A level-readyAll subjects · Biology · WBI15 (Unit 5)
Respiration overview and glycolysis
Respiration releases the energy in glucose in many small, enzyme-controlled steps and traps it in ATP. The first stage, glycolysis, happens in the cytoplasm with or without oxygen.
Aerobic respiration
C6H12O6 + 6O2 → 6CO2 + 6H2O (energy released, much of it trapped in ATP)
Respiration is a many-step process, each step controlled by a specific enzyme. Releasing energy gradually means it can be captured efficiently in ATP rather than lost as heat, and intermediates can be used to make other molecules. The stages are glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation.
Glycolysis
Glycolysis happens in the cytoplasm and does not need oxygen.
1. Phosphorylation: glucose is phosphorylated using 2 ATP, making it more reactive (hexose bisphosphate).
2. Splitting: the 6-carbon sugar splits into two 3-carbon triose phosphate molecules.
3. Oxidation: hydrogen is removed from each triose phosphate and taken up by NAD, forming reduced NAD.
4. Each triose phosphate is converted to pyruvate, producing 2 ATP (4 in total).
Net yield per glucose: 2 ATP, 2 reduced NAD, 2 pyruvate.
| Per glucose | |
|---|---|
| ATP used | 2 |
| ATP made | 4 |
| Net ATP | 2 |
| Reduced NAD | 2 |
| Pyruvate (3C) | 2 |
Without oxygen
Glycolysis happens in both aerobic and anaerobic respiration. Without oxygen, pyruvate cannot enter the mitochondria to be oxidised. Instead:
- in animals, pyruvate is reduced to lactate (lactic acid)
- in plants and yeast, pyruvate is converted to ethanol and carbon dioxide
Either way, reduced NAD gives its hydrogen to pyruvate (or ethanal), regenerating NAD so glycolysis can continue making 2 ATP per glucose.
Worked example
How much ATP is made, and how much is used, in glycolysis for 3 glucose molecules? What is the net yield?
- Per glucose: 4 made, 2 used.
- For 3 glucose: 12 made, 6 used.
- Net = 12 - 6 = 6
Answer: 12 made, 6 used, net 6 ATP.
Check yourself
- What is the net gain of ATP from glycolysis of 5 glucose molecules?
Show answer
10. Net 2 ATP per glucose: 5 x 2 = 10.
- How many molecules of reduced NAD are made in glycolysis from 4 glucose molecules?
Show answer
8. 2 per glucose: 8.
- Where does glycolysis take place?
- Mitochondrial matrix
- Inner mitochondrial membrane
- Stroma of the chloroplast
- Cytoplasm
Show answer
Cytoplasm. Glycolysis enzymes are in the cytoplasm.
- Why is glucose phosphorylated at the start of glycolysis?
- To make it more reactive so it can be split
- To release energy as heat
- To make it leave the cell
- To store it as starch
Show answer
To make it more reactive so it can be split. Adding phosphate from ATP activates the sugar.
- Why is respiration carried out in many small steps?
- It makes respiration slower than necessary
- Each step releases a large burst of heat
- Energy is released gradually and can be trapped efficiently in ATP
- So that no enzymes are needed
Show answer
Energy is released gradually and can be trapped efficiently in ATP. Small energy steps match the energy needed to make ATP.
- In anaerobic respiration in muscle, why is pyruvate converted to lactate?
- To regenerate NAD so glycolysis can continue
- To produce more carbon dioxide
- To make oxygen
- To produce large amounts of ATP in the mitochondria
Show answer
To regenerate NAD so glycolysis can continue. Reduced NAD is reoxidised by passing hydrogen to pyruvate.
- What are the products of anaerobic respiration in yeast?
- Lactate only
- Water and oxygen
- Ethanol and carbon dioxide
- Glucose and carbon dioxide
Show answer
Ethanol and carbon dioxide. Yeast ferments pyruvate to ethanol and CO2.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.