Every movement, thought, and heartbeat requires energy. Cell respiration is how cells convert the chemical energy in glucose into the universal energy currency of the cell — ATP.
When you sprint 100 metres, your muscles demand enormous amounts of energy immediately. But when oxygen can't reach your muscles fast enough, they keep working anyway.
How do your cells continue to produce energy when oxygen is limited? And what price does your body pay for this emergency energy production?
Come back to this at the end of the lesson.
Core Content
Cell respiration is the controlled release of energy from organic compounds to produce ATP. It is not the same as breathing — breathing simply brings oxygen into the body. Cell respiration happens inside cells, converting the chemical energy stored in glucose into ATP, which powers all cellular activities.
Cells can produce ATP through two main pathways depending on oxygen availability. The difference is fundamental to understanding exercise physiology, fermentation, and why oxygen is essential for complex life.
Requires: Oxygen
Location: Cytoplasm + Mitochondria
ATP Yield: ~30-32 per glucose
Products: CO₂ + H₂O
Efficiency: High — complete glucose breakdown
Requires: No oxygen
Location: Cytoplasm only
ATP Yield: 2 per glucose
Products: Lactate (animals) or ethanol + CO₂ (yeast)
Efficiency: Low — incomplete glucose breakdown
Aerobic respiration consists of three main stages, each occurring in a specific cellular location with distinct biochemical reactions. Understanding the location and products of each stage is essential for the HSC.
Location: Cytoplasm
Requires: No oxygen (occurs in both aerobic and anaerobic conditions)
Input: Glucose (6C)
Outputs: 2 Pyruvate (3C), 2 ATP (net), 2 NADH
Glycolysis literally means "sugar splitting." A single glucose molecule (6 carbons) is split into two pyruvate molecules (3 carbons each). This process produces a small amount of ATP through substrate-level phosphorylation and transfers electrons to NAD⁺, forming NADH.
Location: Mitochondrial matrix
Requires: Oxygen (indirectly — it regenerates electron carriers)
Input: Acetyl-CoA (2C, derived from pyruvate)
Outputs: CO₂, 2 ATP, 6 NADH, 2 FADH₂ per glucose
Before entering the Krebs cycle, each pyruvate loses a carbon as CO₂ and combines with coenzyme A to form acetyl-CoA. The Krebs cycle completes the oxidation of glucose, extracting electrons and transferring them to NAD⁺ and FAD. The carbon atoms exit as CO₂.
Location: Inner mitochondrial membrane (cristae)
Requires: Oxygen (as the final electron acceptor)
Input: NADH, FADH₂, O₂, ADP, phosphate
Outputs: ~26-28 ATP, H₂O
This is where the majority of ATP is produced. Electrons from NADH and FADH₂ pass through the electron transport chain, pumping protons across the inner membrane. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. The proton gradient drives ATP synthesis through chemiosmosis.
When oxygen is absent, cells cannot proceed past glycolysis. The pyruvate produced must be converted to regenerate NAD⁺, allowing glycolysis to continue producing small amounts of ATP. Different organisms use different fermentation pathways.
In exercising muscle, when oxygen demand exceeds supply, pyruvate is converted to lactate. This regenerates NAD⁺ so glycolysis can continue. The lactate buildup contributes to muscle fatigue and oxygen debt. Once oxygen returns, lactate is converted back to pyruvate in the liver (the Cori cycle).
Yeast and some plant cells convert pyruvate to ethanol and carbon dioxide. This process is exploited in brewing, winemaking, and bread-making. The CO₂ causes bread to rise; the ethanol evaporates during baking.
Understanding where ATP comes from helps explain why aerobic respiration is so much more efficient than anaerobic respiration.
| Stage | ATP Produced | Method |
|---|---|---|
| Glycolysis | 2 ATP (net) | Substrate-level phosphorylation |
| Krebs Cycle | 2 ATP | Substrate-level phosphorylation |
| Oxidative Phosphorylation | ~26-28 ATP | Chemiosmosis (electron transport chain) |
| Total Aerobic | ~30-32 ATP | Complete oxidation of glucose |
| Anaerobic | 2 ATP | Glycolysis only |
Substrate-level phosphorylation transfers a phosphate directly from a substrate molecule to ADP. Oxidative phosphorylation uses the energy from electron transport to create a proton gradient, which drives ATP synthesis — much more efficient and producing far more ATP.
Misconception: Respiration and breathing are the same process.
Breathing (ventilation) is the mechanical exchange of gases. Respiration is the metabolic process that releases energy from glucose. They are related but distinct.
Misconception: Anaerobic respiration in animals produces alcohol.
