7.1.3Unit 6: Practical Skills in Biology II
Link Reaction and Krebs Cycle — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 21 of 48 Unit quiz
Curriculum Equation Key معادلة التنفس الخلوي الهوائي الموزونة - Balanced Cellular Respiration EquationFast Exam Solution
C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + 36-38 ATP
Word Equation & Conditions: Glucose + Oxygen -> Carbon Dioxide + Water + Energy (36-38 ATP) | جلوكوز + أكسجين ← ثاني أكسيد كربون + ماء + طاقة
Location & Specifications: الميتوكوندريا (Mitochondria) والسيتوسول.
Part 1 of 6The Link Reaction
1
Pyruvate from glycolysis enters the mitochondrial matrix by active transport through a transport protein.
2
Pyruvate is oxidised by removal of hydrogen, which reduces NAD.
3
It is also decarboxylated, losing carbon as carbon dioxide.
4
The remaining two-carbon acetate combines with coenzyme A to make acetyl coenzyme A.
5
No ATP is made, and the reaction happens twice per glucose because there are two pyruvate molecules.
Part 2 of 6Summary Equation
Inputs
- Inputs Pyruvate (3C), NAD and coenzyme A enter the reaction.
Outputs
- Outputs Acetyl coenzyme A (2C), carbon dioxide and reduced NAD are produced.
Role
- Role The reaction links glycolysis to the Krebs cycle by preparing a two-carbon fragment.
Per glucose
- Per glucose This gives two acetyl CoA, two CO₂ and two reduced NAD.
Part 3 of 6The Krebs Cycle
The Krebs Cycle
Entry
- Entry Acetyl CoA gives its two-carbon acetyl group to four-carbon oxaloacetate to make six-carbon citrate.
Regeneration
- Regeneration Citrate is changed through a series of redox reactions back to oxaloacetate, so the cycle can repeat.
Decarboxylation
- Decarboxylation Carbon dioxide is released from citrate and from later intermediates.
Dehydrogenation
- Dehydrogenation Hydrogen removed from intermediates reduces NAD and FAD, which carry it to oxidative phosphorylation.
Substrate-level phosphorylation
- Substrate-level phosphorylation One ATP is formed per turn when a phosphate is transferred from an intermediate to ADP.
Part 4 of 6Yields per Glucose
Use the table below to keep track of the products of each stage per glucose molecule.
Two turns
- Two turns Each glucose gives two acetyl CoA, so the cycle turns twice per glucose.
Krebs cycle products
- Krebs cycle products Two turns give 2 ATP, 6 reduced NAD, 2 reduced FAD and 4 CO₂.
Other substrates
- Other substrates Fats and proteins can also supply acetyl CoA, which enters the cycle directly.
Oxygen link
- Oxygen link Oxygen is not used here, but the cycle stops without it because NAD and FAD are not regenerated.
| Stage | Reduced NAD | Reduced FAD | ATP | CO₂ |
|---|---|---|---|---|
| Link reaction (×2) | 2 | 0 | 0 | 2 |
| Krebs cycle (×2) | 6 | 2 | 2 | 4 |
| Total | 8 | 2 | 2 | 6 |
Part 5 of 6Worked Example
Question Use the table to find the total number of reduced NAD, reduced FAD, ATP and carbon dioxide formed by the link reaction and Krebs cycle from one glucose molecule.
- One glucose gives two pyruvate, so the link reaction and the cycle each run twice.
- Reduced NAD2 (link) + 6 (Krebs) = 8.
- Reduced FAD
- 2;
- ATP: 2;
- CO₂: 2 (link) + 4 (Krebs) = 6.
- 2;
- Check carbon6 CO₂ released matches the 6 carbon atoms of glucose.
Answer 8 reduced NAD, 2 reduced FAD, 2 ATP and 6 CO₂.
Part 6 of 6Exam Checklist & Tips
7.1.3 – Key Points for the Exam
- Decarboxylation Removal of carbon dioxide from a molecule.
- Dehydrogenation Removal of hydrogen from a molecule.
- Oxaloacetate The four-carbon compound that accepts acetyl groups in the Krebs cycle.
- Substrate-level phosphorylation Formation of ATP by direct transfer of a phosphate from an intermediate to ADP.
Common Mistakes
- Doing the calculations per pyruvate rather than per glucose, which halves the answer.
- Saying ATP is made in the link reaction; none is.
- Stating that the Krebs cycle uses oxygen directly; it needs oxygen indirectly to regenerate NAD and FAD.
- Forgetting that oxaloacetate is regenerated, which is why the process is a cycle.
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