Study guide
This chapter is educational content only, not medical advice, and outcomes on any exam depend on many factors beyond any single resource. It follows the AAMC content outline's emphasis on biology and biochemistry at the molecular, cellular, and organismal level, again largely delivered through data- and experiment-based passages that require applying foundational principles to an unfamiliar system.
Passage: An Enzyme Kinetics and Cellular Respiration Investigation
A student researcher, Devan Okafor, studies isolated mitochondria from rat liver cells to investigate cellular respiration. He first measures oxygen consumption (a proxy for electron transport chain activity) in three conditions: (1) mitochondria plus glucose alone, (2) mitochondria plus pyruvate plus ADP and inorganic phosphate, and (3) mitochondria plus pyruvate plus ADP with cyanide added. In condition 1, oxygen consumption stays near baseline, since glucose itself cannot enter mitochondria or be used directly by the electron transport chain; it must first be converted to pyruvate via glycolysis in the cytosol, then transported into the mitochondrial matrix. In condition 2, oxygen consumption rises sharply: pyruvate is converted to acetyl-CoA by pyruvate dehydrogenase, acetyl-CoA enters the citric acid cycle, and the NADH and FADH2 generated donate electrons to the electron transport chain, which uses O2 as the final electron acceptor while pumping protons to build the electrochemical gradient that ATP synthase uses to generate ATP (oxidative phosphorylation). In condition 3, oxygen consumption drops to near zero, because cyanide blocks Complex IV (cytochrome c oxidase), preventing electron transfer to oxygen and backing up the entire chain. Devan then measures the initial rate of the citric acid cycle enzyme succinate dehydrogenase under increasing concentrations of malonate, a molecule structurally similar to succinate. He finds that malonate raises the apparent Km of the reaction without changing Vmax when substrate is in large excess, and that this effect is reduced by adding more succinate, consistent with competitive inhibition at the enzyme's active site. Finally, he separates the same cell lysate on an agarose gel after treating one sample with a restriction enzyme that cuts at a specific palindromic DNA sequence, producing fragments he visualizes by their differential migration, with smaller fragments traveling farther through the gel matrix under an applied electric field.
Questions 1-5 (Passage-Based)
1. Based on the passage, why does oxygen consumption remain at baseline when only glucose is added (condition 1)? (A) glucose is toxic to mitochondria (B) glucose cannot be used directly by the electron transport chain and must first be processed to pyruvate in the cytosol (C) mitochondria lack ADP (D) glucose inhibits Complex IV. The passage states glucose 'cannot enter mitochondria or be used directly' and must be converted to pyruvate first, supporting (B). 2. Cyanide's effect on oxygen consumption (condition 3) occurs because it: (A) speeds up glycolysis (B) blocks Complex IV, preventing electron transfer to the final electron acceptor, oxygen (C) increases ATP synthase activity (D) directly inhibits succinate dehydrogenase. The passage directly states cyanide 'blocks Complex IV (cytochrome c oxidase), preventing electron transfer to oxygen,' matching (B); note this is distinct from the malonate effect on succinate dehydrogenase described separately. 3. The effect of malonate on succinate dehydrogenase, as described, is best classified as: (A) noncompetitive inhibition (B) competitive inhibition (C) allosteric activation (D) irreversible inhibition. The passage states malonate raises apparent Km without changing Vmax and that the effect is 'reduced by adding more succinate,' the defining signature of competitive inhibition, matching (B). 4. In the gel electrophoresis step, smaller DNA fragments migrate farther because: (A) they carry less negative charge overall (B) they experience less resistance moving through the gel matrix under the applied field (C) larger fragments are positively charged (D) smaller fragments dissolve in the buffer. DNA's negatively charged phosphate backbone causes all fragments to migrate toward the positive electrode, but smaller fragments move through the gel's pore structure more easily and thus travel farther in a given time, matching (B). 5. If a mutation eliminated pyruvate dehydrogenase activity entirely, which of the following would most directly result? (A) increased entry of acetyl-CoA into the citric acid cycle (B) impaired conversion of pyruvate to acetyl-CoA, limiting citric acid cycle input from glucose-derived pyruvate (C) increased oxidative phosphorylation (D) no metabolic effect, since glycolysis compensates. Because the passage establishes that pyruvate dehydrogenase converts pyruvate to acetyl-CoA before the citric acid cycle, losing this enzyme would block that entry point, matching (B).
