Practice Test 6 - MCAT Chemical and Physical Foundations of Biological Systems

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

An enzyme-catalyzed redox reaction in aqueous solution is studied under two conditions at 37°C: (1) standard pressure and (2) elevated hydrostatic pressure in a pressure vessel. The reaction decreases total molar volume (products occupy less partial molar volume than reactants). Using Le Châtelier's principle applied to pressure changes, which conclusion is most consistent with the model?
A rigid container holds an ideal gas at constant temperature. The number of moles is decreased by venting half the gas while keeping volume fixed. The question targets the ideal gas law dependence on $n$ . Which prediction is most consistent for pressure after venting? A. Pressure doubles because density decreases B. Pressure halves because $P \propto n$ at fixed $T$ and $V$ C. Pressure is unchanged because $T$ is constant D. Pressure increases because fewer molecules collide less often, reducing repulsion
Investigators compare two 30-bp nucleic acid duplexes at 25°C in 100 mM monovalent salt. Duplex X is DNA:DNA; duplex Y is RNA:RNA with the same base sequence (with U in place of T). Circular dichroism indicates duplex Y adopts an A-form helix under these conditions, whereas duplex X adopts a B-form helix. The core concept is how sugar structure influences nucleic acid helical geometry. Which statement is most consistent with these observations? Assume both duplexes are perfectly complementary and of equal length; no chemical modifications are present.
A human lysosomal enzyme functions at pH 4.5. When expressed recombinantly and assayed at pH 7.4, the enzyme binds substrate (similar $K_m$ ) but shows a 30-fold lower $k_{cat}$ . The catalytic residue is a Glu that must be protonated to act as a general acid. Which statement best explains the catalytic mechanism of the enzyme?
A radiotherapy seed contains $^{125}\text{I}$ , which decays to $^{125}\text{Te}$ primarily via electron capture followed by emission of low-energy photons. The decay constant is $\lambda = 0.0116\ \text{day}^{-1}$ . Based on the decay model, what fraction of the original nuclei is expected to remain undecayed after 60 days?
A lab uses a converging lens to focus a laser beam to a small spot for photothermal therapy experiments. The incoming beam is approximately collimated (object at infinity). The lens has focal length $f=100\,\text{mm}$ . Based on the lens equation as the central concept, where should the sample be placed relative to the lens to be near the focal spot?
In a study of ligand binding to a soluble receptor at $25^\circ\text{C}$ , the measured equilibrium association constant is $K_a = 1.0\times10^7\ \text{M}^{-1}$ in buffer A and decreases to $1.0\times10^6\ \text{M}^{-1}$ in buffer B after adding 200 mM NaCl. Assuming temperature is constant and the receptor–ligand complex is stabilized primarily by electrostatic attraction, which conclusion is most consistent with Debye–Hückel screening of charges in solution?
A group examined leaflet asymmetry by selectively adding a charged amphipathic lipid to the outer leaflet of living cells without immediate flip-flop to the inner leaflet. Within minutes, cells showed increased membrane curvature and budding in regions enriched with the added lipid. Which physical explanation is most consistent with how lipid geometry can influence membrane shape?
In a cell-free assay, a substrate (S) is converted to product (P) by a catalyst C via a rate law empirically found to be $\text{rate}=k[S][C]$ . A technician accidentally halves [S] while doubling [C], keeping temperature constant. Which factor most influences the observed initial rate under these changes?
A researcher synthesizes a short oligonucleotide and reports its backbone as "5′-phosphate terminated" on one end and "3′-hydroxyl terminated" on the other. The core concept is nucleic acid directionality and functional group identity at strand termini. Which statement is most consistent with this description? Assume standard 3′→5′ phosphodiester linkages and no terminal modifications other than those stated.
A patient receives a therapeutic radionuclide that decays by alpha emission. The clinician notes that alpha particles have high linear energy transfer and short range in tissue. Which statement best describes what must be true about the nuclear change in alpha decay?
A sealed container is partially filled with a liquid and contains a small floating plastic bead. The container is taken from sea level to a high-altitude lab, where ambient pressure is lower. The liquid density is effectively unchanged, but the bead is slightly compressible and expands, increasing its external volume while its mass remains constant. Based on Archimedes' Principle, which outcome would be expected given the described changes?
A researcher compared migration of a glycosylated protein (G, 55 kDa, pI 6.0) and its deglycosylated form (DG, 48 kDa, pI 6.0) by SDS-PAGE at pH 8.3, 170 V. DG migrated farther than G. Which factor most directly explains the observed difference in SDS-PAGE migration?
A researcher compared two conditions in isolated mitochondria with identical substrates and O2: (i) high ADP, (ii) no ADP. In (i), oxygen consumption was high and NADH levels were low; in (ii), oxygen consumption was low and NADH levels were high. Which statement best reflects the redox process described?
A thin transparent film (refractive index $n=1.50$ ) coats a glass slide. A laser beam in air strikes the film at $30^\circ$ to the normal and enters the film. Which prediction about the light in the film is most consistent with electromagnetic wave behavior? (Assume no absorption.) Constants: $c = 3.0\times10^8\ \text{m/s}$ .
A defibrillator training device charges a capacitor through a resistor. The charging circuit uses $R_1=2\,\text{k}\Omega$ in series with $R_2=2\,\text{k}\Omega$ (no capacitors in this comparison). If $R_2$ is replaced by a wire (short circuit), which outcome would be expected for the total resistance seen by the battery?
A synthetic protocol generates sulfur dioxide (SO $_2$ ) as a byproduct, and the lab monitors its reactivity toward nucleophiles. SO $_2$ has 18 valence electrons total (S = 6, each O = 6) and is commonly represented by resonance structures with one S=O and one S–O bond in each contributor. Based on the Lewis structure and electron-domain counting around sulfur, what prediction can be made about the molecular shape of SO $_2$ ? (Assume VSEPR applies.)
A lab tests the hypothesis that increasing ionic strength decreases electrostatic attraction between a cationic dye (+1) and a negatively charged polymer surface. The polymer is incubated with dye at 25°C in buffers of identical pH but different NaCl concentrations. After equilibration, the fraction of dye bound to the polymer is measured. Given: higher ionic strength increases charge screening; no covalent bonding occurs. Which result is most consistent with the hypothesis?
A UV–Vis spectrum of a ligand shows $\lambda_{\max}=350\ \text{nm}$ . After binding to a metal center, the complex shows a new strong band at 450 nm while the 350 nm band decreases. Which conclusion is most consistent with molecular absorption?
A spectroscopist analyzes an emission series from a hydrogen-like ion in a calibration lamp. They state that the electron's energy levels are quantized and that emitted photon wavelengths correspond to discrete energy differences rather than a continuum. Which principle best explains the observed discrete spectral lines?

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