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Difficulty: HardLogical Inferences

Microorganisms in deep terrestrial aquifers subsist under extreme energy limitation, often taking centuries to divide. Scientists debate whether these microbes maintain active cellular repair mechanisms or merely persist in a dormant state of gradual decay. To investigate, researchers analyzed intracellular proteins from microbes collected two kilometers below Earth’s surface. They focused on amino acid racemization, a spontaneous process that converts biological L-amino acids into D-amino acids, which cells must expend energy to repair. The team found that the ratio of D- to L-amino acids in the proteins was significantly lower than expected from spontaneous chemical decay over the geological age of the aquifer, suggesting that ______

Which choice most logically completes the text?

  1. A
    the rate of spontaneous amino acid racemization is accelerated by the geological conditions of deep terrestrial aquifers.
  2. these deep-biosphere microbes actively expend metabolic energy to maintain their cellular proteins rather than residing in a state of passive decay.Answer
  3. C
    microorganisms in deep terrestrial aquifers are the only life forms capable of surviving without any source of metabolic energy.
  4. D
    spontaneous chemical decay is the primary mechanism by which deep-subsurface microbes regulate their internal cellular processes.

Answer

these deep-biosphere microbes actively expend metabolic energy to maintain their cellular proteins rather than residing in a state of passive decay.
The correct answer logically follows from the premises in the text. The passage states that L-amino acids spontaneously convert to D-amino acids over time and that cells must expend energy to repair or replace them. Because the observed ratio of D- to L-amino acids in the subterranean microbes was significantly lower than would be expected from spontaneous chemical decay alone over the age of the aquifer, the microbes must be actively expending metabolic energy to repair their proteins. This supports the hypothesis that they maintain active cellular repair rather than existing in a state of passive, dormant decay.

Step-by-Step Solution

1
Identify the central debate and the methodology introduced in the passage.
The debate is whether subsurface microbes are active or dormant, and the methodology uses the ratio of D- to L-amino acids to measure repair activity.
This establishes the framework for drawing a logical inference.
2
Connect the biochemical mechanism to energy expenditure.
Racemization (L to D) is spontaneous, and cells must expend energy to repair or replace these amino acids.
This defines the prerequisite condition for keeping D-amino acid levels low.
3
Evaluate the empirical finding.
The ratio of D- to L-amino acids was significantly lower than expected from spontaneous decay alone.
A lower ratio indicates that active repair has occurred to keep D-amino acid accumulation in check.
4
Synthesize the findings to complete the text.
Since repair occurred and repair requires energy, the microbes must be actively expending metabolic energy to maintain their proteins.
This directly resolves the debate in favor of active cellular repair over passive decay.

Key Concept

Logical Inferences
Estimated Time:2m 0s
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