Practice Test 11 - 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 study tested whether a detergent increases membrane permeability. Identical liposomes containing a fluorescent dye were split into two conditions: buffer only vs buffer + detergent. Fluorescence in the surrounding solution increased only in the detergent condition. What conclusion can be drawn about the variable change?
A microscopy add-on uses a thin diverging lens to expand a beam before it enters the objective. The lens has focal length $f = -5.0\ \text{cm}$ . A small aperture (object) is placed $d_o = 10\ \text{cm}$ in front of the diverging lens. Which outcome is most consistent with image formation by a diverging lens?
In a study of CO $_2$ -dependent vasodilation, dissolved CO $_2$ in plasma equilibrates with bicarbonate via carbonic anhydrase. At physiological pH, the dominant inorganic carbon species is HCO $_3^-$ . For the carbon atom in HCO $_3^-$ , which hybridization state is most consistent with the observed planarity of the O–C–O fragment and rapid resonance delocalization in IR spectra?
A lab uses a magnetic-sector mass spectrometer in which ions of the same speed are separated by their radius of curvature in a magnetic field. For an ion of charge $q$ moving at speed $v$ in a magnetic field $B$ , the radius is $r=\frac{mv}{qB}$ . Two singly charged ions are accelerated to the same speed and enter the same magnetic field. Ion X has $m/z=40$ and Ion Y has $m/z=20$ . The principle involved is that larger $m/z$ yields a larger radius under these conditions. Which outcome is most consistent? (Constants: $e=1.60\times10^{-19}\ \text{C}$ .)
A lab uses the same capillary (fixed $r$ and $L$ ) to measure flow of a Newtonian fluid at two temperatures. At $20^\circ\text{C}$ , the viscosity is $\eta_{20}$ and the measured flow rate is $Q_{20}$ . At $37^\circ\text{C}$ , the viscosity decreases to $0.75\eta_{20}$ while the applied pressure drop $\Delta P$ is kept constant and the flow remains laminar. Based on Poiseuille flow, what is the expected $Q_{37}$ relative to $Q_{20}$ ?
A study team recruits participants for a new drug trial from a clinic where the PI is also the treating physician. Patients are told, "I think this trial is your best option," and the appointment ends with a consent form placed in front of them. The team argues that the physician's recommendation increases enrollment and helps patients access innovative care. Which action best aligns with ethical research practices regarding informed consent?
A single neuron is stimulated twice in rapid succession with identical current pulses at the axon hillock. The first pulse triggers an action potential. The second pulse is delivered 1 ms later, during the early repolarization phase when many voltage-gated Na $^+$ channels are inactivated and voltage-gated K $^+$ channels are open. Which observation is most consistent with the neuron's refractory behavior? Parameters: second stimulus at 1 ms; Na $^+$ channels inactivated; K $^+$ conductance elevated.
A mixture of three small molecules is analyzed by TLC on silica using 70:30 hexanes:ethyl acetate. The observed $R_f$ values are: A = 0.10, B = 0.35, C = 0.70. Which compound is most polar under these conditions? Assume all spots are from single compounds and none ionize.
A ball is thrown straight upward in a physiology lab to calibrate a motion-capture system. Take upward as positive. At the instant the ball reaches its highest point, air resistance is negligible and gravitational acceleration is constant at $a=-9.8\ \text{m/s}^2$ . Which statement is most consistent with the motion variables at the highest point?
A clinician uses Doppler ultrasound to estimate whether blood is flowing toward or away from the probe. The device displays a positive frequency shift for one vessel segment. Without changing any settings, the clinician rotates the probe 180° so it points in the opposite direction along the same vessel segment. Based on the scenario, which frequency shift would be expected?
A chiral drug is resolved on a chiral HPLC column. The isolated fraction is reported as 90% ee of the active enantiomer. In a receptor-binding assay, only the active enantiomer contributes to binding; the inactive enantiomer has negligible affinity. Based on the technique used and the meaning of ee, which conclusion is most consistent with the biological implication?
In a redox-linked enzymatic assay, a mediator couple is represented by the half-reaction $\text{Ox} + e^- \rightarrow \text{Red}$ with $E^\circ=+0.10\,\text{V}$ . The assay is paired in a galvanic cell with the O $_2$ /H $_2$ O couple (acidic conditions): O $_2$ + 4H $^+$ + 4e $^-$ $\rightarrow$ 2H $_2$ O, $E^\circ=+1.23\,\text{V}$ . All activities are ~1. Which outcome would be expected in this electrochemical cell?
A materials group tests whether a polymer's glass transition temperature ( $T_g$ ) changes when a plasticizer is added. The hypothesis: adding plasticizer lowers $T_g$ by increasing chain mobility. They prepare two polymer samples with identical mass and thermal history; the only variable changed is plasticizer content (0% vs 10% by mass). Using differential scanning calorimetry, they observe that the 10% plasticizer sample shows the transition at a lower temperature. What conclusion can be drawn about the variable change?
A racemic alcohol is analyzed by chiral HPLC. The analyst reports two peaks with identical retention times when the chiral column is replaced with an otherwise similar but achiral column. Based on the technique used, which conclusion is most consistent with the results?
A virology lab studies viral entry mediated by binding to host cell-surface glycoconjugates. In untreated cells, viral attachment is high. When cells are pretreated with a heparinase that cleaves heparan sulfate glycosaminoglycans (GAGs), viral attachment decreases markedly. However, when cells are pretreated with a protease under mild conditions that removes many exposed protein domains but leaves the plasma membrane intact, viral attachment decreases only modestly. Which host feature is most likely acting as the primary viral attachment factor in this system?
A biochemist studies aspirin (an ester) hydrolysis in gastric fluid (pH 2). The proposed mechanism includes formation of a tetrahedral intermediate and collapse to expel a leaving group. Which leaving group is most consistent with acid-catalyzed hydrolysis of an ester in water?
A sample of elemental chlorine from a physiological saline preparation shows two mass spectrometry peaks corresponding to $^{35}\text{Cl}$ and $^{37}\text{Cl}$ . The average atomic mass of chlorine in the sample is measured as 35.45 amu. Using $m(^{35}\text{Cl})=34.97$ amu and $m(^{37}\text{Cl})=36.97$ amu, which isotopic abundance of $^{37}\text{Cl}$ is most consistent with the measurement?
A 20-residue peptide is synthesized with the sequence Ac-Ala-Lys-Gly-Asp-Ser-Leu-Val-Phe-Gly-Lys-Leu-Ala-Asp-Gly-Ser-Val-Leu-Lys-NH $_2$ . The peptide is designed to be net neutral near physiological pH due to N-terminal acetylation and C-terminal amidation. At pH 7.4, which alteration to the amino acid sequence would most likely increase the peptides net positive charge?
A medicinal chemistry group uses UV-Vis to compare two small aromatic drugs. Drug 1 shows a strong absorption attributed to a $\pi\to\pi^*$ transition; Drug 2 shows a weaker absorption attributed to an $n\to\pi^*$ transition from a heteroatom lone pair. Both transitions occur within the same molecule without ionization. Which principle best explains the observed electron behavior (stronger vs weaker absorption)?
A researcher performed native PAGE at pH 7.0 on a protein with pI 9.0 (Protein T, 45 kDa). The sample was loaded near the cathode (−). After 30 min at 120 V, the band was faint and remained close to the well, while a negatively charged marker migrated toward the anode (+). Which modification would most likely increase the migration distance of Protein T in the same direction as the marker without changing its mass substantially?

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