Choose all the true statements about the citric acid cycle.
The correct answer and explanation is:
The citric acid cycle (also known as the Krebs cycle or TCA cycle) is a crucial metabolic pathway in cellular respiration. It takes place in the mitochondria of eukaryotic cells and plays a central role in the generation of energy from organic molecules. Let’s analyze each statement:
1. “The major reactants in the citric acid cycle are acetyl-CoA, NAD+, GDP, and FAD.”
True:
- Acetyl-CoA is indeed a key reactant, as it combines with oxaloacetate to form citrate in the first step of the cycle.
- NAD+, FAD, and GDP are also essential. NAD+ and FAD are electron carriers, which are reduced to NADH and FADH2 during the cycle. GDP is converted to GTP, which can then be used to produce ATP in some organisms.
2. “Two molecules of coenzyme A are produced per turn of the citric acid cycle.”
False:
- Only one molecule of coenzyme A (CoA) is involved in each turn of the citric acid cycle. Acetyl-CoA enters the cycle and reacts with oxaloacetate to form citrate. At the end of the cycle, CoA is released when succinyl-CoA is converted to succinate. Therefore, only one CoA molecule is used and regenerated per turn, not two.
3. “The citric acid cycle occurs in the mitochondria of the cell.”
True:
- The citric acid cycle occurs in the mitochondrial matrix in eukaryotic cells. This is the central location for many energy-producing pathways, including the citric acid cycle, oxidative phosphorylation, and fatty acid oxidation.
4. “The citric acid cycle is an aerobic process.”
True:
- Although the citric acid cycle itself does not require oxygen directly, it is considered an aerobic process because it depends on the electron transport chain (ETC), which does require oxygen to function. The ETC uses the electrons generated by NADH and FADH2 from the citric acid cycle to produce ATP.
5. “In the citric acid cycle, pyruvate is degraded to produce acetyl-CoA.”
True:
- Before entering the citric acid cycle, pyruvate (produced from glycolysis) is converted into acetyl-CoA by the enzyme pyruvate dehydrogenase complex in the mitochondria. This reaction also produces NADH and releases CO2.
Summary:
- True Statements: 1, 3, 4, 5
- False Statement: 2
The citric acid cycle is crucial for cellular energy production, where acetyl-CoA is oxidized, releasing energy stored in the form of NADH, FADH2, and GTP (or ATP), which are later used in the electron transport chain to produce ATP.