Krebs cycle and oxidative phosphorylation
Krebs cycle and oxidative phosphorylation
- Acetyl CoA now enters the Krebs cycle in the matrix.
- Then the carried hydrogen powers oxidative phosphorylation — making most of the ATP.
- It all happens inside the mitochondrion.
The Krebs cycle
- The 2C acetyl group joins a 4C oxaloacetate to make a 6C citrate.
- Citrate is changed back to oxaloacetate in small steps, ready for the next acetyl group. Along the way:
- decarboxylation removes carbon as CO₂,
- dehydrogenation removes hydrogen, reducing NAD and FAD.
- The reduced NAD and FAD carry the hydrogen to the inner membrane.
Practice
During the Krebs cycle, what is removed?
Each turn removes CO₂ (decarboxylation) and hydrogen (dehydrogenation), reducing the coenzymes NAD and FAD.
Oxidative phosphorylation
- This stage makes most of the ATP:
- hydrogen splits into protons and energetic electrons.
- electrons pass along the electron transport chain, releasing energy.
- that energy pumps protons across the inner membrane.
- protons flow back through ATP synthase, making ATP (chemiosmosis).
- oxygen is the final electron acceptor, joining electrons and protons to form water.
Practice
Which stage of respiration produces most of the ATP?
Oxidative phosphorylation, using the reduced coenzymes, makes the great majority of the ATP.
Practice
In oxidative phosphorylation, oxygen acts as the:
Oxygen accepts the electrons at the end of the chain and combines with protons to form water.
Practice
ATP is made as protons flow back through:
Protons pumped across the inner membrane flow back through ATP synthase, driving ATP synthesis — chemiosmosis.
Mitochondria, built for the job
- The inner membrane is folded into cristae — a large surface for the electron transport chain and ATP synthase.
- The matrix holds the substances and helpers for the link reaction and Krebs cycle.
Practice
The cristae of a mitochondrion are useful because they:
The folded inner membrane (cristae) maximises the area for the electron carriers and ATP synthase.
You've got it
Key idea
- Krebs cycle: acetyl + oxaloacetate → citrate → back; decarboxylation (CO₂) + dehydrogenation (reduced NAD/FAD)
- oxidative phosphorylation makes most ATP: electron transport chain pumps protons → ATP synthase (chemiosmosis)
- oxygen is the final electron acceptor → water
- cristae give a large surface; the matrix runs the link reaction + Krebs