Link reaction and Krebs cycle
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of glycolysis. You can test your knowledge on this below.
What are the end products of glycolysis?
2 molecules of pyruvate (3C).
What is the net ATP gain during glycolysis per glucose molecule?
2 ATP (4 produced – 2 used).
What coenzyme is reduced during glycolysis?
NAD (forms 2 reduced NAD).
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains link reaction and krebs cycle or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Where Do These Reactions Take Place?
|
Stage |
Location |
|---|---|
|
Link Reaction |
Mitochondrial matrix |
|
Krebs Cycle |
Mitochondrial matrix |
Both require oxygen indirectly (aerobic conditions), even though oxygen is not used directly in the reactions.
The Link Reaction
- Pyruvate (3C) is actively transported into the mitochondrial matrix.
-
Decarboxylation: One carbon is removed as CO₂.
-
Oxidation: The remaining 2-carbon molecule is oxidised; NAD is reduced to NADH.
-
The 2-carbon acetate molecule combines with coenzyme A (CoA) to form acetyl-CoA (2C).
Products of the Link Reaction per glucose molecule (remember 1 x glucose would produce 2 x pyruvate in glycolysis so the link reaction would happen twice):
-
2 acetyl-CoA
-
2 CO₂
-
2 reduced NAD
The Krebs Cycle
-
Acetyl-CoA carries acetate into the Krebs cycle.
-
Co-enzyme A can then return to the Link reaction to be re-used.
-
Acetate (2C) combines with a 4-carbon compound (oxaloacetate) to form a 6- carbon compound (citrate).
-
The 6-carbon compound undergoes a series of redox reactions, producing:
-
2 CO₂ molecules (via decarboxylation)
-
3 reduced NAD
-
1 reduced FAD
-
1 ATP (via substrate-level phosphorylation)
-
The 4-carbon compound (oxaloacetate) is regenerated, allowing the cycle to repeat.
Products of the Krebs Cycle per glucose molecule (2 turns of the Krebs cycle):
-
6 NADH
-
2 FADH₂
-
2 ATP
-
4 CO₂
Summary Table
|
Product |
Link Reaction (per glucose) |
Krebs Cycle (per glucose) |
Total |
|---|---|---|---|
|
CO₂ |
2 |
4 |
6 |
|
Reduced NAD |
2 |
6 |
8 |
|
Reduced FAD |
0 |
2 |
2 |
|
ATP |
0 |
2 |
2 |
|
Acetyl-CoA |
2 |
- |
2 |
Key Terms
-
Decarboxylation: Removal of a carbon atom as carbon dioxide (CO₂).
-
Oxidation: Loss of electrons or hydrogen atoms (results in reduced NAD/FAD).
-
Acetyl-CoA: A 2-carbon molecule that enters the Krebs cycle.
-
Coenzyme A: Combines with acetate to form acetyl-CoA; acts as a carrier.
-
Substrate-level phosphorylation: Direct production of ATP from a reaction involving a molecule that donates phosphate.
No answer provided.
Exam Tip
-
Make sure you distinguish between per glucose and per cycle — many questions ask for total yield per glucose. Don't forget: the Link Reaction and the Krebs cycle happens twice per glucose because glycolysis produces 2 pyruvate molecules.
-
You do not need to learn the names citrate and oxaloacetate for AQA. You can refer to these molecules by carbon number.
-
Learn the respiration reactions in chunks/ stages to make it more manageable: glycolysis → link reaction → Krebs cycle → oxidative phosphorylation.
No answer provided.
1. Add the names of substances P and Q (2 marks)
2. How many carbon atoms are there in P and Q? (2 marks)
3. Name the stages shown on this diagram and give their locations within a cell. (2 marks)
4. Describe how substance P is converted into substance Q (3 marks)
5. By what process is ATP made in the cycle shown? (1 mark)
1. P = Pyruvate
Q = Acetyl Coenzyme A
2. P = 3 carbons
Q = 2 carbons
3. Glycolysis in the cytoplasm
The Link Reaction and the Krebs Cycle in the matrix of the mitochondria
4. Pyruvate is decarboxylated and oxidised
Forming acetate and reduced NAD.
Acetate combines with coenzyme A to form acetyl-CoA.
5. Substrate level phosphorylation
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!