Animal cells produce lactate (lactic acid) during anaerobic respiration. Only yeast and some plants produce ethanol and CO₂.
Misconception: Glycolysis requires oxygen.
Glycolysis occurs in both aerobic and anaerobic conditions. Oxygen is only required for the later stages (Krebs cycle and oxidative phosphorylation).
Misconception: The 38 ATP figure is the actual yield in cells.
The theoretical maximum is 38 ATP, but the actual yield is about 30-32 ATP due to energy costs of transporting NADH into mitochondria and proton leak.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~30-32 ATP
Activities
Draw a simple diagram showing the three stages of aerobic respiration. For each stage, indicate:
Use arrows to show the flow of materials between stages. Label your diagram clearly — this will be useful for revision.
Write any notes here.
Copy and complete this comparison table in your book:
| Feature | Aerobic | Anaerobic (Animals) | Anaerobic (Yeast) |
|---|---|---|---|
| Oxygen required? | |||
| Location | |||
| ATP per glucose | |||
| Final products |
Your completed table:
A marathon runner is competing on a hot day. At the 30km mark, she starts to feel her legs burning and her pace slows. Answer the following:
Write your response here or in your book.
Assessment
1. Which of the following correctly identifies where glycolysis occurs?
2. What is the primary purpose of fermentation in anaerobic respiration?
3. Which of the following is produced during aerobic respiration but NOT during anaerobic respiration in animals?
4. Where does the Krebs cycle occur?
5. A yeast culture is grown in anaerobic conditions. Which products would be detected in the growth medium?
1. Write the balanced chemical equation for aerobic respiration and identify the cellular location where most of the ATP is produced. (3 marks)
1 mark for correct equation; 1 mark for identifying inner mitochondrial membrane; 1 mark for naming oxidative phosphorylation
2. Explain why aerobic respiration produces significantly more ATP than anaerobic respiration. In your answer, refer to the role of oxygen. (3 marks)
1 mark for comparison of ATP yields; 1 mark for explaining complete vs incomplete glucose breakdown; 1 mark for role of oxygen as final electron acceptor
3. A student investigates two identical muscle samples. Sample A is incubated with oxygen; Sample B without oxygen. After 10 minutes, both samples have consumed equal amounts of glucose. Predict and explain the relative ATP production in each sample. (3 marks)
1 mark for prediction of ATP yields; 1 mark for explanation of aerobic pathway; 1 mark for explanation of anaerobic limitation
Answers
SA1 (3 marks):
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~30-32 ATP. Most ATP is produced on the inner mitochondrial membrane (cristae) during oxidative phosphorylation, where the electron transport chain and chemiosmosis generate approximately 26-28 ATP per glucose.
SA2 (3 marks):
Aerobic respiration produces approximately 30-32 ATP per glucose compared to only 2 ATP in anaerobic respiration. This difference occurs because aerobic respiration involves complete oxidation of glucose through the Krebs cycle and oxidative phosphorylation, extracting almost all available energy. Oxygen serves as the final electron acceptor in the electron transport chain, allowing electrons to flow continuously and drive proton pumping for chemiosmosis. Without oxygen, cells can only carry out glycolysis — pyruvate cannot enter the mitochondria, and most of the energy in glucose remains trapped in lactate or ethanol.
SA3 (3 marks):
Sample A (with oxygen) would produce approximately 15-16 times more ATP than Sample B. With oxygen, Sample A undergoes complete aerobic respiration (glycolysis → Krebs cycle → oxidative phosphorylation), yielding ~30-32 ATP per glucose. Without oxygen, Sample B can only perform glycolysis, producing 2 ATP per glucose through substrate-level phosphorylation. The pyruvate produced must be converted to lactate to regenerate NAD⁺, but this yields no additional ATP. Although both samples consume glucose at the same rate, Sample A extracts approximately 94% more usable energy from each glucose molecule.
Earlier you were asked about how cells produce energy when oxygen is limited. Consider your original response:
When oxygen is scarce, cells switch to anaerobic respiration (fermentation). Glycolysis continues in the cytoplasm, producing 2 ATP per glucose. To keep glycolysis running, NAD⁺ must be regenerated — this happens when pyruvate is converted to lactate (in animals) or ethanol and CO₂ (in yeast).
The price your body pays: Anaerobic respiration produces only 2 ATP compared to ~30-32 ATP aerobically — a 94% reduction in efficiency. The lactate buildup lowers pH, contributing to muscle fatigue. This is why you cannot sustain maximum effort indefinitely — your muscles demand more ATP than anaerobic respiration can provide, and the waste products accumulate.