SAT Subject Biology Practice Question 465: Answer and Explanation

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Question: 465

15. β-galactosidase (β-gal) is a bacterial enzyme used in the metabolism of lactose. Three different groups of human intestinal bacteria were cultured in a glucose-based medium. At Time 1, the bacteria were transferred to media containing lactose as a nutrient source instead of glucose. At Time 2, the bacteria were returned to the glucose-based medium. Samples of bacteria were removed at five-minute intervals and tested for the presence of β-gal. The results are presented below.

Why is it an advantage to intestinal bacteria to be able to induce β-gal production in this way?

A. Humans do not consume foods containing lactose all the time.
B. It prevents toxic buildup of lactic acid.
C. β-gal is harmful to bacteria.
D. It prevents the bacteria from using glucose as a nutrient source.
E. Lactose is harmful to bacteria.

Correct Answer: A

Explanation:

A Because humans consume lactose-containing foods irregularly, it makes sense for the bacteria to expend the energy only to make this enzyme when it is needed. None of the choices B, C, D, or E make any sense. Producing the enzyme β-gal would not prevent buildup of lactic acid. It's not as though the lactose spontaneously converts to lactic acid and the enzyme would prevent this. β-gal cannot be harmful to bacteria (choice C). If it were, they might die. Over time, the ability to make β-gal would be lost, because the bacteria that had this ability would die out. The bacteria are still able to use glucose as a nutrient source (choice D). They were grown in glucose originally and were returned to glucose at the end of the experiment. Finally, lactose is a nutrient. It is not harmful to bacteria (choice E).

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