Practice Test 10 - MCAT Chemical and Physical Foundations of Biological Systems

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

A $2.0\,\text{kg}$ block is on a horizontal surface with static friction. A horizontal force is slowly increased from $0$ upward. The block remains at rest until the applied force reaches $10\,\text{N}$ , at which point it begins to slip. Forces on the block include $mg$ , $N$ , applied force, and friction. Based on Newton's Laws, which statement is most consistent just before slipping occurs? Assume the threshold is reached at $10\,\text{N}$ .
A lab used SDS-PAGE to estimate sizes of four secreted proteins at pH 8.3. All samples were fully denatured and SDS-coated. At 110 V for 30 min, the migration distances were: Protein E: 38 mm Protein F: 26 mm Protein G: 17 mm Protein H: 10 mm Which protein is most likely the smallest by molecular weight?
A student measures the RC time constant in a drug-delivery pump controller. Initially, $R=100\,\text{k}\Omega$ and $C=10\,\mu\text{F}$ are in series. The student then adds a second $100\,\text{k}\Omega$ resistor in parallel with the first (capacitance unchanged). Which outcome would be expected for the time constant $\tau$ ?
A microelectrode experiment uses a galvanic cell with H $_2$ (g, 1 atm)|H $^+$ (1.0 M) and Fe $^{3+}$ (1.0 M)/Fe $^{2+}$ (1.0 M) coupled via inert Pt electrodes. Standard reduction potentials: 2H $^+$ + 2e $^-$ $\rightarrow$ H $_2$ , $E^\circ=0.00$ V; Fe $^{3+}$ + e $^-$ $\rightarrow$ Fe $^{2+}$ , $E^\circ=+0.77$ V. Which outcome would be expected?
Researchers compare two reversible enzyme inhibitors that bind the same active site. Inhibitor X has $K_d = 1\,\mu$ M and inhibitor Y has $K_d = 10\,\mu$ M at 25°C. After rapid mixing, X reaches binding equilibrium within 5 ms, whereas Y requires 500 ms. Assume the enzyme concentration is far below inhibitor concentration and that binding is a single-step process $E + I \rightleftharpoons EI$ . Which statement best describes the relationship between binding affinity (thermodynamics, via $K_d$ ) and binding speed (kinetics) in this experiment?
A research group compares EAS outcomes on two fused heteroaromatics under identical nitration conditions ( $\ce{HNO3/H2SO4}$ , 25°C): indole and quinoline. The central concept is relative resonance stabilization of the σ-complex formed during electrophile addition. For indole, electrophilic attack at C3 preserves aromaticity in the benzene ring while delocalizing charge in the five-membered ring. For quinoline, the pyridine-like ring is deactivated toward EAS. Which statement is most consistent with these principles? Assume substitution occurs at the position leading to the most stabilized σ-complex.
A biologist anesthetizes a fish and observes that it begins to sink slowly. The fish then increases the volume of gas in its swim bladder with negligible change in mass. Which outcome is expected based on Archimedes' Principle?
To probe membrane fluidity, researchers measured lateral diffusion of a fluorescent lipid in a model bilayer using FRAP. Diffusion coefficient $D$ was measured at different temperatures. Table: Temperature vs diffusion $T$ (°C): 10, 20, 30, 40, 50 $D$ ( $\mu$ m $^2$ /s): 0.12, 0.21, 0.36, 0.58, 0.90 Which trend in the data is most consistent with the underlying physical principle?
A rigid, sealed vessel contains an ideal gas at 2.0 atm and 400 K. The vessel is cooled to 200 K without changing the amount of gas. The question targets Gay-Lussac's law. Which pressure is most consistent after cooling? A. 4.0 atm B. 2.0 atm C. 1.0 atm D. 0.50 atm
To interpret IR data from a respiratory gas mixture, a researcher compares CO2 and SO2, both detected in trace amounts. Using VSEPR theory, which statement is most consistent with their molecular geometries and the presence or absence of a net dipole moment?
A $2.0\ \text{kg}$ box of lab supplies slides down a rough ramp from a vertical height of $1.5\ \text{m}$ and reaches the bottom with speed $4.0\ \text{m/s}$ . Use $g=9.8\ \text{m/s}^2$ . Which outcome is most consistent with conservation of energy for this non-conservative system?
