Which of the following statements about metabolic rate is correct?

• A. Metabolic rate is independent of temperature in ectothermic organisms.
• B. Metabolic rate decreases as physical activity increases.
• C. Metabolic rate is always higher in smaller animals than in larger animals, regardless of body mass.
• D. Metabolic rate is the amount of energy an organism uses per unit of time.

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the concept of metabolic rate. Metabolic rate refers to the amount of energy an organism uses per unit of time, which is essential for maintaining life processes such as growth, reproduction, and homeostasis.
2. Step 2: Analyze the first statement: 'Metabolic rate is independent of temperature in ectothermic organisms.' Ectothermic organisms (cold-blooded animals) rely on external temperatures to regulate their metabolic rate. Therefore, their metabolic rate is highly dependent on temperature, making this statement incorrect.
3. Step 3: Evaluate the second statement: 'Metabolic rate decreases as physical activity increases.' Physical activity requires energy, and metabolic rate typically increases to meet the energy demands of the activity. Thus, this statement is incorrect.
4. Step 4: Examine the third statement: 'Metabolic rate is always higher in smaller animals than in larger animals, regardless of body mass.' While smaller animals often have higher metabolic rates per unit of body mass due to their surface area-to-volume ratio, this statement is overly generalized and does not account for all factors influencing metabolic rate. Therefore, it is not entirely correct.
5. Step 5: Confirm the correct statement: 'Metabolic rate is the amount of energy an organism uses per unit of time.' This definition accurately describes metabolic rate and is the correct answer.