Practice Test 9 - MCAT Chemical and Physical Foundations of Biological Systems

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

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1. A protein is stabilized by a network of hydrogen bonds within a $β$-sheet. A mutation replaces an Asn side chain that hydrogen-bonds to a backbone carbonyl with Leu (N;2L) at a solvent-exposed position. Which is the most likely local effect on stability?
2. A research note describes bromination of anisole-like heteroaromatics using Br$_2$/FeBr$_3$ at room temperature. The substrate is 2-methylthiophene (thiophene ring with CH$_3$ at C2). The authors report predominant monobromination at a single ring position. Using EAS reasoning (formation of a $sigma$-complex and resonance stabilization), which site is most likely brominated? Numbering: thiophene S = position 1; adjacent carbons are C2 and C5; next are C3 and C4. Substituent CH$_3$ is at C2.
3. A contaminated environmental sample contains $^{137}\text{Cs}$, which decays by $\beta^-$ emission to $^{137m}\text{Ba}$, followed by gamma emission to stable $^{137}\text{Ba}$. The half-life of $^{137}\text{Cs}$ is 30.0 years. Based on the decay model, which nuclear change occurs during the initial $\beta^-$ decay step?
4. A researcher titrates 50.0 mL of 0.10 M benzoic acid (HBz) with 0.10 M NaOH at 25°C to assess buffer range for a topical formulation. For $\mathrm{HBz} \rightleftharpoons \mathrm{H^+} + \mathrm{Bz^-}$, $pK_a = 4.2$. The formulation is intended to keep pH within a range where buffering is effective. Based on the passage, which conclusion about the buffer system is most consistent with effective buffering during titration?
5. An enzyme active site contains a histidine residue that acts as a base by accepting a proton. A mutation replaces histidine with phenylalanine at the same position. Which prediction is most consistent with Brønsted–Lowry acid–base theory regarding catalytic proton transfer at that position?
6. A sealed vial contains $N_0$ nuclei of a medical tracer that decays with half-life 4.0 h. No tracer is added or removed. Based on the decay model, what outcome is most likely after 12 h?
7. A structural analysis of a purified polysaccharide uses IR spectroscopy. The sample shows a broad O–H stretch near 3300 cm$^{-1}$ and strong C–O stretches in the 1000–1150 cm$^{-1}$ region. After chemical acetylation of free hydroxyl groups (converting –OH to –O–C(=O)CH$_3$), the 3300 cm$^{-1}$ band decreases substantially and a new strong absorption appears near 1740 cm$^{-1}$. Which conclusion is most consistent with these spectral changes? Reference: Ester carbonyl (C=O) stretches typically appear near 1735–1750 cm$^{-1}$.
8. A physical therapist applies a constant horizontal force to slide a patient's foot $0.30\ \text{m}$ across a surface. The force is increased, but the displacement is unchanged. Principle tested: work by a constant force. Which prediction is most consistent with how the work done by the therapist changes (assuming force remains parallel to displacement)?
9. A surgical headlamp uses a concave mirror to produce a bright spot at the surgical field. The mirror's focal length is $f = 4.0\ \text{cm}$. The lamp filament is mounted $d_o = 3.0\ \text{cm}$ in front of the mirror (closer than the focal length). Based on spherical mirror image formation, which outcome is most consistent with the described optics principle?
10. A rigid beam is supported by two vertical supports at its ends. Two downward loads are applied at different positions along the beam. The system is observed to have no angular acceleration (rotationally stable) but one support force is larger than the other. Which statement is most consistent with equilibrium principles for this setup?
11. A lab tested the effect of voltage on band resolution in SDS-PAGE (10% gel) for two proteins: A (50 kDa) and B (55 kDa). Samples were denatured with SDS and run in identical buffer at either 80 V for 60 min or 200 V for 24 min (same approximate run length). Band separation (distance between band centers) and band broadening were measured. Results: - 80 V: separation 0.35 cm; average band width 0.18 cm - 200 V: separation 0.30 cm; average band width 0.34 cm Based on the data, which factor most likely reduced separation efficiency at 200 V?
12. A researcher uses a simple circuit to model two parallel ion-channel pathways. Two resistors ($R_1=4\,\text{k}\Omega$, $R_2=12\,\text{k}\Omega$) are connected in parallel across a fixed voltage source. Which statement is most consistent with the current through $R_1$ compared with the current through $R_2$?
13. A mitochondrial preparation was provided with succinate as the only respiratory substrate (Complex II entry) and ADP + Pi. Oxygen consumption was robust. Rotenone (Complex I inhibitor) was then added; oxygen consumption changed minimally. When antimycin A (Complex III inhibitor) was added afterward, oxygen consumption rapidly fell. Which statement best reflects the redox process described?
14. An analytical lab measures optical rotation of two samples of Molecule A, a chiral alcohol with one stereocenter. Sample 1 gives $[\alpha]_D^{20} = +12.0^\circ$ (in ethanol), while Sample 2 gives $[\alpha]_D^{20} = -12.0^\circ$ under identical conditions and concentration. No other impurities are detected by NMR. Which configuration is consistent with the observed optical activity?
15. A roller system transports a lab sample container down a $2.0\ \text{m}$ vertical drop. The container starts from rest and reaches the bottom with speed lower than predicted by a frictionless model. Principle tested: conservation of energy with dissipative forces. Which statement is most consistent with the energy difference?
16. A lab compares two electrode models. Model 1 uses a single capacitor $C=200\ \text{nF}$. Model 2 uses two identical capacitors in series (each $200\ \text{nF}$). Both are charged to the same voltage. Which statement best describes Model 2 relative to Model 1?
17. A DNA sample is treated with DNase I, which preferentially cleaves phosphodiester bonds in double-stranded DNA. After treatment, electrophoresis shows a smear of shorter fragments rather than discrete bands. Which conclusion about nucleic acid function is best supported by this pattern, given the enzymes substrate specificity?
18. An ion-exchange resin in the H+ form is used to acidify a protein sample by exchanging cations. The resin contains fixed sulfonate groups (R–SO3−) with mobile counterions. When a solution containing 10 mM NaCl is passed through, the exchange is represented as: $\mathrm{R{-}SO_3^-H^+ + Na^+ \rightleftharpoons R{-}SO_3^-Na^+ + H^+}$. Which outcome would be expected from this ion interaction in the effluent (the liquid leaving the column)?
19. To model osmotic fragility, red blood cells were placed in NaCl solutions of varying concentration, and percent hemolysis was recorded after 5 minutes. [NaCl] (% w/v) vs Hemolysis (%): - 0.90 → 0 - 0.70 → 5 - 0.50 → 30 - 0.30 → 75 - 0.10 → 98 Based on the data, what conclusion can be drawn?
20. A mixture contains a volatile solvent and a nonvolatile solute (e.g., glucose in water). During simple distillation, which statement is most consistent with what happens to the solute concentration in the boiling flask as distillation proceeds?

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