SAT Subject Biology Practice Question 359: Answer and Explanation

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Question: 359

9. Most bacteria can be grown in the laboratory on agar plates containing glucose as their only carbon source. Some bacteria require additional substances, such as amino acids, to be added to the growth medium. Such bacteria are termed auxotrophs. These bacteria are denoted by the amino acid they require followed by a "-" in superscript (e.g., arg-). Bacteria that do not require that particular amino acid can be indicated by a "+" in superscript.

Different strains of bacteria were grown on several plates containing a variety of nutrients. Figure 1 shows the colonies (numbered) that grew on each plate. The supplements in each plate are indicated.

In a second experiment, Colony 1 was mixed with soft agar and spread over a plate so that an even lawn of bacteria grew. Bacterial lawns appear cloudy on agar plates. A single drop of an unknown organism was placed in the center of the

A. Yes, it is able to grow in the presence of the three amino acids being tested.
B. Yes, it can only grow if glucose is present.
C. No, it is able to grow in the absence of glucose.
D. No, it is able to grow in the absence of any additional amino acids.
E. The data available are insufficient to determine the answer.

Correct Answer: D


D Colony 1 can grow on any of the plates, thus it does not require any additional amino acids. Auxotrophs require additional supplements to their growth media.

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