LESSON IX:
THE KREBS CYCLE
To Address NYS Standard:
5 (Organisms maintain a dynamic equilibrium that sustains life)
Behavioural Objective:
The students will be able to understand how the Krebs cycle works, and how it fits in
with the other two processes of aerobic cellular respiration. They will understand the
products and by-products of the system, and where they ultimately go.
Explanation of Lesson Plan:
This lesson will build upon the previous one, and will detail what happens to the products
of glycolysis, and where the products of the Krebs cycle end up.
Hook: (3 minutes)
“This is the second part of aerobic cellular respiration. This is where we generate a little
ATP, but a lot of reduced compounds. These reduced compounds carry electrons around
like little trucks. They carry them to the ETC.”
Test of Prior Learning: (5 minutes)
1. What happens if the ETC stops working?
2. What would happen if you didn’t have any ATP?
New Learning: (30 minutes)
1. The Krebs cycle is also very complex. There are many steps, which involve the
conversion of many different molecules. These conversions, again, release energy.
2. The pyruvate from glycolysis goes into the mitochondrion from the cytoplasm.
3. Shortly thereafter, a molecule of carbon dioxide is removed, a molecule of NADH is
created, and Acetyl CoA is produced. It is this compound that enters the Krebs cycle.
4. Acetyl CoA has only two carbons. It gets attached to oxaloacetate, to form citric acid,
a six-carbon acid.
5. Two more molecules of carbon dioxide come off, and one molecule of NADH is
created.
6. This leaves a four carbon molecule called succinyl CoA.
7. We make a molecule of ATP, a molecule of FADH2, and one more molecule of
NADH.
8. Through all these reactions, the succinyl CoA is being reorganized. It eventually
becomes oxaloacetate again. This completes the cycle.
9. So we have to, in order to preserve the cycle, regenerate oxaloacetate at the end of
each turn. If we did not, there’d be nothing to hook on Acetyl CoA to.
10. Recall, though, that each glucose yields two pyruvates.
11. This means that there are two turns of the Krebs cycle for each glucose (2 pyruvates).
12. So the totals are 8 NADH, 2 FADH2, and 2 ATP.
13. You also get 6 molecules of carbon dioxide. THIS IS WHY WE BREATH OUT
carbon dioxide!
14. The molecule is transported out of the mitochondrion, into the blood, and is
exchanged in the blood.
15. We will find out why we need oxygen when we get to the ETC.
Test of New Learning: (5 minutes)
1. What are the main products of the Krebs cycle?
2. Why do we need to regenerate oxaloacetate?
Assignment: