Practice Test 3 - MCAT Biological and Biochemical Foundations of Living Systems

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

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1. During an in vitro study of mammalian gastrulation, investigators label epiblast cells at the primitive streak and track their positions 12 hours later. Labeled cells that ingress through the streak are observed to spread between an outer epithelial sheet and an inner epithelial sheet, forming a loosely organized middle layer that later shows increased extracellular matrix deposition and cell motility. Baseline: during gastrulation, three primary germ layers are established—ectoderm (outer), mesoderm (middle), and endoderm (inner). Which embryonic layer are the labeled ingressed cells most likely to contribute to? Simple schematic (cross-section): [Outer sheet] = ectoderm [Middle space] = mesoderm [Inner sheet] = endoderm
2. Researchers investigated oxygen delivery in isolated skeletal muscle microvessels perfused with red blood cells (RBCs). A point mutation was introduced into the β chain of hemoglobin that increased oxygen-binding affinity (left-shifted dissociation behavior) without changing hemoglobin concentration per RBC. Under identical arterial $P_{O_2}$, venous $P_{O_2}$ measured downstream increased and tissue lactate production increased. Based on the scenario, which outcome is most consistent with the mutated protein's nonenzymatic function?
3. A lab measures transcription of a heat-shock operon controlled by an alternative sigma factor ($\sigma^H$). At 30°C, $\sigma^H$ is sequestered by an anti-sigma factor. After a rapid shift to 42°C, misfolded proteins accumulate and titrate the anti-sigma factor, freeing $\sigma^H$ to bind RNA polymerase and initiate transcription at heat-shock promoters. Based on the scenario, which outcome would be expected given the gene regulation process?
4. In a study of antibiotic effects on translation, a compound was found to bind the large ribosomal subunit and reduce peptidyl transferase activity without affecting tRNA charging. In treated cells, polysome profiling showed ribosomes stalled near start codons, and immunoblotting detected a marked decrease in full-length protein, with accumulation of short peptidyl-tRNA species. Which outcome is most directly expected from inhibition of peptide bond formation with respect to primary structure? (Assume mRNA levels are unchanged.)
5. A researcher compares two cytosolic pathways starting from glucose-6-phosphate (G6P) in a proliferating cell: (i) glycolysis to pyruvate under anaerobic conditions and (ii) the oxidative PPP to ribulose-5-phosphate. The researcher is interested in reducing equivalents rather than ATP. Which flux comparison is most consistent with these pathways' primary outputs from a single G6P equivalent entering each pathway?
6. A lab studies the effect of a peptide hormone (G) on hepatocyte glycogen breakdown. G treatment increases glycogen phosphorylase activity within 1 minute. Inhibiting PKA blocks this increase. Inhibiting phosphodiesterase (PDE) prolongs the effect of G, while inhibiting protein phosphatase 1 (PP1) increases basal glycogen phosphorylase activity even without G. Which outcome would be expected if the receptor for G is blocked, but PDE is simultaneously inhibited?
7. Investigators examined the effect of a brainstem vagal stimulation protocol on postprandial insulin secretion in humans. Participants consumed a standardized mixed meal; half received transcutaneous vagal stimulation (tVNS) for 10 minutes starting at meal onset. Plasma insulin and glucose were measured.

   Condition	Glucose (mg/dL) at 30 min	Insulin ($\mu$U/mL) at 30 min
   Meal only	155	48
   Meal + tVNS	150	72

