Biology
Cellular respiration — quick study summary
Cellular respiration converts glucose into ATP, the energy currency of the cell. It runs in four stages: glycolysis in the cytosol, pyruvate oxidation and the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner mitochondrial membrane. Aerobic respiration produces ~30–32 ATP per glucose.
Key points
- Overall: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~30–32 ATP
- Glycolysis: glucose → 2 pyruvate, net 2 ATP and 2 NADH
- Krebs cycle: per glucose makes 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂
- Electron transport chain uses NADH/FADH₂ to pump H⁺ for ATP synthase
- Without O₂, cells switch to fermentation (lactate or ethanol)
In more depth
Glycolysis and the link reaction
Glycolysis happens in the cytoplasm and does not need oxygen. One glucose molecule is split into two pyruvate molecules, with a small net gain of 2 ATP and 2 NADH. When oxygen is available, pyruvate enters the mitochondria, where the link reaction removes carbon dioxide and forms acetyl-CoA, which feeds into the Krebs cycle.
Krebs cycle and oxidative phosphorylation
In the mitochondrial matrix, the Krebs cycle releases carbon dioxide and produces NADH and FADH2. These carriers deliver electrons to the electron transport chain on the inner membrane. The energy released pumps protons across the membrane, and their flow back through ATP synthase makes most of the cell's ATP. Oxygen is the final electron acceptor and forms water.
Anaerobic respiration
Without oxygen, cells rely on glycolysis alone and regenerate NAD+ by fermentation. Animal cells convert pyruvate to lactate; yeast converts it to ethanol and carbon dioxide. This yields far less ATP per glucose than aerobic respiration, which is one reason muscles fatigue during intense exercise.
Common mistakes
- Mixing up breathing and respiration. Breathing moves air in and out; respiration is the chemical release of energy inside cells.
- Saying respiration 'makes energy'. It transfers chemical energy from glucose into ATP.
- Forgetting that plants respire all the time. During the day, photosynthesis simply outpaces respiration.
Practice quiz
Click each question to reveal the answer.
1. Where does glycolysis take place?
- Mitochondrial matrix
- Cytosol
- Nucleus
- Endoplasmic reticulum
Answer: Cytosol
2. What is the final electron acceptor in aerobic respiration?
Answer: Oxygen
Oxygen accepts electrons at the end of the electron transport chain, forming water.
3. How many net ATP does glycolysis produce per glucose?
Answer: 2
Last reviewed: May 2026