LSAT 151 | Section 4 | Logical Reasoning: Q22

LR Question 22 Explanation

QUESTION TEXT: Biologist: When bacteria of a particular species are…

QUESTION TYPE: Weaken

CONCLUSION: The bacteria probably detect red light by checking how much energy their chlorophyll makes.

REASONING: The bacteria’s chlorophyll makes the most energy when they’re in a certain kind of red light. The bacteria tend to move towards this light when there are multiple areas.

ANALYSIS: The biologist has shown that it is good for the bacteria to be in the red light. The reason it is good is because their chlorophyll produce energy.

But that doesn’t mean that energy production is why animals move. When you eat food, you aren’t thinking “ah, this produces the most energy for my brain and the most energy for my muscles”. You think that it tastes good, or that it is filling. Hunger and taste are what decide your actions.

So, to weaken the argument we should look for a similar alternate cause amongst the bacteria. Perhaps they are programmed to seek red light, and would do so even if their chlorophyll were broken.

___________

1. This strengthens the argument. It shows motion is determined entirely by chlorophyll energy production.
2. This just repeats info we largely could guess from the stimulus. The biologist said that the bacteria only go in the red light area. And their chlorophyll produce energy best in this area. So it doesn’t matter if the bacteria could produce some energy in other shades of light: the bacteria are choosing the best place. The stimulus already told us that this is how the bacteria behave.
3. This strengthens the argument by removing an alternate explanation. It’s possible that bacteria find the light by warmth, rather than chlorophyll. This answer removes that possibility.
4. CORRECT. This strongly weakens the argument. If the bacteria were moving based on chlorophyll production, then at least some bacteria should go into the blue light: their energy production is just as high there.
    
   This answer implies that the bacteria are choosing their location based on color shade directly.
5. This sounds a bit promising, but it’s far too vague to be useful. Light could have many effects on a bacteria, and not all of them need to be related to energy production. For example, maybe white light kills some bacteria. In that case, we would expect them to move to any other area to avoid the white light. This is very different from bacteria choosing red light specifically for energy.
    
   The right answer needs to be specific enough to speak directly to the case in the stimulus.