Question 90·Hard·Central Ideas and Details
Environmental engineer Asha Patel set out to determine how certain wetland plants survive in soils laced with high concentrations of mercury. Patel’s team discovered that the plants secrete root exudates unusually rich in thiols, sulfur-containing organic molecules that form exceptionally strong bonds with mercury ions. When the researchers carefully washed away these exudates in a controlled greenhouse experiment, mercury quickly built up to lethal levels inside the plants’ tissues. When purified thiol compounds were added back to the soil, however, the mercury remained in the surrounding sediment as chemically inert complexes and the plants showed no signs of stress. The team concludes that the thiol-based exudates act as a "chemical shield" for the plants.
Based on the study, what primary function do the thiol-based root exudates serve for the wetland plants in mercury-contaminated soils?
For central-idea/detail questions tied to an experiment, track the variable and the outcome: here, the presence/absence of thiol exudates and whether mercury accumulates inside the plant. Then restate the mechanism in plain language (thiols bind mercury so it stays in the sediment) and pick the option that most directly paraphrases that relationship without adding an unmentioned process (storage, transformation, export, microbes, etc.).
Hints
Find the direct description of thiols
Reread the sentence explaining what thiols do to mercury ions. What kind of interaction is described?
Use the wash-away vs. add-back comparison
Compare what happens to mercury levels inside the plant when exudates are removed versus when thiols are added back. Where is the mercury in each case?
Interpret “chemical shield” precisely
A “shield” prevents harm by stopping something from reaching its target. In this study, what keeps mercury from reaching the plant’s tissues?
Step-by-step Explanation
Understand what the question is asking
The question asks for the primary function of the thiol-based root exudates in mercury-contaminated soils—specifically, what role they play in helping the plants survive mercury exposure.
Locate key evidence in the passage
Key lines describe both mechanism and outcome:
- Thiols “form exceptionally strong bonds with mercury ions.”
- When exudates are washed away, mercury “built up to lethal levels inside the plants’ tissues.”
- When purified thiols are added back, mercury remains in the sediment as “chemically inert complexes” and plants show no stress.
These details indicate what the thiols do and why the plants fare differently across conditions.
Translate the evidence into a clear function
If thiols strongly bond to mercury ions, then mercury is held in the soil/sediment as inert complexes rather than moving into the plant. Removing thiols leads to dangerous internal mercury buildup, so the thiols’ protective effect comes from keeping mercury out of the plant.
Choose the option that matches the passage
The choice that matches the passage’s mechanism (binding mercury in the soil so it stays outside the plant) is:
They bind mercury in the soil, preventing its uptake by the plants.