   Which of the following best explains the relationship between the neural stimulus and the hormonal response?
8. A neuropharmacology group tested a compound that inhibits the Na$^+$/K$^+$-ATPase in cultured neurons. Over several minutes, intracellular Na$^+$ increased and intracellular K$^+$ decreased; simultaneously, the resting membrane potential drifted from $-70\ \text{mV}$ to $-55\ \text{mV}$. No acute change in voltage-gated channel kinetics was detected. Which statement best explains the changes in membrane potential observed?
9. In a structural comparison study, investigators treated bacterial cells and cultured mammalian cells with a translation inhibitor that binds the small ribosomal subunit. Protein synthesis in bacteria dropped sharply, while mammalian cytosolic protein synthesis was minimally affected, though mitochondrial protein synthesis decreased. The bacterial cells were confirmed to lack a nucleus and endomembrane system. Core cell theory idea: cells carry out essential life processes, including protein synthesis. Based on these data, which conclusion best applies cell theory to prokaryotic structure?
10. A wildfire reduces a rabbit population to 18 survivors chosen largely by chance with respect to a fur-length locus (L vs l). Before the fire, allele L frequency was 0.55; immediately after, it is 0.22. Over the next year, food availability and predation remain similar to pre-fire conditions, and no fur-length phenotype shows a consistent survival advantage. Based on the scenario, which outcome is most consistent with genetic drift?
11. To probe epithelial barrier integrity, a monolayer of intestinal cells was engineered to express a truncated claudin that reaches the plasma membrane but lacks most of its extracellular loop. Transepithelial electrical resistance (TEER) decreased, while total claudin expression and cell viability were unchanged. Based on this scenario, which statement best describes the protein's role in this system?
12. A longitudinal study followed human fibroblast cultures across repeated passages. Cells were grouped by initial telomere length (long vs short). Cultures with shorter starting telomeres reached irreversible growth arrest earlier and showed increased expression of a DNA damage response marker, while cultures with longer starting telomeres maintained proliferative capacity for more passages. No differences in nutrient conditions were reported. Which outcome is expected concerning aging?
13. A researcher performs fate mapping at the onset of gastrulation in a mammalian embryo. Cells that remain on the surface later contribute to neural plate and epidermis, while cells that internalize first displace the pre-existing inner lining and later form a new inner epithelium of the primitive gut. Cells that internalize later occupy the middle position and spread widely. Based on the passage, which process is most consistent with establishing the endodermal layer?
14. A research group expressed a receptor-associated adaptor protein (Adap1) in mammalian cells. After stimulation with a growth factor, Adap1 shifted to a slower-migrating band on SDS-PAGE, and the shift was eliminated by phosphatase treatment. A mutant Adap1 in which a single serine in a consensus kinase motif was replaced with alanine failed to show the shift and showed prolonged receptor signaling. Based on the findings, what effect would the modification have on Adap1 function?
15. Researchers compare two neurons that both fire action potentials, but one expresses a higher density of voltage-gated K+ channels along the axon. In extracellular recordings, this neuron shows narrower spikes and can sustain higher firing rates during repetitive stimulation. Which statement best describes the role of ion channels in action potential propagation consistent with these observations?
16. A team examined a soil bacterium that survives desiccation by producing an extracellular capsule. In drying conditions, wild-type cells formed hydrated microcolonies and retained viability, while a capsule-deficient mutant showed extensive cell clumping and loss of viability. When both strains were rehydrated, only the wild-type rapidly resumed growth. (Terms: capsule = extracellular polysaccharide layer; desiccation = severe drying.) Which structure is most critical for the prokaryote's function that improved survival in the described environment?
17. In a skeletal muscle fiber, depolarization of the T-tubule activates the dihydropyridine receptor (DHPR), which is mechanically coupled to RyR1 on the SR. A mutation disrupts DHPR–RyR1 coupling without affecting DHPR voltage sensing. The muscle action potential is normal, but the Ca$^{2+}$ transient is greatly reduced. Which factor is most likely to limit muscle contraction in the described conditions?
18. A lab engineered a chimeric receptor with an extracellular domain that binds a peptide ligand and an intracellular domain derived from a receptor tyrosine kinase. Upon ligand addition, cells showed tyrosine phosphorylation and ERK activation, despite the ligand normally signaling through cAMP in wild-type cells. Which cellular response is most consistent with the signal transduction pathway in the chimeric receptor cells?
19. A flowering plant population splits when a new highway creates a long median barrier that prevents pollinators from crossing. On one side, the dominant pollinator is a long-tongued bee; on the other, a short-tongued fly. Over time, flower tube length diverges, and cross-pollination becomes extremely rare. Which event would most likely lead to speciation?
20. During a cold-pressor test, sympathetic activation causes widespread cutaneous vasoconstriction (hemodynamic variation). In one subject, MAP rises from 90 to 105 mmHg while cardiac output remains approximately constant at 5.0 L/min (measured by Doppler). The central physiological principle is MAP–CO–TPR relationship: $\text{MAP} \approx CO \times TPR$. Based on the passage, which conclusion is most consistent with the observed homeostatic response?

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