Practice Test 27 - AP Biology

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

A cell increases in size while keeping the same shape and membrane composition. Nutrients enter by diffusion across the plasma membrane, and nutrient use occurs throughout the cytoplasm at a similar rate per unit volume. As the cell grows, measurements show that nutrient concentration near the center becomes lower than near the membrane. Which explanation best accounts for the spatial gradient inside the larger cell?
A membrane consists of a phospholipid bilayer with no proteins. The hydrophobic interior strongly limits passage of polar and charged molecules. Consider two solutes: formamide (small, strongly polar due to a carbonyl and amine) and carbon monoxide (CO), which is small and relatively nonpolar. Both are uncharged. Temperature and concentration gradients are held constant. Which solute would most likely have greater permeability across the bilayer?
A researcher compares two ligands, A1 and A2, that bind different receptors on the same cell type. A1 causes rapid cAMP increase; A2 causes rapid IP $_3$ increase. When adenylyl cyclase is inhibited, only A1's response is lost; when PLC is inhibited, only A2's response is lost. Which of the following best explains these observations about early transduction specificity?
RNA is more susceptible than DNA to alkaline hydrolysis because:
A beetle population varies in body color, a heritable trait controlled by alleles G (green) and g (brown). In a forest, birds preferentially capture green beetles when leaves are absent during a prolonged drought, because green beetles are more visible on brown branches and leaf litter. Brown beetles are less often captured and more frequently survive to reproduce. The population remains in the same area and continues interbreeding. Which outcome is most likely after several generations of drought?
A fungal pathogen infects a population of wild rabbits. In years when rabbit density is high, field surveys record many rabbits with lesions and a rapid increase in new infections. In years when rabbit density is low, few rabbits show lesions and new infections spread slowly. Temperature and rainfall are similar across years, and the fungus is transmitted by direct contact between rabbits. Which statement best describes the role of the pathogen in regulating rabbit population size?
Two fatty acids are compared: Fatty acid A has 18 carbons and no double bonds; Fatty acid B has 18 carbons and one cis double bond. Both are incorporated into phospholipid tails in separate artificial membranes. At the same temperature, the membrane made with B is less tightly packed. Which feature of B most directly causes reduced packing?
In isolated mitochondria, adding ADP and $P_i$ increases oxygen consumption; adding oligomycin (ATP synthase inhibitor) returns oxygen use to baseline. Which process most directly links ADP availability to increased oxygen consumption in these mitochondria?
A student observes that when a loud sound occurs near a flock of pigeons, most birds take flight within 1–2 seconds. Heart rate measured from a few trained birds increases immediately during the sound and returns near baseline several minutes after the sound stops. No changes in body size or feather structure occur. Which response best explains the rapid increase in heart rate during the loud sound?
Two disaccharides each contain glucose. Disaccharide 1 has a glycosidic bond that leaves one anomeric carbon unlinked, so the molecule can open to a linear form with a free aldehyde group. Disaccharide 2 has a glycosidic bond connecting both anomeric carbons, preventing ring opening and eliminating a free aldehyde in solution. In a lab test, only one disaccharide reduces Cu $^{2+}$ to Cu $^+$ . Which statement best explains the observed difference in reducing ability?
Two populations of a ground beetle were separated by a landslide that blocked a mountain pass for 8,000 years. When the pass reopened, beetles from both sides were observed in the same valley. Mating trials show that individuals attempt to mate, and fertilized eggs form, but most embryos fail to develop past early stages. Adult beetles from both populations are otherwise healthy and occupy similar microhabitats in the valley. Genetic analyses indicate substantial divergence accumulated during separation. Which reproductive barrier most directly maintains separation after secondary contact?
A short RNA molecule folds into a hairpin structure when placed in solution. Sequencing reveals two regions within the same RNA strand that are complementary, allowing intramolecular base pairing. The paired region forms a double-stranded stem stabilized by hydrogen bonds, while the unpaired nucleotides form a loop. The formation of the hairpin depends on which bases can pair with each other under standard conditions. Which feature best explains how a single RNA strand can form a stable hairpin stem?
A researcher measures DNA content and observes spindle formation. Cells with 4C DNA begin forming mitotic spindles and condensing chromosomes. In one experimental group, cells with 2C DNA also begin forming mitotic spindles and condensing chromosomes shortly after the treatment. After division, many daughter cells in the experimental group are missing chromosomes. The treatment did not affect microtubule attachment once spindles formed. Which checkpoint was most likely bypassed by the treatment?
In an isolated mitochondrion preparation, adding ADP and inorganic phosphate (Pi) causes oxygen consumption to increase and ATP concentration to rise. When a chemical uncoupler that allows protons to cross the inner mitochondrial membrane is added, oxygen consumption remains high but ATP production drops sharply. The electron transport chain proteins are still intact, and the inner membrane is otherwise impermeable to ions. The uncoupler does not directly inhibit any electron carriers. This setup focuses on how redox energy is converted into a proton gradient and then into ATP by ATP synthase.
A biologist places a drop of water on a clean glass slide and observes that it beads up rather than spreading into a thin film. Each water molecule is polar, with a partial negative charge near oxygen and partial positive charges near hydrogens. Hydrogen bonds form between neighboring water molecules when the partially positive hydrogen is attracted to the partially negative oxygen of another molecule. At the surface of the drop, many water molecules are pulled inward by hydrogen bonding with neighbors, creating a tight network that resists expansion of the surface. Which statement best explains the beading behavior at the molecular level?
In one species, a regulatory RNA folds back on itself and forms a short double-stranded stem region. The stem forms because nucleotides within the same RNA molecule align in antiparallel orientation and hydrogen-bond using standard RNA base-pairing rules. The folded structure is required for the RNA to bind a protein that recognizes the stem. Which feature best explains how a single-stranded RNA can form a stable stem structure?
In snapdragons, tall stems (T) are completely dominant to short stems (t). A tall plant of unknown genotype is crossed with a short plant (tt). In the offspring, 48 plants are tall and 52 plants are short. Assume random fertilization and a single gene with complete dominance. Which genotype is most likely for the tall parent?
In a cell-free replication reaction, all components are present for normal DNA synthesis, including primers that provide a free 3′ end. DNA polymerase adds nucleotides by complementary base pairing. A competitive inhibitor is added that binds to the active site of DNA polymerase and prevents incoming deoxyribonucleotides from being added, without affecting helicase activity. Which observation is most likely during the reaction after inhibitor addition?
In a marine snail population, shell thickness varies. After the introduction of a crab species that crushes shells, snails with thicker shells are more likely to survive and reproduce. Over 30 generations, the mean shell thickness increases steadily and the frequency of alleles associated with thicker shells rises, while alleles associated with thin shells decline. The population does not show an increase in both extremes; instead, the distribution shifts toward thicker shells. Which selection pattern best matches these observations?
Cell type T secretes cytokine C that acts locally. In a mixed culture, only cells within ~200 µm of T respond. When the medium is continuously stirred, the response disappears even though the total amount of C measured in the dish is unchanged. Which outcome is most likely responsible for the loss of response?

Practice tests