# Krebs cycle

> CIE A-Level Biology · 9700 A2 Energy and Respiration
> Source: https://www.owlsprep.com/study/cie-9700-u13-krebs-cycle/

This module covers the Krebs cycle (citric acid cycle), the second stage of aerobic respiration after glycolysis and the link reaction. We explain its location, key steps, coenzyme yield, and role in energy production.

**Prerequisites:** [Mitochondria structure](https://www.owlsprep.com/study/cie-9700-u12-mitochondria-structure/); [Glycolysis and the link reaction](https://www.owlsprep.com/study/cie-9700-u13-glycolysis-link-reaction/)

## Learning objectives

- Outline the location and overall inputs/outputs of the Krebs cycle
- Explain key steps and oxidative decarboxylation reactions
- Calculate net yield of coenzymes and ATP per glucose molecule
- Identify common exam misconceptions about the cycle

## Location and Overview

The Krebs cycle occurs in the **mitochondrial matrix** of eukaryotic cells (and the cytoplasm of prokaryotes). It follows directly on from the link reaction, which produces acetyl coenzyme A (acetyl-CoA) from pyruvate.

**Krebs Cycle** — A cyclic metabolic pathway that oxidises acetyl-CoA to produce carbon dioxide, reduced coenzymes, and ATP for aerobic respiration.

*Notation:* Also called citric acid cycle / tricarboxylic acid (TCA) cycle

**Worked example:** State the overall inputs and outputs of one full turn of the Krebs cycle

1. Step 1: Identify the input that enters the cycle: one 2-carbon acetyl group bound to coenzyme A (acetyl-CoA)
2. Step 2: Identify the other inputs: 3 NAD+, 1 FAD, 1 ADP and 1 inorganic phosphate (Pi)
3. Step 3: Count the outputs: 2 CO2 molecules released from decarboxylation, 3 reduced NAD (NADH), 1 reduced FAD (FADH2), 1 ATP, and free CoA
4. Step 4: Note that oxaloacetate (the 4-carbon starting molecule) is regenerated, so it is not consumed

> **Exam tip:** Always remember one glucose produces two acetyl-CoA, so two full turns of the cycle occur per glucose

## Key Steps of the Cycle

The cycle consists of 8 enzyme-catalysed reactions, with the most important energy-producing steps summarised below:

1. **Condensation**: Acetyl (2C) combines with oxaloacetate (4C) to form 6-carbon citrate, releasing free CoA for reuse
2. **Two oxidative decarboxylations**: Citrate is rearranged, then two carbon groups are removed as CO2, reducing two NAD+ to NADH, leaving a 4-carbon intermediate
3. **Substrate-level phosphorylation**: A phosphate group is transferred directly to ADP to form ATP
4. **Regeneration of oxaloacetate**: Two more dehydrogenation reactions reduce one FAD to FADH2 and one NAD+ to NADH, regenerating the original 4-carbon oxaloacetate

**Worked example:** Explain why oxaloacetate concentration does not change during steady-state Krebs cycle activity

1. Step 1: One molecule of oxaloacetate reacts with one acetyl-CoA in the first step of the cycle, so one molecule is consumed per turn
2. Step 2: Over the full turn, two 1-carbon units are removed as CO2 from the 6-carbon citrate, leaving a 4-carbon molecule
3. Step 3: The final reaction of the cycle rearranges this 4-carbon molecule to regenerate exactly one oxaloacetate per turn
4. Conclusion: Because one molecule is consumed and one is produced per turn, the overall concentration of oxaloacetate remains constant

## Net Yield Calculations Per Glucose

Calculating the total yield of products from the Krebs cycle is a very common exam question in CIE A-Level Biology. You must remember how many turns of the cycle occur per glucose molecule to get full marks.

> **tip**
>
> Glycolysis produces two pyruvate molecules from one glucose, so two link reactions produce two acetyl-CoA, meaning two full turns of the Krebs cycle per glucose.

