Metabolism

Step 1: Understanding nitrogenase.
Nitrogenase is the enzyme complex responsible for biological nitrogen fixation in diazotrophic organisms. It consists of two components: Fe-protein (dinitrogenase reductase) and MoFe-protein (dinitrogenase).
Step 2: Energy requirement.
The reduction of atmospheric N to ammonia (NH ) is highly energy-intensive and requires 16 ATP molecules per N reduced.
Step 3: Evaluating options.
(A) Incorrect — nitrogenase does not contain Cu-S centers.
(B) Incorrect — the (4Fe–4S) cluster is present but not the correct ATP/FADH usage.
(C) Correct — Fe-protein + MoFe-protein complex and 16 ATP per N .
(D) Incorrect — wrong cofactor and ATP requirement.
Step 4: Conclusion.
Thus, option (C) correctly describes nitrogenase.

Step 1: Recall nitrogenous waste types.
- Ammonotelic animals: Excrete ammonia (aquatic animals like bony fishes).
- Ureotelic animals: Excrete urea (mammals like lions).
- Uricotelic animals: Excrete uric acid (birds and reptiles).
Step 2: Match each.
- Bony fishes → Ammonia (3).
- Lions → Urea (1).
- Birds → Uric acid (2).
Final Answer:

Step 1: Understanding the metabolic effects of a ketogenic diet.
A ketogenic diet promotes the production of ketone bodies from fatty acids. These ketone bodies are used as an alternative energy source when glucose is limited.
Step 2: Analyzing the options.
- (A) Correct: D-β-hydroxybutyrate is one of the primary ketone bodies produced during ketosis, and its levels accumulate in the blood during a ketogenic diet.
- (B) Correct: Acetoacetate is another ketone body that accumulates in the blood during a ketogenic diet, serving as a source of energy for tissues like the brain.
- (C) Incorrect: Pyruvate is a key intermediate in glucose metabolism but does not accumulate during a ketogenic diet. In fact, its production is reduced in favor of ketone bodies.
- (D) Incorrect: Oxaloacetate is involved in gluconeogenesis and is not a product of ketogenesis. Its levels are typically low during a ketogenic diet.
Final Answer:

Step 1: Understanding the 'Futile Cycle'.
A futile cycle is a biochemical process where two opposing metabolic pathways run simultaneously, leading to the hydrolysis of ATP and the dissipation of energy as heat. This process typically involves reversible reactions that, when coupled, result in no net change in the products but lead to the expenditure of energy.
Step 2: Analyzing the options.
- (A) Incorrect: The reaction between glucose and glucose-6-phosphate is part of the glycolysis and gluconeogenesis pathways, but it is not part of the futile cycle, since it does not lead to energy dissipation through opposing reactions.
- (B) Correct: The interconversion between fructose-6-phosphate and fructose-1,6-bisphosphate involves the opposing actions of phosphofructokinase (PFK) and fructose-1,6-bisphosphatase, which is a classic example of a futile cycle. These opposing reactions lead to ATP hydrolysis and heat production without any net change in metabolite concentrations.
- (C) Correct: The reaction between glucose-6-phosphate and fructose-6-phosphate can also be part of a futile cycle, particularly during the process of gluconeogenesis and glycolysis, where opposing reactions involving phosphoglucoisomerase and glucose-6-phosphatase contribute to energy dissipation.
- (D) Incorrect: The reaction between 1,3-bisphosphoglycerate and 3-phosphoglycerate is involved in the glycolysis and gluconeogenesis pathways but does not typically form a futile cycle.
Final Answer: