3.2.7Inquiry Process
The Link Reaction and The Krebs Cycle — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 113 of 160
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.
Section 1 of 4The Link Reaction
The link reaction connects glycolysis in the cytoplasm to the Krebs cycle in the matrix.
1
When oxygen is present, pyruvate from glycolysis is transported into the mitochondrial matrix.
2
Pyruvate is decarboxylated: a carbon atom is removed as carbon dioxide, leaving a two-carbon compound.
3
The two-carbon compound is oxidised, with its hydrogen taken by NAD to form reduced NAD.
4
It combines with coenzyme A to make acetyl coenzyme A, usually written acetyl CoA.
5
Acetyl CoA carries the acetyl group to the Krebs cycle, so the link reaction joins glycolysis to the cycle.
Section 2 of 4The Krebs Cycle
Starting step
- Starting step The two-carbon acetyl group from acetyl CoA joins four-carbon oxaloacetate to form six-carbon citrate, releasing coenzyme A.
Decarboxylation
- Decarboxylation Citrate is converted back to oxaloacetate through a series of steps in which two carbon dioxide molecules are released.
Oxidation
- Oxidation Hydrogen is removed in several steps and picked up by NAD and FAD, which become reduced NAD and reduced FAD.
Substrate-level phosphorylation
- Substrate-level phosphorylation In one step a phosphate is passed from an intermediate to ADP to produce one ATP.
Cycle
- Cycle Because oxaloacetate is regenerated at the end, the cycle can continue as long as acetyl CoA is supplied.
| Product per turn (one acetyl CoA) | Number |
|---|---|
| Carbon dioxide | 2 |
| Reduced NAD | 3 |
| Reduced FAD | 1 |
| ATP | 1 |
Section 3 of 4Totals per Glucose
Totals per Glucose
Two turns
- Two turns One glucose gives two pyruvate, two acetyl CoA and so two turns of the cycle, so all the quantities per turn are doubled.
Link reaction totals
- Link reaction totals Two carbon dioxide and two reduced NAD are made from two pyruvate.
Krebs cycle totals
- Krebs cycle totals Four carbon dioxide, six reduced NAD, two reduced FAD and two ATP are made.
Overall carbon dioxide
- Overall carbon dioxide The six carbon atoms of glucose all leave as six carbon dioxide molecules after both stages.
Role of reduced carriers
- Role of reduced carriers Most energy remains in reduced NAD and reduced FAD, which pass it on to the electron transport chain.
Section 4 of 4Exam Checklist & Tips
3.2.7 – Key Points for the Exam
- Acetyl CoA A compound carrying a two-carbon acetyl group into the Krebs cycle.
- Oxaloacetate The four-carbon compound that accepts the acetyl group.
- Citrate The six-carbon compound formed in the first step of the cycle.
- Decarboxylation Removal of a carbon atom as carbon dioxide.
- Matrix The fluid-filled inner space of a mitochondrion.
Common Mistakes
- Forgetting to double the numbers, since two pyruvate and so two turns of the cycle result from each glucose.
- Placing the Krebs cycle on the cristae instead of in the matrix.
- Writing that the cycle makes lots of ATP directly; it makes only one per turn.
- Mixing up the compoundscitrate has six carbons and oxaloacetate has four.
Last updated:
\n