12-8 Summary of Glycolysis, Link Reaction & Krebs Cycle (Cambridge AS A Level Biology, 9700)

Summary

This video offers a concise breakdown of three major biochemical processes: glycolysis, the link reaction, and the citric acid (Krebs) cycle. It explains how glucose is phosphorylated, broken down into pyruvate, and then converted into acetyl-CoA. The video highlights the key steps of each process, including the production of ATP, NADH, FADH2, and carbon dioxide. Additionally, it emphasizes the importance of hydrogen in powering ATP synthesis during chemiosmosis. The video concludes by summarizing the total energy yield and sets the stage for further discussion on the role of reduced NADH and FADH2.

Takeaways

• 🔬 Glycolysis occurs in the cytoplasm where glucose is phosphorylated with 2 ATP to become fructose 1,6-bisphosphate, making it more reactive.
• ⚡ The fructose 1,6-bisphosphate breaks down into two triose phosphate molecules, leading to the production of 4 ATP and the release of hydrogen.
• 🧪 NAD accepts hydrogen during glycolysis, producing two reduced NAD molecules, and the final product of glycolysis is pyruvate.
• 🏭 The pyruvate enters the mitochondrial matrix for the link reaction, undergoing decarboxylation (releasing CO2) and oxidation, creating a two-carbon acetyl group.
• 🔗 The acetyl group binds with coenzyme A, forming acetyl-CoA, which is the end product of the link reaction and enters the citric acid (Krebs) cycle.
• 🔄 Acetyl-CoA combines with oxaloacetate (a 4-carbon molecule) to form citrate (a 6-carbon molecule) in the mitochondrial matrix during the citric acid cycle.
• 🌿 The citric acid cycle involves decarboxylation and dehydrogenation, producing CO2, reduced NAD, and reduced FAD while regenerating oxaloacetate.
• 🌬 Each glucose molecule is fully broken down during glycolysis, the link reaction, and the citric acid cycle, producing 6 molecules of CO2.
• ⚙️ The purpose of this breakdown is to release hydrogen atoms for chemiosmosis, which powers ATP synthesis.
• 💡 In total, the complete breakdown of one glucose molecule results in 6 ATP, 10 reduced NAD molecules, and 2 reduced FAD molecules.

Q & A

What is the purpose of phosphorylating glucose in glycolysis?

Phosphorylating glucose makes it more reactive by destabilizing the molecule, allowing it to be broken down more easily in subsequent steps. This involves the addition of phosphate groups from ATP, forming fructose-1,6-bisphosphate.

How many ATP molecules are used and produced in glycolysis?

In glycolysis, 2 ATP molecules are used to phosphorylate glucose, and 4 ATP molecules are produced during substrate-level phosphorylation, resulting in a net gain of 2 ATP.

What is the final product of glycolysis?

The final product of glycolysis is pyruvate, which will enter the link reaction in the mitochondrial matrix for further processing.

What happens to pyruvate during the link reaction?

During the link reaction, pyruvate undergoes decarboxylation, releasing carbon dioxide and forming a two-carbon molecule. This two-carbon molecule undergoes dehydrogenation, reducing NAD+ to NADH and forming acetyl-CoA, which enters the citric acid (Krebs) cycle.

Where does the link reaction occur in the cell?

The link reaction occurs in the mitochondrial matrix.

What is the role of acetyl-CoA in the citric acid cycle?

Acetyl-CoA transports the acetyl group, which binds to oxaloacetate (a four-carbon molecule), forming citrate (a six-carbon molecule) at the start of the citric acid cycle.

What are the key products of the citric acid cycle?

The citric acid cycle produces carbon dioxide, reduced NADH and FADH2, ATP (through substrate-level phosphorylation), and regenerates oxaloacetate to maintain the cycle.

How many molecules of carbon dioxide are produced from one glucose molecule during glycolysis, the link reaction, and the citric acid cycle?

A total of six molecules of carbon dioxide are produced, which corresponds to the six carbons originally present in the glucose molecule. These are released during the link reaction and the citric acid cycle.

What is the importance of the hydrogen atoms released during these reactions?

The hydrogen atoms released during dehydrogenation are crucial for chemiosmosis, where they contribute to the formation of ATP in the electron transport chain by powering ATP synthase.

What is the total energy yield from breaking down one glucose molecule by the end of glycolysis, the link reaction, and the citric acid cycle?

From one glucose molecule, a total of 6 ATP molecules, 10 reduced NADH molecules, and 2 reduced FADH2 molecules are produced, which will be used to generate more ATP in the electron transport chain.