18. A water-soluble hormone typically acts on a target cell by:
(A) binding to a receptor inside the cell to alter gene expression.
(B) binding to a cell-surface receptor to trigger an intracellular signal transduction pathway.
(C) diffusing across the plasma membrane to directly activate enzymes.
(D) being converted into a lipid-soluble form before entering the cell.

14. The immediate energy source that directly drives ATP synthesis by ATP synthase during oxidative phosphorylation is the:(A) oxidation of glucose and other organic compounds.(B) flow of electrons down the electron transport chain.(C) H+ concentration gradient across the membrane holding ATP synthase.(D) transfer of phosphate to ADP.

15. How do transposons and retrotransposons differ in their movement mechanisms within the genome?(A) Transposons move via an RNA intermediate, while retrotransposons move via a DNA intermediate.(B) Transposons are less prevalent and move via an RNA intermediate, while retrotransposons are more prevalent and move via a DNA intermediate.(C) Transposons move via a DNA intermediate, while retrotransposons are more prevalent and move via an RNA intermediate.(D) Both transposons and retrotransposons move via a DNA intermediate, but only retrotransposons create new copies.

16. Which mechanism of evolution consistently causes adaptive evolution?(A) Genetic drift (B) Gene flow (C) Natural selection (D) Mutation

17. Which of the following is the primary role of the Calvin cycle in photosynthesis?(A) To produce ATP and NADPH.(B) To capture light energy.(C) To split water and release oxygen.(D) To use chemical energy (ATP and NADPH) to reduce CO2 to sugar.

19. Different species of fruit flies that live in the same parts of the Hawaiian Islands have distinct, elaborate courtship rituals.What type of reproductive isolation does this represent?(A) Habitat isolation (B) Temporal isolation (C) Behavioral isolation (D) Gametic isolation

20. Genes A, B, and C are located on the same chromosome. Testcrosses show that the recombination frequency between A andB is 28% and that between A and C is 12%. Which of the following statements about the order of these genes on thechromosome is most likely correct?(A) The order is A-B-C. (B) The order is A-C-B.(C) The order is B-A-C. (D) The order cannot be determined from this data alone.

21. If a diploid cell in the G1 phase of the cell cycle has a DNA content of 'x', what would be the DNA content of the same cell at metaphase of meiosis I?(A) 0.25x (B) 0.5x (C) x (D) 2x

22. The relatively short interval of time (535–525 million years ago) during which large forms of many present-day animal phyla first appear in the fossil record is known as what?(A) The Permian extinction (B) The Cambrian explosion(C) The Mesozoic era (D) The colonization of land

23. How might an error during meiosis lead to polyploidy in plants?(A) Failure of homologous chromosomes to separate during meiosis I, leading to diploid gametes.(B) Random assortment of chromosomes during metaphase I, resulting in gametes with extra chromosomes.(C) Crossing over between non-sister chromatids, creating recombinant chromosomes.(D) Nondisjunction in mitosis during gamete formation in the diploid organism.

24. A population has an allele 'a' with a frequency of 0.45. If the population is in Hardy-Weinberg equilibrium, what is the expected frequency of the homozygous dominant genotype (AA)?(A) 0.2025 (B) 0.3025 (C) 0.4950 (D) 0.5500