Practice Test 54 - AP Biology

20 questions. Answer them in any order, then submit to see your score and every explanation.

Two steroid hormones differ only by small functional groups attached to the same four fused-ring carbon skeleton. Both molecules are largely nonpolar because of the hydrocarbon rings, but one has an additional hydroxyl group that increases polarity relative to the other. The more polar steroid shows reduced ability to pass through the hydrophobic interior of a pure phospholipid bilayer. Which feature best explains the reduced passage through the bilayer?
In a yeast cell, mating factor F binds GPCR R, activating G protein and a MAPK cascade: kinase M1 activates M2, which activates M3. M3 phosphorylates target protein T, changing its activity within minutes. A scaffold protein holds M1, M2, and M3 in a complex near the receptor. In cells lacking the scaffold, receptor binding and G protein activation occur, but phosphorylation of T is much lower and slower. Which mechanism best explains the scaffold effect?
In a neuron, glutamate opens an ionotropic receptor that allows Ca $^{2+}$ influx. Ca $^{2+}$ binds calmodulin to activate CaMKII. Active CaMKII phosphorylates nearby glutamate receptors, increasing their open probability during glutamate binding and allowing additional Ca $^{2+}$ influx. A peptide that blocks CaMKII phosphorylation of the receptor reduces the sustained Ca $^{2+}$ rise after repeated glutamate pulses. Which change would most likely result from removing this feedback step?
An enzyme catalyzes the breakdown of a polymer into its monomers by adding water across the covalent bond between adjacent subunits. The polymer's monomers each have an amino group and a carboxyl group, and the covalent bond between monomers formed when water was removed during synthesis. At the molecular level, adding water reverses that synthesis reaction. Which statement best describes the reaction used to break down this polymer?
A population of field mice includes two alleles at a coat-color gene: L (light) and D (dark). After a new predator arrives, biologists sample the population every 2 years. Over 10 years, allele D increases from 0.22 to 0.55, while the habitat and migration rates remain similar. Which observation best demonstrates evolution occurring in this mouse population?
In a population of 2,000 insects, genotype counts at a locus are 1,280 CC, 640 Cc, and 80 cc. Conditions for Hardy-Weinberg equilibrium are met. Which allele frequency is expected for c in this population?
Two cell membranes contain identical transport proteins but differ in phospholipid tail composition. Membrane Q has a higher proportion of saturated fatty acids; Membrane R has a higher proportion of unsaturated fatty acids. At the same temperature, lateral movement of membrane proteins is slower in Membrane Q than in Membrane R. Which membrane feature best explains the reduced protein mobility in Membrane Q?
In a photosynthetic cell, the thylakoid membrane separates the thylakoid lumen from the stroma. Light-driven electron transport increases proton concentration in the thylakoid lumen relative to the stroma. When the thylakoid membrane is experimentally punctured, the proton difference rapidly disappears and ATP synthesis decreases. Which feature best explains how this compartmentalization supports photosynthetic ATP production?
In an experiment, a cell is placed in a solution containing a fluorescent ligand that binds a specific receptor. Fluorescence appears in small internal vesicles minutes later. Which outcome is most likely explained by receptor location in the plasma membrane?
Neurons in culture respond to neurotransmitter X with a rapid influx of Ca $^{2+}$ . X binds to a receptor embedded in the plasma membrane. In the presence of a competitive antagonist that occupies the same binding site, adding X produces no Ca $^{2+}$ influx. However, directly depolarizing the membrane with high extracellular K $^+$ still produces Ca $^{2+}$ influx. Which of the following best explains the role of the X receptor in signal reception?
Researchers compare two internal compartments in a eukaryotic cell. Compartment X is bounded by a single membrane and is continuous with the outer membrane of the nucleus; proteins in its membrane match plasma membrane proteins in amino acid sequence. Compartment Y is bounded by two membranes, contains circular DNA, and has ribosomes that differ from cytosolic ribosomes. Which claim is best supported about the origin of compartment X?
Animal cells (0.35 M internal nonpenetrating solute) are placed in a 0.35 M nonpenetrating solute solution. Water can cross the membrane, but solute cannot. After 10 minutes, the cells look unchanged. Which outcome is most likely?
A lysosome contains hydrolytic enzymes that function best at pH 5, while the cytosol remains near pH 7.2. Proton pumps in the lysosomal membrane maintain the acidic lumen. When a drug inhibits these pumps, lysosomal pH rises toward cytosolic pH and macromolecule digestion slows, even though the enzyme concentration in the lysosome is unchanged. Which feature best explains how compartmentalization provides control of cellular reactions?
A diploid cell contains two homologous chromosome pairs. Pair 1 carries alleles M and m; pair 2 carries alleles N and n. No crossing over occurs. The instructor states that gametes can still differ because each pair's homologs align independently at metaphase I. In a class activity, students list possible gamete chromosome sets produced after meiosis II. Which set of gamete types best supports the instructor's statement about genetic variation arising at the chromosomal level?
In a cat breed, fur color is controlled by an X-linked gene. Allele $X^O$ produces orange fur and allele $X^o$ produces black fur. Heterozygous females ( $X^OX^o$ ) have patches of orange and black fur. A patched female ( $X^OX^o$ ) is crossed with a black male ( $X^oY$ ). Which outcome best predicts the phenotypes of female offspring?
In a lab, students compare two artificial membranes made of phospholipid bilayers with embedded proteins. Membrane 1 contains mostly saturated fatty acid tails, while Membrane 2 contains mostly unsaturated tails. Both membranes have the same cholesterol content and the same number of channel proteins. At the same temperature, small nonpolar molecules (e.g., O2) cross Membrane 2 faster than Membrane 1. Which membrane feature best explains the difference in permeability?
A unicellular organism exchanges ammonia waste with its environment by diffusion across the plasma membrane. Two individuals of the same species are observed: one is small and one is large, with the large cell having about 8 times the volume of the small cell. Both generate ammonia at similar rates per unit volume. In the same environment, the larger cell accumulates higher internal ammonia concentration. Which explanation best accounts for the accumulation?
In dogs, the allele for curly fur (C) is completely dominant to straight fur (c). A breeder crosses two curly-furred dogs and obtains some straight-furred puppies. Assume a single autosomal gene and complete dominance. Based on the appearance of straight-furred offspring, which parental genotype combination is most likely?
A marine invertebrate releases sperm and eggs into seawater for external fertilization. Two populations spawn in the same bay on the same nights, and currents mix gametes. Laboratory assays show that sperm from each population fertilizes eggs from its own population at a much higher rate than it fertilizes eggs from the other population, even when equal sperm concentrations are used. When cross-fertilization does occur, resulting larvae develop normally. Which reproductive isolating mechanism most directly reduces gene flow between the populations?
A diploid organism has $2n=8$ . During anaphase II of meiosis, sister chromatids separate and move to opposite poles in each of the two cells formed after meiosis I. Which description best accounts for how chromosome number per cell changes from metaphase II to the end of meiosis II?

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