The citric acid cycle :

Options

A. must occur twice for each glucose molecule metabolised
B. produces 2 ATP directly from the cycle intermediates
C. produces 4 carbon dioxide molecules per glucose molecule
D. all of the above

Answer

### Step-by-Step Solution for the Question on the Citric Acid Cycle:

1. **Understanding the Starting Point**:
- The citric acid cycle (also known as the Krebs cycle) begins after glycolysis. Glycolysis converts one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each).

2. **Conversion of Pyruvate to Acetyl CoA**:
- Each pyruvate molecule is then converted into acetyl CoA (2 carbons) before entering the citric acid cycle. Since one glucose molecule produces two pyruvate molecules, two acetyl CoA molecules are produced from one glucose molecule.

3. **Cycle Completion**:
- Each acetyl CoA enters the citric acid cycle once. Therefore, for one glucose molecule, the citric acid cycle must occur twice (once for each acetyl CoA).

4. **Products of the Citric Acid Cycle**:
- For each turn of the cycle (per acetyl CoA), the following products are generated:
- 3 NADH (Nicotinamide adenine dinucleotide)
- 1 FADH2 (Flavin adenine dinucleotide)
- 1 ATP (Adenosine triphosphate)
- 2 CO2 (Carbon dioxide)

5. **Total Products from One Glucose Molecule**:
- Since the cycle occurs twice for one glucose molecule, the total products will be:
- 6 NADH (3 from each cycle)
- 2 FADH2 (1 from each cycle)
- 2 ATP (1 from each cycle)
- 4 CO2 (2 from each cycle)

6. **Conclusion**:
- Therefore, the citric acid cycle must occur twice for each glucose molecule metabolized, produces 2 ATP directly from the cycle intermediates, and produces 4 carbon dioxide molecules per glucose molecule.

7. **Final Answer**:
- The correct answer to the question is "all of the above".