Q22 - Biologist: When bacteria of a particular species are p

[ohthatpatrick]

Question Type:
Weaken

Stimulus Breakdown:
Conclusion: Bacteria detect this special red by monitoring how much energy their chlorophyll is producing.
Evidence: When you throw bacteria into a test tube light show, they all move to the red zone. This red allows them to produce energy (via chlorophyll) most effectively.

Answer Anticipation:
This falls into the classic "CURIOUS FACT --> AUTHOR'S EXPLANATION" template.

CURIOUS FACT: How do the bacteria all find the red zone?
AUTHOR'S EXPLANATION: since the red zone gives them their optimal energy-producing level, they must detect the red by monitoring their rate of energy-production.

We always think through these with same two questions:
1. Is there some OTHER WAY to explain the curious fact? (how else might they detect the red light? eyes? smell? vibration?)
2. How plausible is the AUTHOR'S WAY? (can a bacteria really tell that it's producing more energy? does it just try out all the colors of light until it finds the most energetic one?)

On Weaken, Flaw, and Necessary Assumption, it's much more common to see a #1 answer. On Strengthen, it's more common to see a #2.

Correct Answer:
D

Answer Choice Analysis:
(A) This is a big strengthener (plausibility: covariation). It heavily suggests that chlorophyll is causally related to finding the red zone, since NO chlorophyll, NO red zone.

(B) This somewhat decreases the plausibility of the author's story. If the bacteria prefer darkness to other energy-providing colors, then they're probably not picking where they hang out based on maximizing energy.

(C) This is a strengthener (rules out an OTHER WAY).

(D) This is better than (B) … it's stronger (NO tendency vs. LITTLE tendency) and it's a better match for the author's explanation. If blue is providing the same energy as red, the bacteria should be equally attracted (according to the author's theory). Since they're not, it looks like the author's theory is wrong.

(E) We're not going to beat (D) by talking about OTHER types of bacteria that DON'T have chlorophyll. These bacteria don't even have a clear color preference, so the relevance is very weak.

Takeaway/Pattern: This correct answer deals with plausibility of the AUTHOR'S EXPLANATION and uses the most common plausibility idea: covariation. (D) undermines the author's story with an example where the supposed CAUSE is present, but the EFFECT is absent.

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