MCAT B/B Study Guide — Biological and Biochemical Foundations of Living Systems

Section: Biological and Biochemical Foundations of Living Systems (B/B) Coverage: Foundational Concepts 1–3, content categories 1A–3B Status: Draft awaiting SME review Note on use: This guide is original instructional prose grounded in standard introductory biology and first-semester biochemistry. It emphasizes the reasoning the MCAT rewards (application, data interpretation) over isolated recall.

Foundational Concept 1 — Biomolecules: structure, function, and the chemistry of life

Biomolecules have unique properties that determine how they contribute to the structure and function of cells, and how they participate in the processes necessary to maintain life.

1A — Structure and function of proteins and their constituent amino acids

Amino acids. All 20 standard amino acids share a central (alpha) carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable R group (side chain). Glycine (R = H) is the only achiral standard amino acid; the other 19 are L-stereoisomers. Side-chain chemistry drives almost everything the MCAT asks: nonpolar/hydrophobic residues (Ala, Val, Leu, Ile, Met, Phe, Trp, Pro) bury in protein cores; polar uncharged residues (Ser, Thr, Cys, Tyr, Asn, Gln) hydrogen-bond; acidic residues (Asp, Glu) carry negative charge near physiological pH; basic residues (Lys, Arg, His) carry positive charge. Two special cases recur on the exam: proline kinks the backbone and disrupts helices because its side chain loops back to the backbone nitrogen, and cysteine forms disulfide bonds that covalently cross-link tertiary/quaternary structure.

Acid–base behavior. Each ionizable group has a pKa; a group is predominantly protonated when pH < pKa and deprotonated when pH > pKa. The isoelectric point (pI) is the pH at which the molecule carries no net charge, computed as the average of the two pKa values flanking the neutral species. For an amino acid with an ionizable side chain, identify which two pKa values bracket the zwitterion before averaging. At pH above its pI a residue/protein is net negative (migrates toward the anode in electrophoresis); below pI, net positive.

Peptide bonds. Formed by condensation (dehydration) between one residue's carboxyl and the next residue's amino group; hydrolysis reverses this. The peptide bond has partial double-bond character from resonance, making it planar and rigid and favoring the trans configuration.

The four levels of structure. Primary = the amino-acid sequence (peptide bonds). Secondary = local backbone hydrogen-bonding patterns (alpha-helices, beta-sheets). Tertiary = the overall 3-D fold of a single chain, stabilized by hydrophobic interactions (the dominant driving force in aqueous solution), hydrogen bonds, ionic/salt bridges, and disulfide bonds. Quaternary = assembly of multiple polypeptide subunits (e.g., hemoglobin's four chains). Denaturation disrupts secondary/tertiary/quaternary structure (heat, pH extremes, urea, detergents) without breaking peptide bonds; the primary sequence is preserved.

Enzymes. Enzymes are biological catalysts that lower activation energy without changing the equilibrium or the overall ΔG of a reaction. The active site binds substrate; induced fit describes the conformational adjustment of enzyme and substrate upon binding. Classic regulation themes: allosteric regulation (effectors bind a site distinct from the active site, shifting between high- and low-affinity conformations and producing sigmoidal kinetics), covalent modification (e.g., reversible phosphorylation), zymogens (inactive precursors activated by cleavage, e.g., trypsinogen → trypsin), and feedback inhibition (a pathway's product inhibits an upstream enzyme).

Enzyme kinetics (high-yield). Michaelis–Menten kinetics: rate v = (Vmax·[S]) / (Km + [S]). Vmax is the maximal rate at saturating substrate; Km is the substrate concentration at half Vmax and is an inverse proxy for substrate affinity (low Km = high affinity). On a Lineweaver–Burk (double-reciprocal) plot, the x-intercept = −1/Km and the y-intercept = 1/Vmax.

Inhibition type	Binds	Km (apparent)	Vmax	LB plot signature
Competitive	active site (free E)	increases	unchanged	lines meet on y-axis
Noncompetitive	allosteric (E and ES)	unchanged	decreases	lines meet on x-axis
Uncompetitive	only ES complex	decreases	decreases	parallel lines
Mixed	E and ES, unequal affinity	varies	decreases	meet off both axes

How the MCAT tests this: A passage gives a data table of v vs. [S] with and without an inhibitor and asks you to classify the inhibitor. Reason from the table: if adding more substrate fully restores Vmax, the inhibitor competes for the active site (competitive). If Vmax falls no matter how much substrate you add, the inhibitor binds elsewhere (noncompetitive). Don't reach for the formula until you've reasoned about which kinetic parameter changed.

