SAT Subject Biology Practice Question 330: Answer and Explanation

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Question: 330

20. Three different cell types were observed under the microscope. The observations are summarized in Table 1.

Table 1

The three cell types were grown in separate cultures with plenty of oxygen and nutrients available. Figure 1 shows their rates of growth. At Time 1, oxygen was no longer available to the cells.

Figure 1

Consider Figure 1. Which of the following statements best describes the reason for the difference between the curves for cell Type B and cell Type C ?

A. Cell Type B is unable to survive in the presence of oxygen, while cell Type C can ferment.
B. The products of fermentation in cell Type C are toxic to the cells and they are dying.
C. Cell Type B is an obligate aerobe while cell Type C is able to ferment.
D. Cell Type B is a facultative anaerobe, while cell Type C is an obligate aerobe.
E. Cell Type C is an obligate aerobe, while cell Type B is an obligate anaerobe.

Correct Answer: D


D This is a great question to do some answer predicting on. At Time 1 the oxygen was removed from the cultures and cell Type C died. Clearly it is an obligate aerobe. Thus we can eliminate choices A, B, and C. Because cell Type B was growing well in the presence of oxygen, it cannot be an obligate anaerobe, thus choice E is eliminated. Cell Type B must be a facultative anaerobe, using oxygen when it is available and fermenting when oxygen is not available. The decrease in growth of cell Type B after Time 1 is most likely because energy is produced during fermentation than during aerobic metabolism.

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