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

Summary

This video explains the key steps of the Krebs cycle (Citric Acid Cycle), a crucial part of aerobic respiration. It starts with acetyl-CoA entering the cycle, where it combines with oxaloacetate to form citrate. The cycle involves decarboxylation (removal of CO2) and dehydrogenation (oxidation), which generates NADH and FADH2. These energy carriers are essential for ATP production. The cycle ends by regenerating oxaloacetate, releasing CO2, and producing ATP. The lecturer emphasizes key steps, including the role of NAD and FAD, while simplifying the names of intermediate molecules to focus on the overall process.

Takeaways

• 🔬 The link reaction leads to the breakdown of the organic molecule, with acetyl-CoA being formed.
• 🧬 Glycolysis breaks glucose into pyruvate, and through the link reaction, pyruvate becomes acetyl-CoA.
• 🏭 The Krebs cycle occurs in the mitochondrial matrix, where acetyl-CoA is fully broken down.
• ⚛️ Acetyl-CoA (2-carbon) combines with oxaloacetate (4-carbon) to form citrate (6-carbon).
• 🧪 Decarboxylation (carbon dioxide release) and dehydrogenation (oxidation) occur twice during the Krebs cycle.
• 💡 The cycle regenerates oxaloacetate to ensure the process can continue.
• 💥 Energy is released during substrate-level phosphorylation, generating ATP.
• 🧫 NAD and FAD accept hydrogen atoms to form reduced NAD and reduced FAD.
• ⚙️ Two acetyl-CoA molecules enter the cycle, producing four carbon dioxide molecules, six reduced NAD, two reduced FAD, and two ATP in total.
• 🧠 You don’t need to memorize all the intermediate molecules but should focus on key steps and products.

Q & A

What is the primary function of the Krebs cycle in aerobic respiration?

The primary function of the Krebs cycle is to fully break down the acetyl group, releasing energy stored in the form of NADH, FADH2, and ATP, as well as producing carbon dioxide as a byproduct.

Where does the Krebs cycle occur within the cell?

The Krebs cycle takes place in the mitochondrial matrix, which is the innermost compartment of the mitochondria.

What happens to the acetyl-CoA molecule at the beginning of the Krebs cycle?

At the beginning of the Krebs cycle, the acetyl group from acetyl-CoA is combined with a four-carbon molecule called oxaloacetate, forming a six-carbon molecule called citrate.

What is the significance of oxaloacetate in the Krebs cycle?

Oxaloacetate is crucial because it starts the cycle by combining with the acetyl group, and it is regenerated at the end of the cycle, ensuring the cycle can continue.

What is decarboxylation, and how does it occur in the Krebs cycle?

Decarboxylation is the removal of a carbon atom from a molecule in the form of carbon dioxide (CO2). In the Krebs cycle, it occurs twice, reducing the six-carbon citrate to a five-carbon and then to a four-carbon molecule.

What role does NAD+ play in the Krebs cycle?

NAD+ acts as an electron acceptor, becoming reduced to NADH by gaining hydrogen atoms during the oxidation steps of the Krebs cycle. This NADH later carries electrons to the electron transport chain for ATP production.

Do students need to memorize the names of all the intermediate molecules in the Krebs cycle?

No, students do not need to memorize the names of every intermediate molecule in the Krebs cycle. However, it is important to know that citrate is the first molecule formed and the general steps of the cycle.

Why is FAD used as an electron acceptor in step five of the Krebs cycle instead of NAD+?

The reason FAD is used instead of NAD+ in step five is due to the specific energetics of the reaction. Some reactions release less energy, making it more suitable for FAD to accept the electrons, but further detail is not necessary for this level of understanding.

How many molecules of ATP are produced in the Krebs cycle per glucose molecule?

The Krebs cycle produces 2 ATP molecules per glucose molecule, as each glucose results in two acetyl-CoA molecules entering the cycle.

What is the overall output of the Krebs cycle in terms of NADH, FADH2, ATP, and CO2?

For every two acetyl-CoA molecules entering the Krebs cycle, the output is 6 NADH, 2 FADH2, 2 ATP, and 4 CO2 molecules.