The Chemical and Physical Foundations of Biological Systems section gives you 59 passage-based and discrete questions in 95 minutes, with a periodic table provided. Prepare by anchoring each calculation to the target quantity's units: write the units you need, pick the relationship that produces them, verify its assumptions, then substitute numbers. Build one equation page per content category, practice selection on mixed sets, and audit errors by cause rather than by count.
What the Chemical and Physical Foundations section assesses
It is one of the four scored sections of the MCAT: 59 questions in 95 minutes, mixing passage-based sets with discrete questions, with a periodic table provided.
The AAMC describes this section as testing the mechanical, physical, and biochemical functions of human tissues, organs, and organ systems, and the basic chemical and physical principles underlying them. You solve problems by combining science knowledge with scientific inquiry and reasoning skills, applied to living systems.
Content is drawn from introductory-level biology, general chemistry, organic chemistry, and physics, plus first-semester biochemistry and molecular biology topics. Research methods and statistics concepts also appear, applied to the natural sciences. The four reasoning skills below run across all of it.
- Skill 1 — Knowledge of Scientific Concepts and Principles: demonstrating understanding of concepts and the relationships between closely related ideas.
- Skill 2 — Scientific Reasoning and Problem-Solving: reasoning about principles, theories, and models, and analyzing explanations and predictions.
- Skill 3 — Reasoning About the Design and Execution of Research: understanding research components and ethical issues in research.
- Skill 4 — Data-Based and Statistical Reasoning: interpreting patterns in tables, figures, and graphs, and drawing conclusions from them.
| Section logistics | Value |
|---|---|
| Number of questions | 59, a combination of passage-based and discrete questions |
| Time | 95 minutes |
| Provided resource | Periodic table |
| Calculator | Not mentioned in the retrieved content documents; confirm with the AAMC's official exam materials |
Sources: Chemical and Physical Foundations of Biological Systems Section: Overview | Students & Residents — AAMC What's on the MCAT Exam? section overview page; Chemical and Physical Foundations of Biological Systems (AAMC content download) — Official section content PDF, © 2020 AAMC
How the tested content is organized
The same 59 questions are counted three ways: by discipline, by foundational concept, and by reasoning skill. These are three views of one section, not separate topic totals.
By discipline, general chemistry carries the largest share at about 30 percent, followed by first-semester biochemistry and introductory physics at about 25 percent each, organic chemistry at about 15 percent, and introductory biology at about 5 percent. The AAMC rounds these to the nearest 5 percent, and the actual distribution varies between test forms because of difficulty balancing, shared passages, and unscored field-test questions.
By foundational concept, about 40 percent of questions come from Concept 4 (physical principles in living systems) and about 60 percent from Concept 5 (chemical interactions and reactions). By skill, the approximate split is 35/45/10/10 across Skills 1 through 4. Concept 4 covers translational motion, forces, work, energy, and equilibrium (4A); fluids in circulation and gas exchange (4B); electrochemistry and circuits (4C); light and sound interacting with matter (4D); and atoms, nuclear decay, and electronic structure (4E). Concept 5 covers water, solutions, and acid–base behavior (5A); molecules and intermolecular interactions (5B); separation and purification methods (5C); structure and reactivity of biologically relevant molecules (5D); and chemical thermodynamics and kinetics, including enzymes (5E).
- Download the AAMC's official section content PDF for the complete topic and subtopic lists; the exam writers use the same content list.
- The PDF's course abbreviations (BC, BIO, GC, OC, PHY) tell you the course level at which each topic is tested.
| Discipline | Approximate share of questions |
|---|---|
| General chemistry | 30% |
| First-semester biochemistry | 25% |
| Introductory physics | 25% |
| Organic chemistry | 15% |
| Introductory biology | 5% |
Sources: Chemical and Physical Foundations of Biological Systems Section: Overview | Students & Residents — AAMC What's on the MCAT Exam? section overview page; Chemical and Physical Foundations of Biological Systems (AAMC content download) — Official section content PDF, © 2020 AAMC
Math skills you are expected to use
Questions require unit conversion, estimation, and data reading at a level that explicitly excludes calculus.
The AAMC lists the mathematical techniques this section can ask of you: recognizing and interpreting linear, semilog, and log-log scales and calculating slopes; using significant digits and reasonable numerical estimates; converting within the metric system and between metric and English units, with conversion factors provided when needed; arithmetic with probability, proportion, ratio, percentage, and square-root estimation; exponentials and logarithms at an Algebra II level; basic trigonometry with standard values for common angles; and vector addition and subtraction.
Two official notes save preparation time: an understanding of calculus is not required, and a periodic table is provided during the exam. These skills matter as much as the science content, because every passage-based calculation passes through them.
Pacing when unit conversions slow you down
The official format allows 95 minutes for 59 questions, so average time per question is under 100 seconds — and passages shift real time toward reading. Treat conversion speed as a trainable, separate skill.
Those averages are uneven in practice: a passage-based set requires reading several paragraphs before any question can be answered, while discrete items can be quicker. When a calculation stalls, the useful diagnostic is what kind of stall it is. A conceptual stall means you have not identified the target quantity or the relationship that produces it; a mechanical stall means you know the relationship but the unit conversion or arithmetic is slow.
