Practice Test 1 - MCAT Chemical and Physical Foundations of Biological Systems

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

Researchers studied oxygen binding to hemoglobin (Hb) in a sealed tonometer at 37°C. Hb binds oxygen cooperatively: $\theta = \dfrac{(P_{O_2})^n}{(P_{50})^n + (P_{O_2})^n}$ , where $\theta$ is fractional saturation. In one trial, $P_{50}=26\ \text{mmHg}$ and $n=2.8$ . The researchers increased dissolved CO $_2$ (lowering pH) while holding total Hb constant and observed a right shift of the binding curve (higher $P_{50}$ ). Based on Le Châtelier's principle applied to the equilibrium $\text{Hb} + \text{O}_2 \rightleftharpoons \text{HbO}_2$ coupled to proton binding, which outcome is most consistent with the shift at a fixed $P_{O_2}=40\ \text{mmHg}$ ?
An electrophysiology experiment measured the magnitude of ionic current through a ligand-gated channel at different ligand concentrations while holding membrane potential constant. Which statement is most consistent with the pattern in the data?
A lab investigated whether increasing magnetic field strength changes the radius of curvature of an electron beam. Hypothesis: at fixed electron speed, increasing $B$ decreases the circular path radius. Methods: Electrons were accelerated through the same potential difference in both trials. Condition 1: magnetic field $B$ . Condition 2: magnetic field $2B$. The beam radius was measured from a fluorescent screen. Results: The radius was smaller in the $2B$ condition. What conclusion can be drawn about the variable change?
A chemist tests whether adding an inert gas affects equilibrium composition. At 500 K, the reaction $\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)$ is at equilibrium in a sealed rigid container. An inert gas is added, increasing total pressure but not changing temperature or volume. Which prediction is most consistent with equilibrium principles for the equilibrium ratio $[\text{NO}_2]^2/[\text{N}_2\text{O}_4]$ ?
In a cochlear-model experiment, a traveling wave on a flexible membrane is driven at two different frequencies while the membrane tension and linear mass density remain constant. The measured wave speed along the membrane is unchanged between trials. Based on the given conditions, what is most consistent with the observed wave phenomenon regarding wavelength $\lambda$ when the driving frequency is doubled?
In a structural analysis of a 16-nt single-stranded nucleic acid isolated from a viral particle, enzymatic digestion showed the polymer was resistant to RNase A but was degraded by DNase I. Acid hydrolysis released only deoxyribonucleosides. The strand contained 6 guanine, 4 cytosine, 3 adenine, and 3 thymine residues. The core concept tested is base pairing and structural implications of nucleotide composition. Which statement is most consistent with the nucleotide structure described?
A student performs an extraction and mistakenly vents a separatory funnel while the stopper is still in place but the stopcock is closed; pressure builds due to volatile solvent. Based on correct extraction practice, which action is most consistent with safely relieving pressure while maintaining phase separation?
A lab tests a galvanic cell for powering a microfluidic pump: Sn(s)|Sn $^{2+}$ (1.0 M) coupled to Pb $^{2+}$ (1.0 M)|Pb(s). Standard reduction potentials: $E^\circ$ (Sn $^{2+}$ /Sn) = -0.14 V; $E^\circ$ (Pb $^{2+}$ /Pb) = -0.13 V. Which statement best reflects the redox principle illustrated?
A compound is measured by UV–Vis in two solvents. In solvent 1, $\lambda_{\max}=300\ \text{nm}$ ; in solvent 2, $\lambda_{\max}=290\ \text{nm}$ . If solvent 2 is more polar and preferentially stabilizes the ground state more than the excited state, which statement is most consistent with the observed shift?
An enzyme assay uses Tris buffer, modeled as $\mathrm{BH^+ \rightleftharpoons H^+ + B}$ with $pK_a = 8.1$ at the assay temperature. The buffer is prepared at total concentration $[\mathrm{BH^+}]+[\mathrm{B}] = 50\ \mathrm{mM}$ and adjusted to pH 8.1. During the reaction, the enzyme produces 2.0 mmol of $\mathrm{H^+}$ in 1.0 L. Which statement best describes the expected change in buffer composition and pH?
A researcher observes that a hydrogen discharge tube emits a line at 656 nm. When the tube is cooled, the line remains at 656 nm but becomes dimmer. Which explanation is most consistent with the core principle of line spectra?
