Master MCAT
Bio/Biochem
Biological & Biochemical Foundations of Living Systems: 59 questions, 95 minutes, and the highest content volume on the exam. Here’s how to tame the breadth.
95 min
Section Time
59
Questions
118–132
Score Scale
Section overview
The big-content section
B/B is the third section, taken after lunch, and covers more content than any other — five biology sub-areas plus a heavy dose of biochemistry. The upside: with organized review it is the section where thorough preparation most reliably converts into points.
10
Passages (44 questions)
15
Discrete questions
~96 s
Per question average
B/B is experiment-heavy: many passages describe an unfamiliar study, and the questions test whether you can read the design and the figures rather than recite a fact. Treat every passage like a mini research paper — what was manipulated, what was measured, what the result shows.
Scoring: scaled 118–132 (median ~125), a quarter of your 472–528 total. No penalty for wrong answers — never leave a question blank.
Content breakdown
What’s actually tested
Five content areas — bars scaled to each area’s approximate share of the section. Biochemistry and molecular biology together make up half.
Biochemistry
~25%Amino acids and protein structure, enzyme kinetics and regulation, and metabolism — glycolysis, the citric-acid cycle, oxidative phosphorylation, gluconeogenesis and fatty-acid oxidation. The connective tissue between this section and Chem/Phys.
Molecular biology
~25%The central dogma — DNA replication, transcription, translation and gene regulation — plus the lab toolkit: PCR, gel electrophoresis, blotting, sequencing and cloning. Technique questions are a reliable source of points.
Cellular biology
~20%Membranes and transport, organelle function, the cytoskeleton, the cell cycle and apoptosis, and signal transduction cascades. Signaling pathways recur constantly in passages.
Physiology
~20%Organ systems as integrated wholes — nervous, endocrine, cardiovascular, respiratory, renal, immune, digestive and reproductive. Tested at the systems level with heavy emphasis on feedback and regulation.
Genetics
~10%Mendelian inheritance and pedigrees, linkage and recombination, mutations, and population genetics (Hardy–Weinberg). Often paired with a molecular-biology or physiology passage.
Percentages are approximate; the AAMC publishes B/B as biochemistry ~25%, biology ~65% (split here across molecular, cellular, physiology and genetics) and general/organic chemistry ~10%. Test forms vary.
How it’s asked
The four AAMC skills
Every B/B question targets one of four scientific inquiry and reasoning skills. Breadth of recall matters here more than anywhere — but reasoning still carries the largest weight.
Knowledge of scientific concepts
Recall and recognition across a vast content base — pathways, structures, definitions. B/B’s sheer breadth means this section rewards thorough, well-organized content review more than any other.
Scientific reasoning & problem-solving
Apply concepts to new biological scenarios and predict how a system responds to a perturbation — knock out an enzyme, block a receptor — and trace the downstream effects.
Reasoning about research design
B/B is experiment-dense. Read a study’s design, identify controls and variables, and reason about what a result does and does not show.
Data-based & statistical reasoning
Interpret gels, Western blots, growth curves and dose–response data. Read the figure and its axes before the question stem.
Where the points are
High-yield systems
The content that recurs test after test. Learn these as regulated systems — what turns them up, what turns them down — not as static diagrams.
Biochemistry
- The 20 amino acids: structures, pKa values, charge at a given pH
- Protein folding, denaturation and quaternary structure
- Enzyme kinetics, inhibition and allosteric regulation
- Glycolysis, TCA cycle and oxidative phosphorylation — key enzymes and regulation
- Metabolic integration: fed vs fasted state, insulin and glucagon
Molecular biology
- DNA replication, transcription and translation — enzymes and directionality
- Gene regulation: operons, promoters, enhancers, epigenetics
- Lab techniques: PCR, gel electrophoresis, Southern/Northern/Western blots
- Cloning, restriction enzymes and DNA sequencing
Cell biology & signaling
- Membrane transport: passive, active, and secondary active
- Second messengers: cAMP, IP3/DAG, calcium, receptor tyrosine kinases
- The cell cycle, checkpoints and apoptosis
- Organelles and the secretory pathway
Physiology
- Neurons: action potentials, synaptic transmission, neurotransmitters
- Endocrine axes and negative feedback (HPA, HPT, HPG)
- Cardiovascular, respiratory and renal regulation of homeostasis
- Immune system: innate vs adaptive, antibodies, clonal selection
Decode the research passage
B/B is the most experiment-dense section on the MCAT. The students who score highest aren’t the ones who memorized the most — they’re the ones who read an unfamiliar study fast and correctly. These four habits are how.
