3.6 Cellular Respiration

Releasing energy from glucose

Cellular respiration breaks down glucose to make ATP: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP. It's essentially the reverse of photosynthesis, releasing the energy stored in sugar.

The stages

Glycolysis (in the cytoplasm) splits glucose into two pyruvate, making a little ATP and NADH. The Krebs cycle (mitochondrial matrix) releases CO₂ and more electron carriers. The electron transport chain (inner mitochondrial membrane) uses oxygen as the final electron acceptor and makes the most ATP by chemiosmosis. Without oxygen, fermentation regenerates NAD⁺ so glycolysis can continue.

Key terms

Cellular respiration

Breaking down glucose to make ATP: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP.

Glycolysis

Splits glucose into two pyruvate in the cytoplasm; makes a small amount of ATP and NADH.

Krebs cycle

In the mitochondrial matrix; releases CO₂ and produces electron carriers (NADH, FADH₂).

Electron transport chain

On the inner mitochondrial membrane; uses O₂ as the final acceptor and makes the most ATP.

Fermentation

Anaerobic pathway that regenerates NAD⁺ so glycolysis can keep running without oxygen.

Try an AP-style question

What happens to the H⁺ gradient and ATP production?

1. ATP synthase begins pumping H⁺ to replace the chain
2. Glycolysis stops completely, but the gradient is unchanged
3. The gradient grows, so ATP production increases
4. The gradient dissipates and ATP synthesis by ATP synthase stops