An enzymology lab evaluates how strongly a metal center polarizes a bound water molecule (promoting deprotonation). They compare Zn2+, Mg2+, Na+, and K+. Which ion is most consistent with the greatest polarizing power based on charge density trends? Assume similar coordination numbers are possible.
In a titration, 25.0 mL of $0.100\ \mathrm{M}$ acetic acid ( $pK_a = 4.76$ ) is titrated with $0.100\ \mathrm{M}$ NaOH. Reaction: $\mathrm{CH_3COOH + OH^- \rightarrow CH_3COO^- + H_2O}$ . A pH electrode records a smooth titration curve. At the half-equivalence point, the solution contains appreciable amounts of both $\mathrm{CH_3COOH}$ and $\mathrm{CH_3COO^-}$ . Based on titration behavior of a weak acid with a strong base, what is most consistent with the pH at the half-equivalence point? (Assume 25°C and ideal behavior.)
A formulation scientist tests reversible covalent inhibition via carbonyl chemistry. A protease is incubated with an electrophilic inhibitor containing either a nitrile (R–C≡N) or an aldehyde (R–CHO). Under identical aqueous conditions (pH 7.4, 37°C), the aldehyde inhibitor shows time-dependent loss of enzyme activity that is partially reversed upon dilution, consistent with a reversible covalent adduct at the active-site serine. The central concept is nucleophilic addition to carbonyls. Which adduct is most consistent with serine attacking the aldehyde?
In a capillary-flow experiment, a Newtonian fluid is driven through a rigid tube at low Reynolds number (laminar). The investigator wants to increase the volumetric flow rate $Q$ without changing the tube geometry ( $r$ and $L$ fixed). Two interventions are considered: 1) Increase the pressure drop from $\Delta P$ to $2\Delta P$ . 2) Add a solute that doubles viscosity from $\eta$ to $2\eta$ . If both interventions are applied simultaneously and the system remains laminar, what is the net effect on $Q$ according to Poiseuille's law $Q \propto \Delta P/\eta$ (with geometry fixed)?
A researcher adds 1.0 mL of 1.0 M HCl to 1.0 L of a buffer containing 0.50 mol HA and 0.50 mol A $^-$ . Assume volume change is negligible. Which statement best describes the expected pH behavior compared with adding the same HCl to pure water?
Researchers prepare a saturated solution of $\text{CaF}_2$ at 25°C: $\text{CaF}_2(s) \rightleftharpoons \text{Ca}^{2+} + 2\text{F}^-$ . They use $K_{sp}=3.9\times10^{-11}$ . After equilibrium is established, they add NaF to raise $[\text{F}^-]$ (from the salt) without changing volume appreciably. Which shift in equilibrium is most consistent with adding the common ion $\text{F}^-$ ?
A patient receives an IV infusion through a narrow catheter. The pressure at a point in the flowing fluid is related to height by hydrostatics: $P = P_0 + \rho g h$ (for a static column). If the IV bag is raised higher above the patient's vein while fluid density is unchanged, which result is most consistent with the relation as an approximation for driving pressure?
A physiology study used native gel electrophoresis (pH 8.3 buffer) to compare a patient's serum proteins before and after acute dehydration. The gel was run at 100 V for 30 min with wells near the cathode (−). Albumin (pI 4.7) and a basic peptide hormone carrier protein (pI 9.6) were monitored by immunostaining. Densitometry indicated both proteins were present at similar concentrations in both samples, but their migration distances differed. Observed migration distance toward the anode (cm): - Albumin: 4.5 (baseline), 4.5 (dehydration) - Carrier protein: 0.2 (baseline), 0.2 (dehydration) Which conclusion is most consistent with these electrophoresis results?
In a focused experiment on action potential propagation, a single myelinated motor neuron is stimulated at the axon hillock. Resting membrane potential is $V_m=-70\ \text{mV}$ and threshold is $-55\ \text{mV}$ . Voltage-gated Na $^+$ channels open rapidly when threshold is reached, and voltage-gated K $^+$ channels open with a delay to repolarize the membrane. The axon is partially demyelinated over a 2-mm segment, increasing membrane capacitance in that segment from $C_m=1.0\ \mu\text{F}/\text{cm}^2$ to $2.5\ \mu\text{F}/\text{cm}^2$ while leaving ion channel densities unchanged. Assume intracellular and extracellular ion concentrations remain constant during a single spike and that the neuron is otherwise healthy. Which outcome is most consistent with the effect of the demyelinated segment on action potential propagation along the axon?

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