calculate RQ.

1. Overview of Respiration

Think of glucose as a stick of dynamite. If you set it off all at once, you get a huge, uncontrolled release of heat — enough to denature every enzyme in the cell. Respiration is the cell's way of "defusing" that dynamite gradually, stage by stage, using enzymes to control exactly how much energy is released at each step, and capturing that energy in a useful, portable form: ATP.

The overall equation:

Aerobic Respiration glucose + oxygen → carbon dioxide + water + energy
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 2870 kJ

That energy is used to phosphorylate ADP into ATP (adding a phosphate group). ATP is the "energy currency" that powers everything else in the cell — muscle contraction, active transport, protein synthesis, you name it.

The Four Stages — and WHERE They Happen

Stage	What happens	Location
1. Glycolysis	Phosphorylation and splitting of glucose	Cytoplasm
2. Link reaction	Decarboxylation and dehydrogenation of pyruvate	Matrix of mitochondria
3. Krebs cycle	Cyclical pathway of enzyme-controlled reactions	Matrix of mitochondria
4. Oxidative phosphorylation	ATP production via oxidation of hydrogen atoms	Inner membrane of mitochondria