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Practice Test 55 - AP Biology

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

  1. A freshwater fish is transferred from a tank with low salt concentration to a tank with higher salt concentration. Within hours, the fish drinks more water and produces a smaller volume of more concentrated urine than before. After returning the fish to the original tank, drinking decreases and urine becomes more dilute. Which response best explains these changes in drinking and urine concentration?
  2. Mendel's law of segregation describes how alleles for a single gene are distributed into gametes. This law is physically explained by the behavior of chromosomes during which phase of meiosis?
  3. A unicellular eukaryote contains a photosynthetic organelle surrounded by four membranes. The two innermost membranes resemble cyanobacterial membranes, and the organelle contains circular DNA. Between the second and third membranes, researchers detect remnants of a membrane-bound compartment with ribosomes distinct from the host cytosol. Which inference best accounts for the origin of this compartmentalization pattern?
  4. Two spherical cells have the same membrane composition and internal solute concentration. Cell 1 has radius 5 µm; Cell 2 has radius 10 µm. Both rely on diffusion across the plasma membrane for O2 entry and CO2 exit. If external O2 concentration is held constant, measurements show Cell 2 develops a lower internal O2 concentration than Cell 1 under the same conditions. Which explanation best accounts for the difference in internal O2 concentration between the two cells?
  5. A scientist uses CRISPR-Cas9 to edit a gene in cultured cells. A guide RNA is designed to match a 20-nt sequence in exon 2, adjacent to a PAM site. After introducing Cas9 and the guide RNA, the cell repairs the double-strand break using nonhomologous end joining (NHEJ). Sequencing of the target region from edited cells shows small insertions and deletions near the cut site. Which outcome best explains how NHEJ can alter the encoded protein?
  6. In a lab, genetically identical mice are raised from birth in either an enriched cage (toys, running wheel) or a standard cage. At 8 weeks, neurons from the hippocampus are analyzed. Mice from enriched cages show higher levels of BDNF mRNA and more dendritic branching, while DNA sequencing shows no differences between groups. Which explanation best accounts for the increased dendritic branching in enriched-cage mice?
  7. An enzyme catalyzes hydrolysis of a macromolecule by adding water across the bond between two subunits, splitting them apart. The macromolecule is known to be a polymer with a repeating backbone formed by dehydration reactions between phosphate and sugar groups. Based on the type of repeating linkage being broken, which macromolecule is most directly being hydrolyzed?
  8. A cell membrane contains many nonpolar lipid molecules. When a culture medium is supplemented with a small nonpolar gas, its concentration becomes higher outside the cell than inside. Within seconds, the gas concentration equalizes across the membrane. The rate is unchanged when protein channels are blocked and when ATP production is inhibited. Which process is most likely occurring as the gas enters the cell?
  9. A researcher synthesizes a short nucleic acid strand with the sequence 5'-AUGC-3'. In solution, it binds tightly to a second strand through hydrogen bonding between complementary bases, forming a short double-stranded region. The binding requires antiparallel alignment of the two strands. Which sequence best represents the strand that will base-pair with 5'-AUGC-3'?
  10. A farmer saves seeds only from corn plants with the highest kernel protein content. Protein content is associated with allele P (higher) versus p (lower). Starting frequencies are P = 0.25 and p = 0.75. After ten generations, the farmer observes that nearly all plants show high protein content and fewer plants show low protein content. Which outcome is most likely in the corn population?
  11. Two cube-shaped cells have identical membrane permeability to a small lipid-soluble molecule. Cell P has side length 5 µm; Cell Q has side length 15 µm. Both start with zero of the molecule inside and are placed in the same external concentration. After a short time interval, the increase in internal concentration is greater in Cell P. Which explanation best accounts for the greater increase in Cell P?
  12. In a neuron preparation, neurotransmitter N binds a ligand-gated ion channel receptor that opens to allow Na+ influx, depolarizing the membrane. Depolarization activates a voltage-gated Ca2+ channel, increasing cytosolic Ca2+. Ca2+ binds synaptotagmin on synaptic vesicles, promoting vesicle fusion and additional N release. A Ca2+ pump in the plasma membrane lowers cytosolic Ca2+ after stimulation. A drug is added that inhibits the Ca2+ pump without affecting the ion channel receptors. After a brief N pulse, cytosolic Ca2+ remains elevated longer than normal. Which effect is most likely in the presence of the pump inhibitor?
  13. A membrane is composed of a phospholipid bilayer with embedded proteins. The hydrophobic tails form an interior barrier to ions, while some transmembrane proteins form ion channels that allow specific ions to cross. In a lab, cells are placed in a solution with high NaCl, and the rate of Na+ entry is measured. A drug is added that blocks the pore of a Na+-specific channel protein without removing the protein from the membrane. Which feature best explains the expected change in Na+ permeability?
  14. A fish population in a lake is estimated at 5,000 adults in spring. During the next year, 1,200 adults die, 900 adult fish immigrate, 700 adults emigrate, and 1,500 juveniles survive to adulthood. Which statement best predicts the adult population size next spring?
  15. A membrane contains phospholipids and a transmembrane protein that functions as a gated ion channel. When the channel is closed, Na+ movement across the membrane is minimal, even with a steep Na+ concentration gradient. When the channel is open, Na+ movement increases greatly. The phospholipid bilayer remains unchanged. Which feature best explains the difference in Na+ transport between the two conditions?
  16. In a lake fish population, gill-raker number affects feeding. Individuals with very low or very high gill-raker counts each produce about 20 surviving juveniles per breeding season, while individuals with intermediate counts produce about 7. This difference persists for 15 generations, and the population remains large with no detected migration. Over time, the frequency of intermediate gill-raker phenotypes declines, while both extreme phenotypes become more common. Which pattern best illustrates the type of selection acting on gill-raker number?
  17. A DNA fragment is analyzed and found to be 60% guanine plus cytosine (G+C) by nucleotide composition. Another fragment of equal length is 40% G+C. Both are double-stranded and have the same ionic conditions. The researcher predicts one fragment will require a higher temperature to separate the strands. Which feature best explains the difference in strand-separation temperature?
  18. A freshwater enzyme is tested in buffers with increasing NaCl while temperature and pH remain constant. Reaction rate is 25 units at 0.05 M NaCl and 6 units at 0.80 M NaCl. Which explanation best accounts for the decreased rate at high salinity?
  19. A student models diffusion by placing agar cubes containing a pH indicator into an acidic solution. The acid must diffuse in through the cube's surface to change the indicator color. Cube 1 has side length 1 cm and cube 2 has side length 3 cm. After the same incubation time, cube 1 is fully color-changed, while cube 2 remains unchanged in its center. Which explanation best accounts for the difference in how completely the cubes change color?
  20. A lab models cells as cubes with the same membrane composition. Each cube cell absorbs a dye by diffusion across its surface, and dye must reach the center to be detected. Two cube cells are tested in identical dye solution: Cube X has side length 22 units; Cube Y has side length 66 units. After the same time, dye is detected at the center of Cube X but not at the center of Cube Y. Which explanation best accounts for the different results?

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