1B — Transmission of genetic information from the gene to the protein

Nucleic acid structure. DNA is an antiparallel double helix; A pairs with T (two H-bonds) and G pairs with C (three H-bonds), so GC-rich regions are more thermally stable (higher melting temperature). RNA is typically single-stranded and uses uracil instead of thymine. The 5'→3' directionality of synthesis is a recurring constraint.

Replication is semiconservative: each daughter duplex has one parental and one new strand. DNA polymerase synthesizes only 5'→3', so the leading strand is continuous and the lagging strand is built in Okazaki fragments later joined by ligase. Proofreading (3'→5' exonuclease) and mismatch/excision repair maintain fidelity.

Transcription and processing. RNA polymerase reads the template strand 3'→5' and builds mRNA 5'→3'; the coding (sense) strand matches the mRNA sequence (with U for T). Eukaryotic pre-mRNA is processed: a 5' cap, a 3' poly-A tail, and splicing that removes introns and joins exons. Alternative splicing lets one gene yield multiple protein products.

Translation. Ribosomes read mRNA codons 5'→3'; tRNAs deliver amino acids via anticodon–codon pairing. The genetic code is degenerate (multiple codons per amino acid, mostly differing at the wobble third position) and nearly universal. AUG is start (Met); UAA/UAG/UGA are stop codons.

Gene regulation. Prokaryotes use operons: the lac operon is inducible (turned on by lactose/allolactose removing the repressor; further tuned by CAP-cAMP when glucose is low), while the trp operon is repressible (tryptophan acts as corepressor). Eukaryotic control is layered across chromatin remodeling, transcription factors and enhancers, RNA processing, and translational/post-translational steps.

1C — Heritable information across generations and the processes that increase diversity

Mendelian genetics. Segregation and independent assortment underlie monohybrid and dihybrid ratios (3:1, 9:3:3:1). Watch for incomplete dominance (blended heterozygote), codominance (both alleles expressed, e.g., AB blood type), sex linkage (X-linked recessive traits affect males more), and pleiotropy/epistasis. Pedigree reasoning is a frequent SIRS task.

Meiosis and variability. Meiosis halves ploidy and generates diversity through crossing over (recombination) in prophase I and independent assortment of homologs in metaphase I. Nondisjunction produces aneuploidy.

Mutations. Point mutations: silent (no amino-acid change, often wobble), missense (one amino-acid change), nonsense (premature stop). Frameshifts from insertions/deletions not divisible by three usually have the largest effect. Mutations in germ cells are heritable; somatic mutations are not.

Evolution and population genetics. Natural selection acts on heritable variation that affects reproductive success (fitness). The Hardy–Weinberg equilibrium (p + q = 1; p² + 2pq + q² = 1) provides a null model; deviation implies evolution is occurring. The five conditions for equilibrium are no mutation, no migration, no selection, random mating, and large population size. Speciation often follows reproductive isolation.

How the MCAT tests this: given an allele frequency or the frequency of affected (homozygous recessive) individuals, you'll back-calculate carrier frequency (2pq) using q² as the starting point.

1D — Bioenergetics and fuel-molecule metabolism

Thermodynamics. Spontaneity is set by ΔG: negative ΔG = exergonic/spontaneous. ΔG° relates to the equilibrium constant (ΔG° = −RT ln Keq); the actual ΔG depends on real concentrations. Cells drive unfavorable reactions by coupling them to ATP hydrolysis (a large negative ΔG).

ATP and phosphoryl transfer. ATP is the cell's energy currency; hydrolysis of its phosphoanhydride bonds is strongly exergonic, enabling group transfers and conformational work.

Central pathways.

