Practice Test 48 - AP Biology

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

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1. A bacterial cell membrane contains a channel protein selective for glycerol. When extracellular glycerol is 50 mM and intracellular glycerol is 5 mM, glycerol enters rapidly. When extracellular glycerol is reduced to 5 mM, net glycerol movement stops even though the channel remains present. ATP levels are unchanged across treatments. Which explanation best accounts for the change in net glycerol movement?
2. In a lab, a student isolates nuclei from a skin cell and a neuron from the same person and confirms their DNA sequences match. RNA sequencing shows the neuron contains abundant mRNA for a voltage-gated sodium channel gene, while the skin cell contains abundant mRNA for a keratin gene; the other gene's mRNA is near zero in each cell type. Both cell types contain RNA polymerase and ribosomes. Which explanation best accounts for the different proteins produced by the two cell types?
3. A DNA-binding protein functions as a dimer: two identical polypeptide subunits associate through complementary charged and hydrophobic regions, creating a quaternary structure that positions two helix-turn-helix motifs to contact adjacent major grooves. A mutation changes several surface glutamate residues at the dimer interface to valine, leaving each subunit's length unchanged. In vitro, each subunit still folds, but the protein binds DNA weakly. Which statement best describes why DNA binding is reduced?
4. Cells are engineered to express a membrane channel permeable to K+ but not Na+. Extracellular K+ is 8 mM and intracellular K+ is 140 mM. When the channel is opened, net K+ movement is out of the cell, and the rate is unchanged by blocking ATP synthesis. Which explanation best accounts for the direction and energy requirement of K+ movement?
5. A polymer is synthesized from monomers that each include a phosphate group, a five-carbon sugar, and a nitrogen-containing ring structure. Polymerization occurs by dehydration reactions that connect the phosphate of one monomer to the sugar of the next, producing a directional backbone. Which statement best classifies the polymer based on its monomers and covalent linkages?
6. A newly identified organelle in a unicellular eukaryote has two membranes and contains enzymes for anaerobic ATP production. Sequencing reveals a small circular genome inside the organelle, and phylogenetic analysis groups several organelle genes with bacterial genes rather than nuclear genes. The organelle's ribosomes resemble bacterial ribosomes in size. Which evidence best supports the mechanism by which this organelle originated?
7. In a cell-free system, receptor R activates kinase K, which phosphorylates substrate S. Adding a small amount of active K increases phosphorylated S rapidly, indicating amplification. When a competitive inhibitor of K is added, phosphorylated S decreases even though R remains active. Which statement best describes why the inhibitor reduces signal output?
8. A meadow food chain has these annual energy values stored as new biomass: plants 18,000 kJ/m$^2$/yr, rabbits 900 kJ/m$^2$/yr, hawks 45 kJ/m$^2$/yr. Treat each value as energy available to the next trophic level. Which conclusion is best supported by the data?
9. A student examines a single homologous chromosome pair carrying linked genes K and L. One homolog has KL and the other has kl. The student predicts that if no crossing over occurs between K and L, then gametes will carry only KL or kl chromatids. The student then observes gametes carrying Kl and kL chromatids after meiosis. Assume chromosome number is normal and segregation proceeds correctly. Which meiotic event best explains the appearance of Kl and kL chromatids in the gametes?
10. A desert plant population shows variation in seed size. In years with frequent drought, seedlings from very small seeds die at high rates, and seedlings from very large seeds are more likely to be eaten by rodents. Plants producing intermediate-sized seeds contribute the most offspring to the next generation. After multiple generations, the mean seed size changes little, but extreme seed sizes become rarer. Which type of selection is most consistent with these observations?
11. Two spherical cells are identical except size. Cell X has radius $6\,\mu m$ and Cell Y has radius $18\,\mu m$. A lipid-soluble signaling molecule crosses the membrane by simple diffusion and binds to an intracellular target. The external molecule concentration is constant, and binding does not change membrane permeability. Which explanation best accounts for why Cell Y takes longer to reach the same intracellular molecule concentration as Cell X?
12. In a diploid cell (2n = 6), homologous pairs align at metaphase I. If nondisjunction occurs in meiosis I for one homologous pair while the other pairs segregate normally, which outcome is most likely for the resulting gametes' chromosome numbers?
13. A DNA fragment is rich in G-C base pairs, which form three hydrogen bonds per pair, while A-T pairs form two; both use a sugar-phosphate backbone linked by phosphodiester bonds. In a lab, two equal-length double-stranded DNA fragments are heated under identical salt conditions. Fragment 1 contains 70% G-C pairs, and Fragment 2 contains 30% G-C pairs. The strands separate when enough hydrogen bonds between paired bases are disrupted by thermal motion. Which fragment will require a higher temperature to separate into single strands, and why?
14. In corn, smooth kernels (S) are completely dominant to wrinkled kernels (s). A plant with genotype SS is crossed with a plant with genotype ss. Each offspring receives one allele from each parent. Assume a single gene controls kernel texture and inheritance is Mendelian with complete dominance. Which outcome is expected among the offspring?
15. A ligand binds a receptor and triggers rapid activation of Protein S by phosphorylation. When cells are treated with a phosphatase inhibitor, Protein S phosphorylation becomes higher and persists longer after ligand removal. Ligand binding kinetics to the receptor are unchanged by the inhibitor. Which of the following best explains the role of phosphatases in early signal transduction here?
16. A eukaryotic gene has two alternative 3′ splice sites within the same intron. In cell type 1, splicing uses the upstream 3′ splice site; in cell type 2, splicing uses the downstream 3′ splice site. The 5′ splice site is identical in both cell types, and transcription rates are equal. When mature RNAs are compared, cell type 2 produces an mRNA that is shorter by 18 nucleotides. Which explanation best accounts for the 18-nucleotide difference?
17. A membrane receptor activates a heterotrimeric G protein. The released Gβγ subunits open a K$^+$ channel, increasing K$^+$ efflux and decreasing cytosolic K$^+$. A kinase Z is inhibited when cytosolic K$^+$ is low, so Z activity decreases rapidly after ligand addition. A regulator of G protein signaling (RGS) protein accelerates Gα GTP hydrolysis, promoting reassociation of Gα with Gβγ. In cells overexpressing RGS, the decrease in Z activity is smaller and shorter than in control cells at the same ligand concentration. Which mechanism most likely causes the reduced response in RGS-overexpressing cells?
18. In a sensory cell, a ligand-gated Na$^+$ channel opens when odorant binds, depolarizing the membrane. Depolarization opens voltage-gated Ca$^{2+}$ channels, raising cytosolic Ca$^{2+}$. Ca$^{2+}$ activates a kinase that phosphorylates the odorant-gated Na$^+$ channel, decreasing its open probability even if odorant remains bound. When kinase activity is blocked, depolarization lasts longer for the same odorant pulse. Which outcome best illustrates the feedback's effect on pathway activity?
19. In a eukaryotic cell, enzymes that function best at low pH are enclosed within lysosomes, while many cytosolic enzymes operate near neutral pH. When a weak base that diffuses across membranes is added, lysosomal pH rises and lysosomal digestion of macromolecules decreases, even though the enzyme amount is unchanged. The cytosolic pH remains near neutral. Which feature best explains how compartmentalization affects the efficiency of lysosomal digestion?
20. A phospholipid bilayer (no proteins) is exposed to equal concentrations of Mg$^{2+}$ (24.3 Da, charged) and neon (20.2 Da, nonpolar). Which molecule would most likely cross the bilayer faster by simple diffusion?

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