Map the experiment before the questions
For each passage, note in one line: what was manipulated (independent variable), what was measured (dependent variable), and what the control was. Most B/B passage questions test whether you understood the setup.
Read the figure, then the stem
Gels, blots and curves carry the answer. Read the axes and legend, note the trend, and predict what a question could ask before you read it — the figure is doing the arguing.
Think in perturbations
B/B loves "what happens if we knock this out / add an inhibitor / mutate this residue?" Trace the pathway forward and backward: what accumulates, what disappears, what compensates.
Anchor new proteins to known scaffolds
Passages invent enzymes and receptors you’ve never seen. Slot each into a familiar category (kinase, channel, transcription factor) and reason from what that class always does.
The one-line experiment map
For every passage, jot: manipulated = what the researchers changed · measured = the readout · control = the baseline comparison. Nine times out of ten, the questions are testing exactly those three things.
Beat the clock
Timing & pacing
The trap in B/B is drowning in dense passages. Read for structure, bank the recall discretes, and spend your minutes where reasoning actually earns points.
10 passages + 15 discretes in 95 minutes
About 96 seconds per question, but budget by block: ~8.5 minutes per passage and 30–45 seconds per discrete. B/B discretes are often pure recall — bank quick points there and give passages the reading time.
Don’t drown in the passage
B/B passages are dense with invented detail. Skim for structure — what’s being studied and how — then let each question pull you back to the exact figure or paragraph. You do not need to absorb every sentence up front.
Separate recall from reasoning
If a question is pure recall and you don’t know it, guess and move — no amount of staring recovers a fact you didn’t review. Spend your time on the reasoning questions, where working through the passage actually pays off.
Checkpoint rule: at the 48-minute mark you should be starting passage 6 (or have the discretes banked). Recall you don’t have won’t come with more time — spend the clock on the reasoning questions instead.
Avoid the traps
Common B/B mistakes
The same errors recur across test-takers. Check your practice review against this list.
Under-studying biochemistry because it feels like Chem/Phys’ job — it is the single biggest slice of B/B and it is tested in both sections.
Memorizing pathways as isolated diagrams instead of understanding regulation — the MCAT asks what speeds up, slows down or compensates.
Skipping physiology integration — questions cross systems (how the kidney and lungs jointly manage pH), not single organs in isolation.
Ignoring lab techniques — PCR, blots and electrophoresis are high-yield, learnable points that many students leave on the table.
Trying to memorize every invented protein in a passage rather than categorizing it and reasoning from the class.
Reviewing only content, never experimental design — a large share of B/B is Skill 2/3 reasoning about studies.
Build the section
How to prep for B/B
Content first, passages second, integration last. It slots directly into the full 3–6 month MCAT timeline.
Content pass with a systems spine (weeks 1–6)
Work through biochem, molecular, cell, physiology and genetics with chapter questions. Build one-page pathway maps (glycolysis, the central dogma, each endocrine axis) you can redraw from memory.
Drill and dissect passages (weeks 6–12)
Move to the AAMC B/B Qpack and Section Bank plus a commercial bank. For every experimental passage, write the one-line experiment map; review why each wrong choice is wrong.
Integrate on full-lengths (weeks 12+)
Take AAMC full-lengths and track your B/B subscore. Late prep should feel like reasoning through unfamiliar research, not re-reading notes — stability at target across three exams signals readiness.
AAMC Bio/Biochem Qpack
Official questions at real-exam register — the primary drilling resource for the section.
AAMC Section Bank (B/B)
Dense, research-heavy passages, harder than the real thing — ideal for training experiment reasoning.
Full-lengths FL1–FL5
Your calibration benchmark. Track the B/B subscore across exams; stability at target = ready.
Keep going
Master the other three sections
Chemical & Physical Foundations
Gen chem, physics, biochem and orgo — math under pressure, no calculator.
Read the breakdownCritical Analysis & Reasoning Skills
Humanities passages, zero outside knowledge — pure reading precision.
Read the breakdownPsychological, Social & Biological Foundations
Vocabulary-heavy psych and soc — the fastest section to raise.
Read the breakdownQuestions
Bio/Biochem FAQ
B/B is one section of four
Zoom back out to the full picture — scoring, school benchmarks, the prep timeline and when to retake — in the complete MCAT guide.