Molecular Biology: DNA, Replication, and Gene Expression
DNA stores genetic information as a sequence of four nucleotide bases (adenine, thymine, guanine, cytosine) arranged in an antiparallel double helix, held together by hydrogen bonds between complementary base pairs (A with T via two hydrogen bonds, G with C via three), which is why G-C-rich DNA has a higher melting temperature. DNA replication is semiconservative: each new double helix retains one original (parental) strand and one newly synthesized strand. Replication requires helicase to unwind the double helix, single-strand binding proteins to keep strands separated, primase to lay down a short RNA primer, and DNA polymerase to extend the new strand by adding nucleotides complementary to the template, always in the 5' to 3' direction; because the two template strands run antiparallel, one new strand (the leading strand) is synthesized continuously while the other (the lagging strand) is synthesized discontinuously in Okazaki fragments, later joined by DNA ligase. Gene expression proceeds from DNA to RNA (transcription) to protein (translation), the molecular biology's central dogma. RNA polymerase transcribes a gene into pre-mRNA, which in eukaryotes is modified by addition of a 5' cap and a poly-A tail and by splicing, in which introns (non-coding sequences) are removed and exons (coding sequences) are joined, sometimes in alternative combinations that let a single gene produce multiple protein products. Translation occurs at the ribosome, where transfer RNA (tRNA) molecules, each charged with a specific amino acid and bearing a three-base anticodon, pair with complementary mRNA codons to add amino acids to a growing polypeptide in the order the genetic code specifies; because the code is degenerate (multiple codons can specify the same amino acid) but unambiguous (each codon specifies only one amino acid), a single point mutation may be silent, missense, or nonsense depending on which codon results and where in the reading frame it falls.
Genetics: Inheritance Patterns and Population Genetics
Classical genetics traces how traits pass between generations. For a single autosomal gene with a dominant allele (A) and recessive allele (a), a cross between two heterozygotes (Aa x Aa) produces offspring in a 1:2:1 genotype ratio (AA:Aa:aa) and, since the dominant phenotype appears whenever at least one A allele is present, a 3:1 phenotype ratio, a pattern easily tracked with a Punnett square. X-linked recessive traits (such as the gene for a hypothetical trait Devan studies in fruit flies) show a distinctive inheritance pattern: because males have only one X chromosome, a single recessive allele on that X produces the recessive phenotype, so X-linked recessive conditions appear far more often in males, while a female typically needs two copies of the recessive allele (one from each parent) to express the trait. Pedigree analysis uses these patterns, plus the presence or absence of male-to-male transmission (impossible for X-linked traits, since fathers pass their Y, not their X, to sons), to infer a trait's mode of inheritance across a family tree. At the population level, the Hardy-Weinberg principle models allele and genotype frequencies in a theoretical population that is not evolving: with allele frequencies p (dominant allele) and q (recessive allele), where p + q = 1, genotype frequencies follow p^2 (homozygous dominant) + 2pq (heterozygous) + q^2 (homozygous recessive) = 1. Deviation from Hardy-Weinberg proportions signals that one of the model's assumptions is violated in a real population: natural selection, non-random mating, genetic drift (random allele frequency changes, pronounced in small populations), mutation, or migration (gene flow) between populations. Recognizing which force is most plausible given a described scenario, such as a small founder population or a trait under strong selective pressure, is a frequently tested application of this framework.