Glycolysis (cytosol): glucose → 2 pyruvate, net 2 ATP and 2 NADH; investment phase consumes 2 ATP, payoff phase yields 4. Regulated at phosphofructokinase-1 (PFK-1), the committed step.
Gluconeogenesis reverses glycolysis using bypass enzymes; it is reciprocally regulated with glycolysis (fructose-2,6-bisphosphate is the key signal).
Pyruvate → acetyl-CoA (pyruvate dehydrogenase, mitochondrion) links glycolysis to the citric acid cycle.
Citric acid (Krebs) cycle (mitochondrial matrix): per acetyl-CoA, yields 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂.
Oxidative phosphorylation: NADH and FADH₂ feed electrons into the electron transport chain, pumping protons to build a gradient; ATP synthase uses the proton-motive force to make ATP (chemiosmosis). O₂ is the terminal electron acceptor.

Other fuels. Fatty acids undergo beta-oxidation to acetyl-CoA (high ATP yield per gram); amino acids are deaminated and funneled into the cycle. Hormones coordinate fuel use: insulin promotes storage (glycogenesis, lipogenesis); glucagon and epinephrine promote mobilization (glycogenolysis, gluconeogenesis, lipolysis).

How the MCAT tests this: you may be asked to predict how a poison or uncoupler changes ATP output. An ETC inhibitor stops proton pumping and ATP synthesis; an uncoupler (e.g., a protonophore) dissipates the gradient so electron transport and O₂ consumption continue but ATP synthesis falls and energy is released as heat.

FC1 retrieval prompts

A noncompetitive inhibitor changes which kinetic parameter, and how would that appear on a Lineweaver–Burk plot?

Trace one glucose molecule from cytosolic glucose to ATP via oxidative phosphorylation, naming the NADH/FADH₂ produced at each stage.

If 4% of a population has a recessive disease, what fraction are carriers (assume Hardy–Weinberg)?

Foundational Concept 2 — Organized assemblies: cells, microbes, and cell division

Highly organized assemblies of molecules, cells, and organs interact to carry out the functions of living organisms.

2A — Assemblies of molecules, cells, and groups of cells

Plasma membrane. A fluid-mosaic phospholipid bilayer with embedded proteins; amphipathic phospholipids orient hydrophilic heads outward and hydrophobic tails inward. Cholesterol buffers fluidity. Small nonpolar molecules cross freely; ions and large polar molecules require transport.

Transport. Passive: simple and facilitated diffusion and osmosis move down gradients without ATP. Active: primary active transport (e.g., the Na⁺/K⁺-ATPase pumps 3 Na⁺ out, 2 K⁺ in per ATP) and secondary active transport (symport/antiport powered by an existing gradient). The Na⁺/K⁺-ATPase establishes the gradients underlying the resting membrane potential.

Cytoskeleton and organelles. Microfilaments (actin), intermediate filaments, and microtubules provide shape, transport tracks, and motility. Know organelle functions: nucleus (genetic control), mitochondria (ATP), rough/smooth ER (protein/lipid synthesis), Golgi (modification and sorting), lysosomes (hydrolytic degradation).

Junctions and ECM. Tight junctions seal, desmosomes anchor, gap junctions allow direct cytoplasmic communication. The extracellular matrix (collagen, etc.) provides structural and signaling context.

2B — Prokaryotes and viruses

Prokaryotes. No membrane-bound nucleus or organelles; a single circular chromosome plus plasmids; a cell wall (peptidoglycan in bacteria). Growth follows lag, log/exponential, stationary, and death phases. Genetic exchange occurs via conjugation (direct transfer through a pilus, often plasmid-borne), transformation (uptake of free DNA), and transduction (phage-mediated transfer).

Viruses. Acellular: nucleic acid (DNA or RNA) in a protein capsid, sometimes enveloped; obligate intracellular parasites. The lytic cycle replicates and lyses the host; the lysogenic cycle integrates as a prophage. Retroviruses use reverse transcriptase to make DNA from an RNA genome.

2C — Cell division, differentiation, and specialization

Cell cycle and mitosis. Interphase (G1, S, G2) precedes mitosis (PMAT) and cytokinesis. Checkpoints (G1/S, G2/M, spindle/M) gate progression; cyclins and cyclin-dependent kinases drive transitions. Mitosis yields two genetically identical diploid daughter cells.

Signaling and apoptosis. Cells respond to signals via receptors and transduction cascades (e.g., GPCRs, receptor tyrosine kinases, second messengers like cAMP and Ca²⁺). Apoptosis is programmed, controlled cell death (membrane blebbing, DNA fragmentation), contrasted with messy, inflammatory necrosis.