**Worked example:** Calculate the total net yield of NADH, FADH2 and ATP from the Krebs cycle per molecule of glucose

1. Step 1: Confirm number of turns: 2 turns per glucose (one per acetyl-CoA)
2. Step 2: Recall yield per turn: 3 NADH, 1 FADH2, 1 ATP per turn
3. Step 3: Multiply each yield by 2 for two turns: 3 × 2 = 6 NADH, 1 × 2 = 2 FADH2, 1 × 2 = 2 ATP
4. Step 4: Note that this ATP is produced via substrate-level phosphorylation, and NADH/FADH2 go on to oxidative phosphorylation

> **Exam tip:** Always check if the question asks for yield from only the Krebs cycle, or from all stages of respiration combined

## Role in Aerobic Respiration

The Krebs cycle acts as the central hub of cellular energy metabolism. It processes acetyl-CoA from not only carbohydrate breakdown (glycolysis) but also from fatty acid and amino acid breakdown when other energy sources are available.

Its primary role is to produce reduced coenzymes (NADH and FADH2) that carry high-energy electrons to the electron transport chain for oxidative phosphorylation, which generates ~90% of the ATP in aerobic respiration.

**Check your understanding**

Test your understanding:

1. What is the main biological purpose of the Krebs cycle?

   - Produce all ATP directly for the cell
   - Produce reduced coenzymes for oxidative phosphorylation
   - Break down glucose into pyruvate
   - Remove waste carbon from the cell

   *Answer:* Produce reduced coenzymes for oxidative phosphorylation

   *Why:* Correct. While the cycle produces 2 ATP per glucose, its core function is generating NADH and FADH2 for the electron transport chain.

## Common pitfalls

- **Wrong:** Stating that one turn of the Krebs cycle processes one glucose, giving a total yield of 3 NADH per glucose
  - Why it fails: Confuses the number of acetyl-CoA produced per glucose molecule
  - Correct: One glucose produces two acetyl-CoA, so two turns of the cycle, giving a total yield of 6 NADH per glucose
- **Wrong:** Claiming oxygen is a direct reactant used in the Krebs cycle
  - Why it fails: Oxygen is only used in the final step of oxidative phosphorylation, not the Krebs cycle itself
  - Correct: The Krebs cycle depends on oxygen to regenerate NAD+ via oxidative phosphorylation, so it only runs in aerobic conditions, but does not use oxygen directly
- **Wrong:** Locating the Krebs cycle in the inner mitochondrial membrane
  - Why it fails: Confuses the location of the Krebs cycle with the location of the electron transport chain
  - Correct: In eukaryotes, all Krebs cycle enzymes are found in the mitochondrial matrix, while the electron transport chain is embedded in the inner membrane
- **Wrong:** Claiming oxaloacetate is used up over the course of the cycle
  - Why it fails: Forgets that oxaloacetate is the starting material that is regenerated at the end of every turn
  - Correct: One molecule of oxaloacetate is produced for every one consumed, so its concentration remains constant and it is not used up

## Cheatsheet

| Property | Per acetyl-CoA (1 turn) | Per glucose (2 turns) |
| --- | --- | --- |
| Location (eukaryotes) | Mitochondrial matrix | Mitochondrial matrix |
| CO₂ produced | 2 | 4 |
| NADH produced | 3 | 6 |
| FADH₂ produced | 1 | 2 |
| ATP (substrate-level) | 1 | 2 |
| Oxaloacetate | Regenerated | Regenerated |

## What's next

The reduced coenzymes produced by the Krebs cycle feed directly into the next stage of aerobic respiration: oxidative phosphorylation and chemiosmosis, which generates the vast majority of ATP for the cell. Mastering the steps and yield of the Krebs cycle is essential for explaining how cells extract energy from organic molecules, and for comparing aerobic and anaerobic respiration. The Krebs cycle also connects to other metabolic pathways, processing breakdown products of lipids and proteins as well as carbohydrates.

- [Oxidative Phosphorylation & Chemiosmosis](https://www.owlsprep.com/study/cie-9700-u13-oxidative-phosphorylation/)
- [Anaerobic Respiration](https://www.owlsprep.com/study/cie-9700-u13-anaerobic-respiration/)
- [Respiratory substrates](https://www.owlsprep.com/study/cie-9700-u13-respiratory-substrates/)

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