Citric-acid cycle and oxidative phosphorylation

Why this matters

This lesson closes the loop that started three lessons ago. It lets you finally answer how much energy a glucose molecule is actually worth, and therefore why a fermenting plaque bacterium must consume so much more sugar — and make so much more acid — than one of your own cells to get the same work done. It also explains a set of tumours, an inherited cause of stained teeth, and the smell that should make you check a patient's blood sugar.

The big picture

The pyruvate dehydrogenase complex handed you acetyl-CoA. The citric acid cycle takes that two-carbon group, strips its electrons onto carriers over eight steps, and releases the carbon as carbon dioxide. The carriers then feed the machinery you already know. The cycle is the collection point between fuel and the respiratory chain — and it doubles as the supply depot for half of biosynthesis.

The 14 cards in this lesson

1. 01The missing middle
2. 02One turn, eight steps
3. 03Why one step uses FAD and three use NAD
4. 04The carbons that leave are not the ones that arrived
5. 05Oxaloacetate is a catalyst
6. 06The three control points
7. 07Why cyanide stops the cycle
8. 08The full account, glucose to carbon dioxide
9. 09Where 36 and 38 came from, and why they are gone
10. 10Acetyl-CoA is a junction, not just a fuel
11. 11Clinical alert — in the lesson
12. 12The cycle as a supply depot
13. 13Clinical alert — in the lesson
14. 14Why fermentation makes so much acid

You have got it when

You are ready to continue when you can reproduce one turn of the cycle's ledger, explain why cyanide stops the cycle without touching a cycle enzyme, and say what the fifteen-fold energy gap between respiration and fermentation means for caries.