IB Biology SL A1.1 Water questions

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Question 1

Multiple choiceHard1 mark

In a plant, aqueous phloem sap carries substance P to cells, where a cytoplasmic enzyme uses P as a substrate. Molecule L is found in the hydrophobic interior of the cell-surface membrane. Which explanation links water’s solvent properties to these roles?

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Answer: A. Hydrophilic substances dissolve in water, supporting transport and availability in aqueous environments; hydrophobic substances are insoluble and can associate with a membrane’s hydrophobic interior. P dissolves in aqueous phloem sap and is present in aqueous cytoplasm, where enzymes catalyse reactions. L’s insolubility is consistent with its membrane location.

B: Treats dissolution as preventing enzyme–substrate interaction; enzymes catalyse reactions in aqueous solution. C: Reverses the effect of solubility on transport: dissolution enables P to be carried in water-based phloem sap. D: Reverses the hydrophilic and hydrophobic roles; hydrophobic P would not dissolve in aqueous sap, and hydrophilic L is not suited to the membrane interior.

Remember: water-soluble molecules can be transported in aqueous fluids and participate in metabolism in aqueous solution.

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