Biochemistry Fundamentals: Proteins, Enzymes, and Metabolism
Proteins are built from twenty standard amino acids joined by peptide bonds, formed through a dehydration (condensation) reaction between the carboxyl group of one amino acid and the amino group of the next. A protein's function depends on its three-dimensional structure, organized in a hierarchy: primary structure is the linear amino acid sequence; secondary structure describes local folding patterns (the alpha helix and beta sheet) stabilized by hydrogen bonds along the polypeptide backbone; tertiary structure is the overall three-dimensional shape of a single polypeptide, stabilized by interactions among side chains (hydrophobic packing away from water, hydrogen bonds, ionic bonds, and disulfide bonds between cysteine residues); and quaternary structure describes the arrangement of multiple polypeptide subunits into a functional complex, as in hemoglobin's four subunits. Enzymes are biological catalysts, almost always proteins, that accelerate reactions by stabilizing the transition state and lowering activation energy without being consumed themselves or altering the reaction's equilibrium; the induced-fit model describes how an enzyme's active site changes shape slightly upon substrate binding to maximize the interactions that stabilize the transition state. Enzyme activity is regulated in several ways relevant to metabolism as a whole: allosteric regulation, in which a molecule binds a site other than the active site to change enzyme shape and activity (often the basis of feedback inhibition, where a pathway's end product inhibits an early enzyme in its own synthesis); covalent modification, such as phosphorylation by a kinase; and changes in enzyme concentration through altered gene expression. Metabolism is organized around coupled reactions: catabolic pathways (such as glycolysis and the citric acid cycle, described in the passage above) break down molecules and release energy, much of it captured in ATP or reduced electron carriers (NADH, FADH2), while anabolic pathways consume energy to build complex molecules, and the cell balances these opposing demands continuously based on energy charge and substrate availability.
Key terms
- Oxidative phosphorylation
- — ATP synthesis driven by the proton gradient the electron transport chain builds across the inner mitochondrial membrane, using O2 as the final electron acceptor.
- Competitive inhibition
- — Enzyme inhibition in which a substrate-like molecule binds the active site, raising apparent Km while Vmax is unchanged and substrate excess can restore activity.
- Semiconservative replication
- — DNA replication producing daughter helices each containing one original parental strand and one newly synthesized strand.
- Okazaki fragments
- — Short DNA segments synthesized discontinuously on the lagging strand during replication, later joined by DNA ligase.
- Splicing
- — Removal of introns and joining of exons from pre-mRNA in eukaryotes, sometimes alternatively to produce multiple protein products from one gene.
- Codon degeneracy
- — The property that multiple three-base mRNA codons can specify the same amino acid, while each individual codon remains unambiguous.
- Hardy-Weinberg principle
- — A model, p^2 + 2pq + q^2 = 1, predicting stable allele and genotype frequencies in a population absent evolutionary forces.
- Genetic drift
- — Random change in allele frequency across generations, with disproportionately large effects in small populations.
- Tertiary structure
- — A single polypeptide's overall three-dimensional folded shape, stabilized by side-chain interactions including hydrophobic packing and disulfide bonds.
- Allosteric regulation
- — Enzyme regulation in which a molecule binds a site other than the active site, changing enzyme shape and activity, often underlying feedback inhibition.
- Feedback inhibition
- — A regulatory pattern in which a pathway's end product inhibits an earlier enzyme in the same pathway, limiting further product formation.
- Induced fit
- — A model of enzyme-substrate binding in which the active site's shape adjusts upon substrate binding to better stabilize the transition state.
Exam tips
- In a metabolism passage, trace exactly which molecule is added or blocked at each step before answering — many questions hinge on knowing the specific point in the pathway a manipulation affects.
- Distinguish competitive inhibition (Km up, Vmax unchanged, reversible by excess substrate) from noncompetitive inhibition (Vmax down, Km unchanged) by the pattern described, not by the inhibitor's name alone.
- For inheritance questions, first determine the pattern (autosomal vs. X-linked, dominant vs. recessive) from the data given, such as which sex is affected more often, before building a Punnett square or pedigree.
- In Hardy-Weinberg problems, solve for q (or q^2) from the given data first, since p + q = 1 and p^2 + 2pq + q^2 = 1 let every other frequency be derived from that one value.
- When a passage introduces an unfamiliar enzyme or organism, apply general principles (kinetics, central dogma, structure-function logic) rather than searching memory for the specific named system, since MCAT passages are frequently built around invented or simplified examples.