In an electrophysiology rig, a parallel-plate capacitor ( $C=2.0\ \mu\text{F}$ ) is charged to $V_0=6.0\ \text{V}$ and then disconnected from the battery (open circuit). A technician inserts a dielectric slab fully between the plates, increasing the capacitance to $C'=6.0\ \mu\text{F}$ without providing any conductive path between plates. Which outcome is most consistent with changes in electric potential in this system? (Use $Q=CV$ .)
A biomechanics lab tracks a runner's center of mass along a straight hallway. At $t=0$ s the runner is moving at $+6\ \text{m/s}$ . For the next 2 s, the runner maintains a constant velocity (no net horizontal force). At $t=2$ s, the runner suddenly begins decelerating at a constant $-2\ \text{m/s}^2$ for 1 s. Based on the scenario, which prediction aligns with the principle that acceleration is the time rate of change of velocity?
A research group models the geometry of nitrite (NO $_2^-$ ) in aqueous solution to predict its interactions with metal centers. The Lewis structure that minimizes formal charges places N in the center with one N=O double bond, one N–O single bond, and one lone pair on N; two equivalent resonance forms exist. Based on the Lewis structure, what prediction can be made regarding the N–O bond lengths in NO $_2^-$ ?
A DNA sample was treated with a reagent that specifically modifies cytosine (C) bases, preventing them from forming normal hydrogen bonds. After treatment, duplex formation with a complementary strand was impaired. Which statement is most consistent with the nucleotide structure described? Assume: C pairs with G via specific hydrogen-bond donors/acceptors; disrupting these interactions reduces duplex stability.
A researcher studies resonance in the external auditory canal by modeling it as a tube that is open at the entrance and effectively closed at the eardrum. A tone of adjustable frequency is played near the ear, and sound intensity at the eardrum is monitored. Assume the speed of sound in air is $v = 340\ \text{m/s}$ and the canal length is $L = 2.5\ \text{cm}$ . Which frequency is most consistent with the lowest resonant mode of this open-closed tube?
A soluble human hydrolase (E) catalyzes cleavage of an ester substrate (S) in buffered aqueous solution at 37°C. Initial rates were measured at varying $[S]$ with and without 10 µM inhibitor X. The data are summarized: without X, $V_{\max}=120$ nM·s $^{-1}$ and $K_m=15$ µM; with X, $V_{\max}=118$ nM·s $^{-1}$ and apparent $K_m=60$ µM. Assume enzyme concentration is unchanged and product inhibition is negligible. Based on the vignette, which outcome is most consistent with the presence of a competitive inhibitor? Constants: none needed beyond values provided.
In a medicinal chemistry study modeling lysine side-chain modification, a buffered aqueous solution (pH 5.0, 25°C) contains acetone ( $\ce{(CH3)2C=O}$ , 10 mM) and methylamine ( $\ce{CH3NH2}$ , 10 mM). The mixture is treated with a mild reducing agent, sodium cyanoborohydride ( $\ce{NaBH3CN}$ , 20 mM), and stirred for 1 hour. No strong acid is added; pH is maintained at 5.0 throughout. The central carbonyl reactivity concept is nucleophilic addition to form an iminium intermediate followed by selective reduction (reductive amination). Which product is most likely formed in highest yield under these conditions? Assume $\ce{NaBH3CN}$ selectively reduces iminium ions over unprotonated ketones at pH 5, and acetone does not undergo self-aldol under these conditions.
In an experiment modeling oxygen delivery, human hemoglobin (Hb) solutions were equilibrated at 37°C with different concentrations of 2,3-BPG while keeping total Hb constant. The $P_{50}$ (partial pressure of O $_2$ at 50% saturation) was measured. Data (2,3-BPG vs $P_{50}$ ): - 0.0 mM → 22 mmHg - 2.0 mM → 26 mmHg - 4.0 mM → 31 mmHg - 6.0 mM → 38 mmHg Which conclusion is most consistent with the data presented?
A materials test relevant to orthopedic implants measures the stress required to produce 1% strain in a polymer scaffold after different UV crosslinking times. UV time (min) vs Stress at 1% strain (MPa): - 0 → 0.9 - 2 → 1.4 - 4 → 2.1 - 6 → 2.8 - 8 → 3.2 Which conclusion can be drawn from the data?

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