Problem 7 - Campbell Biology

Question

Most \(\mathrm{CO}_{2}\) from catabolism is released during (A) glycolysis. (B) the citric acid cycle. (C) lactate fermentation. (D) electron transport.

Step-by-Step Solution

Answer

(B) the citric acid cycle.

Step 1: Understanding the Processes

To determine where the most \(\text{CO}_{2}\) is released during catabolism, review the steps involved in each process: glycolysis, the citric acid cycle, lactate fermentation, and electron transport.

Step 2: Glycolysis

Glycolysis occurs in the cytoplasm and converts one glucose molecule into two pyruvate molecules with a net production of 2 ATP and 2 NADH. Importantly, it does not release \(\text{CO}_{2}\).

Step 3: Citric Acid Cycle

The citric acid cycle (Krebs cycle) takes place in the mitochondria. During this cycle, each acetyl-CoA molecule entering the cycle results in the release of two \(\text{CO}_{2}\) molecules. Given that two acetyl-CoA molecules are produced from one glucose molecule, the citric acid cycle releases a total of four \(\text{CO}_{2}\) molecules per glucose.

Step 4: Lactate Fermentation

Lactate fermentation converts pyruvate to lactate in the absence of oxygen and does not produce any \(\text{CO}_{2}\).

Step 5: Electron Transport Chain

The electron transport chain occurs in the inner mitochondrial membrane and involves the transfer of electrons from NADH and FADH2 to oxygen, producing water. This process does not directly release \(\text{CO}_{2}\).

Step 6: Conclusion

Based on the description of each process, the citric acid cycle is the only stage of catabolism that significantly releases \(\text{CO}_{2}\).

Key Concepts

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm of the cell. It breaks one glucose molecule into two pyruvate molecules, generating a net gain of 2 ATP and 2 NADH. This step does not produce \text{CO}_2 \(\text{carbon dioxide}\). The main purpose of glycolysis is to extract energy from glucose by breaking its bonds. Here are the key points to remember about glycolysis:

• Occurs in the cytoplasm
• Breaks down glucose into two pyruvate molecules
• Produces a net gain of 2 ATP and 2 NADH
• No \(\text{CO}_2\) released

Glycolysis is an anaerobic process, meaning it doesn't require oxygen. It is essential because it prepares the pyruvate molecules for further energy extraction in the mitochondria. Despite not producing \(\text{CO}_2\), glycolysis is a critical step in the overall process of cellular respiration.

Lactate Fermentation

Lactate fermentation occurs when oxygen is scarce, such as during intense exercise. In this process, pyruvate from glycolysis is converted into lactate. This helps regenerate NAD+, which is needed for glycolysis to continue. Here are the key points:

• Occurs in the absence of oxygen
• Converts pyruvate into lactate
• Regenerates NAD+ for glycolysis
• No \(\text{CO}_2\) released

Lactate fermentation is particularly important in muscle cells during vigorous activity. When oxygen delivery cannot keep up with demand, lactate fermentation provides a temporary solution to keep producing energy. However, it is less efficient than aerobic respiration. The lactate produced can later be converted back to pyruvate when oxygen becomes available.

Electron Transport Chain

The Electron Transport Chain (ETC) is the final stage of cellular respiration and occurs in the inner mitochondrial membrane. Here, electrons from NADH and FADH2 are transferred through a series of proteins, ultimately combining with oxygen to form water. Here are the key points:

• Occurs in the inner mitochondrial membrane
• Involves the transfer of electrons from NADH and FADH2
• Produces water by combining electrons with oxygen
• No \(\text{CO}_2\) released

The main function of the ETC is to generate a proton gradient across the mitochondrial membrane. This gradient drives the production of ATP through an enzyme called ATP synthase. Although the ETC does not release \(\text{CO}_2\) itself, it is crucial for the cells' energy production, generating the bulk of ATP during cellular respiration.