PrepTest 148, Section 3, Question 23

What we’re looking for

Working the answers

• A
  Correct

  Without phenazines, the colony forms wrinkles that put more bacteria directly in contact with the surrounding environment. That gives more cells access to the environment without needing nutrients carried inward to them. The change makes sense as a way to compensate for lacking the proposed nutrient pipelines: if nutrients can't be transported to as many cells, arrange the colony so more cells can reach the outside themselves. This isn't direct observation of phenazines carrying nutrients, and it doesn't prove that explanation of the wrinkles. But it connects the absence of phenazines to another way of obtaining the access they are hypothesized to provide. That's support for the proposed second function.

• B

  This links production to the number of foreign bacteria nearby, which fits the defensive function we already know about. It doesn't establish a nutrient-transport function. We also shouldn't overread it: a production rate determined by outside bacteria doesn't necessarily mean phenazines have only one use or are never produced without a threat. B simply doesn't provide the needed support for the new hypothesis.

• C

  Equal growth in nutrient-rich soil doesn't identify phenazines as nutrient pipelines. The colonies without phenazines might have another way to obtain sufficient nutrients in those conditions. The result therefore doesn't establish the proposed function, but neither does it automatically prove that phenazines never help transport nutrients. We need evidence connecting their presence to that function, not merely equal growth under one set of conditions.

• D

  Better defense is the established role, not the hypothesized additional role. Showing that phenazine-producing colonies fend off other bacteria more successfully tells us nothing here about how nutrients reach their interior cells.

• E

  Greater death rates in the interior don't give us evidence that phenazines transport nutrients there. We're not told why those cells die. Even if nutrients matter, phenazines might provide some help without making interior cells as well supplied as cells along the edges. E doesn't support the hypothesis, but we also shouldn't turn this comparison into proof that no nutrients reach the interior.