12-7 Krebs Cycle (Cambridge AS A Level Biology, 9700)

Krebs Cycle (Citric Acid Cycle) Overview
📌 The Krebs cycle follows the link reaction, where acetyl $\text{CoA}$ is completely broken down from the initial glucose molecule.
⚙️ The cycle occurs in the mitochondrial matrix, starting with the two-carbon acetyl group combining with the four-carbon molecule, oxaloacetate.
🔄 The core principle is the regeneration of oxaloacetate after the acetyl group is processed, maintaining the cycle.

Krebs Cycle Steps and Transformations
⚛️ Step 1: The acetyl group joins oxaloacetate to form the six-carbon molecule, citrate; the coenzyme A detaches.
💨 Steps 2 & 3: These involve decarboxylation (removal of $\text{CO}_2$) and dehydrogenation (oxidation), reducing the molecule from six carbons to five, and then to four carbons.
⚡ Step 4: A substrate-level phosphorylation step synthesizes a small amount of energy, producing $2\text{ ATP}$ per cycle turn (or per acetyl $\text{CoA}$).

Electron Carriers and Products Summary
🔬 Steps 5 & 6 (Oxidation): These dehydrogenation steps produce electron carriers; one hydrogen molecule is accepted by $\text{FAD}$ (forming reduced $\text{FAD}$), and another by $\text{NAD}^+$ (forming reduced $\text{NAD}$).
📊 For two acetyl $\text{CoA}$ molecules processed (representing one glucose molecule), the cycle yields a total of four $\text{CO}_2$ molecules, six reduced $\text{NAD}$, two reduced $\text{FAD}$, and two $\text{ATP}$.

Key Points & Insights
➡️ Students must recall that the key intermediate formed by acetyl group addition to oxaloacetate is citrate, a six-carbon molecule.
➡️ Decarboxylation is the removal of carbon in the form of carbon dioxide ($\text{CO}_2$), marking the breakdown of the acetyl group.
➡️ Understanding the roles of $\text{NAD}^+$ and $\text{FAD}$ as hydrogen acceptors is crucial, noting that $\text{FAD}$ accepts hydrogen specifically during Step 5.
➡️ The function of coenzyme A is solely to transport the acetyl group into the cycle before detaching.

📸 Video summarized with SummaryTube.com on Mar 18, 2026, 15:02 UTC