Stem cells and differentiation. Stem cells are defined by self-renewal and potency (totipotent → pluripotent → multipotent). Differentiation reflects differential gene expression, not loss of genes. Gametogenesis produces sperm and eggs through meiosis.

FC2 retrieval prompts

How many Na⁺ and K⁺ ions does the Na⁺/K⁺-ATPase move per ATP, and in which directions?

Distinguish conjugation, transformation, and transduction in bacteria.

What distinguishes apoptosis from necrosis, and why does it matter physiologically?

Foundational Concept 3 — Integrated organ systems and homeostasis

Complex systems of tissues and organs sense the internal and external environments... and maintain a stable internal environment.

3A — Nervous and endocrine systems

Neurons and signaling. A neuron maintains a resting potential (~ −70 mV) via the Na⁺/K⁺-ATPase and selective K⁺ permeability. An action potential fires when depolarization reaches threshold: voltage-gated Na⁺ channels open (depolarization), then close and inactivate while voltage-gated K⁺ channels open (repolarization), with a brief hyperpolarization and refractory period ensuring one-way propagation. Myelin speeds conduction via saltatory jumping between nodes of Ranvier. At the synapse, depolarization triggers Ca²⁺ influx, neurotransmitter release, and binding to postsynaptic receptors (excitatory or inhibitory).

Nervous organization. CNS (brain, spinal cord) and PNS (somatic + autonomic, the latter split into sympathetic "fight or flight" and parasympathetic "rest and digest"). Reflex arcs allow rapid responses without brain processing.

Endocrine system. Hormones travel in blood to target tissues. Peptide hormones (hydrophilic) bind surface receptors and act fast via second messengers; steroid hormones (lipophilic) cross membranes to intracellular receptors and alter transcription (slower, longer-lasting). Most axes use negative feedback (e.g., hypothalamus → pituitary → target gland, with the product inhibiting upstream release). Know key players: insulin/glucagon (glucose), thyroid hormone (metabolic rate), cortisol (stress), ADH and aldosterone (water/Na⁺ balance).

3B — Integrative functions of the main organ systems

Circulatory and respiratory. A double circuit (pulmonary + systemic) moves blood; gas exchange occurs across alveolar and capillary walls down partial-pressure gradients. Hemoglobin's sigmoidal O₂-binding curve reflects cooperativity; the Bohr effect (lower pH/higher CO₂ in active tissue) right-shifts the curve to release more O₂ where it's needed.

Digestive and excretory. The GI tract mechanically and enzymatically breaks down food (mouth, stomach, small intestine) and absorbs nutrients. The nephron filters blood and fine-tunes water/solute balance via filtration, reabsorption, and secretion; ADH and aldosterone modulate water and Na⁺ handling.

Musculoskeletal. Skeletal muscle contracts by the sliding-filament mechanism: an action potential triggers Ca²⁺ release from the sarcoplasmic reticulum, Ca²⁺ binds troponin to move tropomyosin off actin's binding sites, and myosin cross-bridges cycle using ATP.

Immune system. Innate immunity is fast and nonspecific (barriers, phagocytes, inflammation); adaptive immunity is specific and develops memory (B cells/antibodies for humoral responses, T cells for cell-mediated responses). Vaccination exploits adaptive memory.

Reproduction and homeostasis. Reproductive anatomy and hormonal cycles (e.g., the menstrual cycle's FSH/LH/estrogen/progesterone interplay) are testable. Skin contributes to thermoregulation and serves as a barrier; homeostatic loops (thermoregulation, osmoregulation, glucose control) integrate multiple systems.

How the MCAT tests this: expect data on the O₂-hemoglobin curve and questions about how exercise, altitude, or pH shift it, or a feedback-loop scenario where you predict the hormonal response to a perturbation.

FC3 retrieval prompts

Order the ion-channel events of an action potential and explain why the refractory period enforces one-way propagation.

Contrast peptide and steroid hormone mechanisms in receptor location, speed, and duration.

Why does the Bohr effect improve O₂ delivery to exercising muscle?

End of Wave 1 guide. All content categories 1A–3B are covered at a high-yield level suitable for SME validation.