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Practice Test 92 - AP Biology

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

  1. In a redox reaction occurring in a cell extract, molecule M donates electrons to molecule N. After the reaction, M is oxidized and N is reduced. Measurements show that the reaction releases free energy that can be used to drive ATP synthesis in a coupled system. Electron transfer from a higher-energy electron donor to a lower-energy electron acceptor can be exergonic. The more electronegative acceptor stabilizes electrons at lower potential energy. Which statement best explains why the electron transfer from M to N can release usable energy?
  2. A purified enzyme shows maximal activity at pH 7.5. When assayed at pH 5.0 with the same substrate and enzyme concentrations, the initial reaction rate is much lower, but the enzyme is not permanently denatured and activity returns after shifting back to pH 7.5. No inhibitor is present. Which explanation best accounts for the reduced rate at pH 5.0?
  3. A frog population in a pond was 600 adults at the start of spring. Over the next two months, 180 adults died and 60 new adults joined the breeding population from metamorphosis. No immigration or emigration occurred. Which statement best describes the net population change over the two months?
  4. A molecular clock makes one branch look much older than the rest. What could do this?
  5. A scientist introduces a premature stop codon into an mRNA at codon 3. In this system, stop codons are bound by release factors that trigger hydrolysis of the bond between the polypeptide and the tRNA in the P site, releasing the polypeptide. The ribosome then dissociates from the mRNA. Which explanation best accounts for why the polypeptide produced is truncated?
  6. In a cell-free system, an RNA polymer is synthesized using a DNA template strand. Analysis shows the RNA contains uracil and has a 5′ end and a 3′ end. The RNA sequence is complementary to the DNA template and is produced by adding ribonucleotides to the growing RNA chain. The directionality of strand synthesis depends on the chemical groups available on the ribose sugar of the incoming nucleotides and on the growing chain. Which feature best explains why the RNA polymer grows only in the 5′→3′ direction?
  7. DNA polymerase adds nucleotides to a growing DNA strand by forming a phosphodiester bond. Which feature best explains why synthesis proceeds only in the 5′→3′ direction?
  8. An enzyme's initial rate is measured at constant enzyme concentration with a fixed substrate amount. In Trial 1, no product is present initially. In Trial 2, a high concentration of product is added before starting the reaction; the initial rate is lower, but adding more enzyme partially restores the initial rate. pH and temperature are unchanged, and substrate is not limiting. Which explanation best accounts for the lower initial rate in Trial 2?
  9. A beetle population contains two heritable color morphs: green and brown. After an invasive plant spreads, the ground becomes covered with brown leaf litter year-round. Birds capture green beetles more often than brown beetles, and brown beetles produce more offspring each season. Both colors still occur among hatchlings. Which outcome is most likely in this beetle population over time?
  10. A comparative genomics study examines a noncoding DNA region near a developmental gene in four species of frogs. Species 1 and 2 share a 12-base deletion at the exact same position; Species 3 and 4 lack the deletion and have the longer sequence. Sequencing coverage is high and rules out error. No phenotype information is included. Which inference is best supported by the shared deletion?
  11. Two bird populations occupy adjacent habitats and occasionally interbreed in a narrow contact zone. Hybrids hatch and survive to adulthood, but long-term banding shows hybrids produce fewer offspring than either parental population because their songs are intermediate and attract fewer mates. Outside the contact zone, each population has a distinct song type, and genetic data show increasing divergence at loci associated with song learning. No physical barrier prevents movement across habitats. Which process most directly promotes further reproductive isolation between the populations?
  12. A ligand binds a receptor and triggers rapid production of reactive oxygen species (ROS) at the inner surface of the plasma membrane. When a specific membrane-localized enzyme is inhibited, ligand binding occurs normally but ROS production does not increase. The inhibitor does not enter the nucleus. Which of the following best explains the earliest step linking receptor activation to ROS production?
  13. A bright light is shined on a freshwater planarian; within seconds it turns and moves away. Which outcome is most likely caused by this stimulus?
  14. A researcher compares two phospholipids: both contain C, H, and O, but only one includes a phosphate group with P and multiple negatively charged oxygens. In water, the phospholipid with P forms stable bilayers in which the phosphate-containing region faces outward and the hydrocarbon chains face inward. The other lipid, lacking P, tends to form oil droplets instead of bilayers. The phosphate group's electronegative oxygens create a polar, hydrophilic head that interacts with water via ion-dipole attractions and hydrogen bonding, while the C–H chains remain nonpolar. Which feature best explains the bilayer-forming behavior of the phospholipid containing P?
  15. A researcher studies a signaling pathway in animal cells where ligand M causes a rapid opening of a specific ion channel in the plasma membrane, increasing cytosolic Na+^+. The ligand binds to receptor T on the cell surface. In cells expressing a receptor T variant lacking most of its cytosolic tail, ligand binding still occurs, but the Na+^+ increase is greatly reduced. Direct application of a channel-opening drug restores Na+^+ influx in both cell types. Which of the following best explains the function of receptor T's cytosolic tail in early signal transduction?
  16. A student compares DNA and RNA nucleotides. Both contain a pentose sugar, a phosphate group, and a nitrogenous base, but one type of nucleic acid is more chemically stable in alkaline conditions. The difference is traced to a functional group on the sugar that can participate in intramolecular reactions that break the backbone. Which feature best explains why RNA is generally less stable than DNA under basic conditions?
  17. A plant leaf cell contains chloroplasts with extensive thylakoid membranes arranged in stacks. When light intensity increases, the cell's rate of ATP formation inside chloroplasts increases. Which feature best explains why stacked thylakoid membranes support higher ATP formation rates?
  18. Red cells lyse in 0.3 M urea but not in 0.15 M NaCl. Why?
  19. To model protocell formation, fatty acids were added to water in two treatments: (1) pure water and (2) water containing dissolved organic molecules (nucleotides and amino acids). In both treatments, fatty acids spontaneously assembled into vesicles, but only in treatment (2) did vesicles persist longer and retain a higher fraction of the dissolved organics after gentle mixing. Which conclusion is best supported regarding hypotheses about early cell-like structures?
  20. A researcher compares two populations of secretory cells. Population 1 contains abundant rough endoplasmic reticulum (RER) and many Golgi stacks; Population 2 has relatively little RER and a small Golgi region. Both populations have similar numbers of mitochondria. The RER consists of membrane sacs studded with ribosomes, and the Golgi modifies and packages proteins into vesicles that fuse with the plasma membrane. Which outcome is most likely for Population 1 compared with Population 2?

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