IB Biology SL Theme A: Unity and diversity questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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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.

Question 2

Multiple choiceHard1 mark

In a beetle population, 20% of adults have dark shells before the substrate darkens. After the change, 70% of the same adults have dark shells. Adults of both shades contribute equally to an offspring cohort, of which 20% have dark shells when raised on the original substrate. Which interpretation applies the definition of evolution to shell shade?

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Answer: C. Evolution is a change in the heritable characteristics of a population, not a change acquired by individuals during their lives. The proportion of dark-shelled adults rises from 20% to 70%, but their offspring, raised on the original substrate, have the initial 20% frequency. Thus, no inherited shift in shell shade is demonstrated. A: treats a change in adult appearance as an evolutionary change in heritable population characteristics. B: applies Lamarckian inheritance, but the offspring do not retain the increase in dark shells. D: confuses some offspring having a characteristic with the frequency of that characteristic changing across generations. Remember: evolution requires a change in heritable population characteristics across generations.

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