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Difficulty: MediumEvaluating the Impact of New Evidence

### Surviving Extreme Desiccation

Tardigrades are microscopic animals famous for their ability to survive extreme environmental conditions by entering a state of suspended animation called cryptobiosis. During cryptobiosis, tardigrades lose up to 97% of their body water. Two hypotheses have been proposed to explain how tardigrades protect their cellular components from damage during this desiccation process.

Hypothesis 1
Tardigrades produce high levels of tardigrade-unique intrinsically disordered proteins (TDPs) during desiccation. TDPs lack a fixed three-dimensional structure and instead form a glass-like solid (vitrification) inside the cytoplasm. This vitrified matrix physically traps and stabilizes cellular membranes and proteins, preventing them from unfolding and aggregating. TDPs are the primary molecules responsible for this protection; other molecules, such as the disaccharide trehalose, play no significant role in cellular survival.

Hypothesis 2
Tardigrades protect their cells by synthesizing large amounts of the sugar trehalose during desiccation. Trehalose molecules physically replace water molecules by forming hydrogen bonds directly with cellular proteins and membrane lipids. This chemical substitution maintains the native structure of these macromolecules, preventing denaturation and membrane fusion. Trehalose is the primary protective agent during desiccation; TDPs do not play a significant role.

Suppose researchers identify a mutant strain of tardigrade that is incapable of synthesizing TDPs but produces normal levels of trehalose. When subjected to desiccation, these mutant tardigrades experience widespread cellular collapse and do not survive.

Which of the following statements best describes how this finding affects Hypothesis 1 and Hypothesis 2?

  1. It supports Hypothesis 1 and weakens Hypothesis 2, because it shows that TDPs are essential for surviving desiccation, whereas trehalose alone is insufficient.Answer
  2. B
    It weakens Hypothesis 1 and supports Hypothesis 2, because it shows that tardigrades can maintain normal trehalose production even when TDP genes are knocked out.
  3. C
    It supports both Hypothesis 1 and Hypothesis 2, because it shows that the presence of trehalose is dependent on the function of TDPs.
  4. D
    It weakens both Hypothesis 1 and Hypothesis 2, because it shows that neither TDPs nor trehalose is involved in the protection of cellular structures.

Answer

The finding supports Hypothesis 1 and weakens Hypothesis 2, because it shows that TDPs are essential for surviving desiccation, whereas trehalose alone is insufficient.
The correct answer is that the finding supports Hypothesis 1 and weakens Hypothesis 2. Hypothesis 1 states that TDPs are primary for protection, while Hypothesis 2 states that trehalose is primary. Because tardigrades lacking TDPs cannot survive desiccation despite having normal trehalose levels, it shows TDPs are necessary (supporting Hypothesis 1) and that trehalose alone is not sufficient (weakening Hypothesis 2).

Step-by-Step Solution

1
Identify the key differences between Hypothesis 1 and Hypothesis 2.
Hypothesis 1 claims TDPs are the primary molecules for desiccation survival and trehalose plays no role, while Hypothesis 2 claims trehalose is the primary molecule and TDPs play no role.
Understanding the conflicting assertions of the two hypotheses is necessary to evaluate the impact of new evidence.
2
Analyze the results of the new experiment.
Mutant tardigrades lacking TDPs but having normal trehalose levels suffered cellular collapse and died during desiccation.
Determining the outcomes of the new evidence shows which mechanisms failed or succeeded under the experimental conditions.
3
Evaluate the impact of the experimental results on each hypothesis.
Since the absence of TDPs led to failure and death, TDPs are essential, which supports Hypothesis 1. Since the presence of normal trehalose was not enough to save the cells, trehalose is not the sole primary protective agent, which weakens Hypothesis 2.
Linking the experimental findings back to the core assertions of the hypotheses determines if they are supported or weakened.

Key Concept

Evaluating the Impact of New Evidence
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Topics

Evaluating the Impact of New Evidence

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