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Difficulty: MediumNecessary Assumption

Plant physiologist: When exposed to intense ultraviolet radiation, certain arid-zone succulents synthesize elevated concentrations of betalain pigments. It was long hypothesized that betalains protect chloroplast DNA from UV damage. However, recent laboratory trials demonstrated that mutant succulents incapable of producing betalains sustained no more UV-induced DNA fragmentation than did wild-type plants exposed to identical radiation levels. Thus, the production of betalains in these succulents must serve a purpose entirely unrelated to protection against ultraviolet radiation.

Which one of the following is an assumption on which the plant physiologist's argument depends?

  1. A
    The mutant succulents used in the trials were exposed to levels of ultraviolet radiation typical of their natural desert environment.
  2. B
    Any plant pigment whose synthesis is triggered by ultraviolet radiation confers substantial protection against environmental stressors.
  3. Betalains do not protect the succulents from varieties of ultraviolet-induced damage other than chloroplast DNA fragmentation.Answer
  4. D
    Arid-zone succulents that synthesize betalains rely exclusively on those pigments to prevent cellular damage caused by solar radiation.
  5. E
    If a pigment fails to protect plant DNA from fragmentation, it cannot serve any adaptive physiological function in desert plants.

Answer

Betalains do not protect the succulents from varieties of ultraviolet-induced damage other than chloroplast DNA fragmentation.
The author concludes that betalain production is entirely unrelated to ultraviolet defense based solely on finding no effect on chloroplast DNA fragmentation. For this conclusion to stand, it is essential that betalains do not protect against any other manifestations of UV damage (such as photosystem degradation or membrane lipid peroxidation). Negating the correct statement establishes that betalains do protect against other forms of UV damage, which completely invalidates the conclusion.

Step-by-Step Solution

1
Analyze the argument structure
Premise: Mutants lacking betalains show no greater UV-induced DNA fragmentation than wild-type plants. Conclusion: Betalain production must serve a purpose entirely unrelated to UV radiation protection.
Deconstructing the argument into premises and conclusion reveals the gap between the specific evidence and the broad claim.
2
Identify the logical leap
The author equates failing to protect against one specific type of damage (chloroplast DNA fragmentation) with providing no protection against ultraviolet radiation in general.
A necessary assumption must bridge this gap by eliminating other possible UV-protective functions.
3
Apply the Negation Test to candidate assumptions
Negating the statement produces: 'Betalains do protect the succulents from varieties of ultraviolet-induced damage other than chloroplast DNA fragmentation.' This directly shatters the conclusion that betalains serve no UV-protective purpose.
A statement whose negation destroys the author's conclusion is a necessary assumption of the argument.

Key Concept

Necessary Assumption (Defender Assumption and Negation Test)
Estimated Time:1m 15s
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Topics

Necessary Assumption

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