Krebs / citric acid cycle | Cellular respiration | Biology | Khan Academy

Summary

This video provides a detailed explanation of cellular respiration, focusing on the breakdown of glucose into pyruvate during glycolysis, followed by the preparation of pyruvate for the Krebs cycle. It describes how pyruvate is oxidized to acetyl-CoA before entering the citric acid cycle, which generates ATP, NADH, FADH2, and carbon dioxide. The video emphasizes the importance of NADH and FADH2 as inputs for the electron transport chain, where the majority of ATP is produced. The process is explained in both biochemical detail and with an overview of how proteins and fats can also contribute to ATP generation.

Takeaways

• 😀 Glycolysis breaks down glucose (a 6-carbon molecule) into two pyruvate molecules (3-carbons each) in the cytoplasm, yielding 2 ATP and 2 NADH.
• 😀 Glycolysis occurs in the cytoplasm of the cell and does not require oxygen (anaerobic process).
• 😀 The mitochondria are essential for the next steps in cellular respiration, including the Krebs cycle (citric acid cycle) and oxidative phosphorylation.
• 😀 The mitochondria have two membranes: an outer membrane and an inner membrane that folds into cristae, which increase surface area for energy production.
• 😀 Pyruvate oxidation occurs in the mitochondria, where pyruvate is converted into acetyl-CoA, producing NADH and CO2 as a byproduct.
• 😀 The Krebs cycle involves the combination of acetyl-CoA with oxaloacetic acid to form citric acid, which is then oxidized to release CO2 and produce NADH, FADH2, and ATP.
• 😀 Each glucose molecule generates two pyruvates, and thus the Krebs cycle runs twice per glucose molecule, producing a total of 6 NADH, 2 ATP, and 2 FADH2.
• 😀 NADH and FADH2 are crucial for the electron transport chain, where most ATP production occurs in the final stage of cellular respiration.
• 😀 The electron transport chain uses NADH and FADH2 to generate a total of 34 ATPs, while glycolysis and the Krebs cycle contribute 4 ATPs directly.
• 😀 Theoretical cellular respiration from one glucose molecule yields a maximum of 38 ATPs under ideal conditions, though actual yield may vary in different cells.
• 😀 The Krebs cycle and pyruvate oxidation are central to the catabolism of not just carbohydrates, but also fats and proteins, through acetyl-CoA as a key intermediate.

Q & A

What is the main outcome of glycolysis?

Glycolysis splits a 6-carbon glucose molecule into two 3-carbon pyruvate molecules. It also produces 2 ATPs (net gain) and 2 NADHs.

Where does glycolysis take place in the cell?

Glycolysis occurs in the cytoplasm of the cell.

What is the role of NADH in cellular respiration?

NADH is an electron carrier that plays a crucial role in the electron transport chain, where it helps generate ATP. Each NADH can produce three ATP molecules.

What is the process of pyruvate oxidation, and why is it important?

Pyruvate oxidation converts pyruvate into acetyl-CoA, a 2-carbon molecule. This process is necessary for the pyruvate to enter the Krebs cycle and generate more ATP.

How is acetyl-CoA involved in the Krebs cycle?

Acetyl-CoA combines with oxaloacetate to form citric acid in the Krebs cycle. The cycle generates NADH, FADH2, ATP, and releases CO2.

How is ATP produced in the Krebs cycle?

The Krebs cycle produces ATP through substrate-level phosphorylation, where ADP is converted into ATP during specific reactions in the cycle.

What are FADH2 and its significance in cellular respiration?

FADH2 is another electron carrier that is produced in the Krebs cycle. Each FADH2 can produce two ATP molecules in the electron transport chain.

How does the electron transport chain contribute to ATP production?

The electron transport chain uses electrons from NADH and FADH2 to create a proton gradient across the inner mitochondrial membrane, which drives the production of ATP through chemiosmosis.

How many ATP molecules are theoretically produced from one glucose molecule?

A theoretical maximum of 38 ATP molecules can be produced from one glucose molecule, accounting for ATP generated in glycolysis, the Krebs cycle, and the electron transport chain.

Why are proteins and fats important in cellular respiration, aside from glucose?

Proteins and fats can be broken down into intermediates like acetyl-CoA, which can enter the Krebs cycle to produce ATP, making them important fuel sources for the cell when glucose is not available.