生命科學(一) Ch10-3 Cellular Respiration and Fermentation

Summary

This section covers the Citric Acid Cycle, the second stage of aerobic respiration following glycolysis. It explains how pyruvate, the product of glycolysis, enters the mitochondria of eukaryotic cells, where it is converted into Acetyl-CoA through pyruvate oxidation. The Citric Acid Cycle then proceeds through eight steps, producing ATP, NADH, and FADH2, which are vital for energy generation in the electron transport chain. The cycle also releases CO2 and regenerates oxaloacetate to restart the process. Key reactions, enzymes, and energy products are emphasized throughout the cycle.

Takeaways

• 🔄 The Citric Acid Cycle, also known as the Krebs Cycle, is the second step of aerobic respiration following glycolysis.
• 🏃 Pyruvate produced during glycolysis enters the mitochondria for aerobic respiration, which only occurs in eukaryotic cells.
• 🔧 Pyruvate must undergo oxidation to become Acetyl-CoA before it can enter the Citric Acid Cycle.
• 🔗 The process of Pyruvate Oxidation connects glycolysis to the Citric Acid Cycle, generating Acetyl-CoA, NADH, and releasing CO₂.
• ⚙️ The Citric Acid Cycle consists of eight steps and completes the conversion of pyruvate into CO₂, generating energy intermediates.
• 💥 Each cycle yields one ATP, three NADH, and one FADH₂, which are crucial for the Electron Transport Chain.
• 🌍 The cycle regenerates Oxaloacetate, enabling it to combine with Acetyl-CoA to restart the cycle in a continuous loop.
• 🧬 ATP generated during the Citric Acid Cycle is produced through substrate-level phosphorylation.
• 📊 The process also releases two molecules of CO₂ per cycle as Acetyl-CoA is fully oxidized.
• 🔋 The NADH and FADH₂ generated carry high-energy electrons for use in the Electron Transport Chain to further produce ATP.

Q & A

What is the Citric Acid Cycle, and why is it important?

The Citric Acid Cycle, also known as the Krebs Cycle, is the second step of aerobic respiration. It is essential because it completes the breakdown of pyruvate into CO2, generating high-energy molecules such as NADH and FADH2, which are crucial for the Electron Transport Chain and ATP production.

What is the role of pyruvate in the Citric Acid Cycle?

Pyruvate, produced from glycolysis, enters the mitochondria and undergoes pyruvate oxidation to become Acetyl-CoA. Acetyl-CoA is then used as the starting molecule for the Citric Acid Cycle.

What happens during pyruvate oxidation?

During pyruvate oxidation, pyruvate is converted into Acetyl-CoA. This process involves three steps, producing CO2, NADH, and attaching a Coenzyme A molecule to pyruvate.

Why is Coenzyme A important in the Citric Acid Cycle?

Coenzyme A is crucial because it binds with pyruvate to form Acetyl-CoA, which is the molecule that enters the Citric Acid Cycle, linking glycolysis with aerobic respiration.

How many steps are involved in the Citric Acid Cycle, and what are its main products?

The Citric Acid Cycle consists of 8 steps. Each cycle produces 1 ATP, 3 NADH, 1 FADH2, and releases 2 CO2 molecules.

What is the role of NADH and FADH2 in the Citric Acid Cycle?

NADH and FADH2 are high-energy electron carriers produced during the Citric Acid Cycle. They are later used in the Electron Transport Chain to generate ATP through oxidative phosphorylation.

What is the significance of the first step of the Citric Acid Cycle?

In the first step, Acetyl-CoA combines with oxaloacetate to form citrate (citric acid), which marks the beginning of the cycle. This step sets the stage for subsequent reactions that will extract energy from the Acetyl-CoA molecule.

How does the Citric Acid Cycle contribute to the production of CO2?

The Citric Acid Cycle releases two molecules of CO2 during the third and fourth steps of the cycle as byproducts of the breakdown of Acetyl-CoA.

How is ATP produced during the Citric Acid Cycle?

ATP is produced in the fifth step of the Citric Acid Cycle through substrate-level phosphorylation, where a phosphate group is directly transferred to ADP to form ATP.

What happens to oxaloacetate at the end of the Citric Acid Cycle?

At the end of the Citric Acid Cycle, oxaloacetate is regenerated and combines with a new Acetyl-CoA molecule to start the cycle again, making the process continuous.