Both respond to different practice. For mechanical speed, drill conversions as short standalone warm-ups — mg to g, mmHg to Pa, cm³ to L — and practice estimating magnitudes before computing, since the exam explicitly expects reasonable numerical estimates. For conceptual speed, practice complete original calculations untimed until the selection step is automatic, then move to timed passage sets. This is preparation advice, not an official method; the official facts are only the question count, the time limit, and the listed math skills.
- Write the target quantity and its units before reading the answer choices.
- Label every given pressure as total, partial, or gauge before converting it.
- Estimate first to catch order-of-magnitude slips early.
- Sort timed-set misses into conceptual stalls, conversion stalls, and arithmetic slips, and drill the largest category.
Sources: Chemical and Physical Foundations of Biological Systems Section: Overview | Students & Residents — AAMC What's on the MCAT Exam? section overview page; Chemical and Physical Foundations of Biological Systems (AAMC content download) — Official section content PDF, © 2020 AAMC
Choosing an equation: units first, assumptions second
Unit analysis is a selection tool, not just a check. Writing the target quantity's units narrows the candidate equations before you read the numbers.
Start a calculation by writing the target quantity and its units. Then identify the quantities given in the passage and choose a relationship that connects them. Check the assumptions before substituting: continuity and Bernoulli address different aspects of ideal fluid flow, while Poiseuille describes viscous flow through a tube under its stated conditions.
Dimensional analysis can expose a setup error: pressure has units of force per area, so an expression in joules per second cannot be a pressure. Use the table as a study prompt, then check the full equation and assumptions in your science reference. The AAMC outline establishes the exam scope; the OpenStax chapters linked here support these explanations.
| Question asks about... | Primary relationship | Key assumption to verify |
|---|---|---|
| Velocity change with area change | Continuity (A₁v₁ = A₂v₂) | Steady incompressible flow without side branches or leaks |
| Pressure at a point in flowing fluid | Bernoulli's equation | Steady, incompressible, nonviscous flow along a streamline |
| Flow rate vs. radius or viscosity | Poiseuille's relation | Steady laminar flow of an incompressible Newtonian fluid in a rigid circular tube |
| Link between cell potential, free energy, and equilibrium | ΔG = −nFE; ΔG° = −RT ln K | n equals electrons transferred in the balanced reaction |
Sources: Chemical and Physical Foundations of Biological Systems (AAMC content download) — Official section content PDF, © 2020 AAMC; 12.1 Flow Rate and Its Relation to Velocity - College Physics 2e | OpenStax — https://openstax.org/books/college-physics-2e/pages/12-1-flow-rate-and-its-relation-to-velocity; 12.2 Bernoulli’s Equation - College Physics 2e | OpenStax — https://openstax.org/books/college-physics-2e/pages/12-2-bernoullis-equation; 12.4 Viscosity and Laminar Flow; Poiseuille’s Law - College Physics 2e | OpenStax — https://openstax.org/books/college-physics-2e/pages/12-4-viscosity-and-laminar-flow-poiseuilles-law; 17.4 Potential, Free Energy, and Equilibrium - Chemistry 2e | OpenStax — https://openstax.org/books/chemistry-2e/pages/17-4-potential-free-energy-and-equilibrium
Fluids: continuity, Bernoulli, and Poiseuille answer different questions
Continuity conserves flow rate across an area change; Bernoulli relates pressure, speed, and height for ideal flow; Poiseuille links viscosity and radius to resistance along a vessel.
Read the question's target to choose. If it asks how velocity changes when a vessel narrows, continuity (A₁v₁ = A₂v₂) answers directly. If it asks how pressure changes at that narrowing, you move to Bernoulli on top of the continuity result. If it asks how flow rate changes when radius or viscosity changes, Poiseuille's relation, with its fourth-power dependence on radius, is the relevant tool. Circulation passages can layer all three, so annotating each sentence with the quantity it describes tells you which relation each sentence feeds.
The complete practice problem below shows the failure mode: a Poiseuille-based intuition answers a different question than the one asked.
Practice problem
An ideal fluid flows steadily at 0.50 m/s through a horizontal tube of radius 3.0 mm, narrowing to a radius of 1.5 mm. Assume incompressible, nonviscous flow with density 1060 kg/m³, no branches or leaks, and no height change. Find (a) the speed in the narrow segment and (b) P_narrow − P_wide, the pressure change from the wide segment to the narrow one.
Show answer
(a) 2.0 m/s. (b) P_narrow − P_wide ≈ −2.0 × 10³ Pa: the pressure is lower in the narrow segment.
Cross-sectional area scales with radius squared. Halving the radius makes the area one quarter as large, so continuity gives four times the speed: 4 × 0.50 = 2.0 m/s. At equal height, Bernoulli gives P_narrow − P_wide = ½ρ(v_wide² − v_narrow²) = ½ × 1060 × (0.25 − 4.00) = −1987.5 Pa. The negative sign identifies a decrease in static pressure. Poiseuille’s law answers a different question about viscous pressure loss along a tube; the supplied assumptions explicitly neglect viscosity. This is an idealized physics problem, not a prediction of pressure in a patient’s artery.
Sources: Chemical and Physical Foundations of Biological Systems (AAMC content download) — Official section content PDF, © 2020 AAMC; 12.1 Flow Rate and Its Relation to Velocity - College Physics 2e | OpenStax — https://openstax.org/books/college-physics-2e/pages/12-1-flow-rate-and-its-relation-to-velocity; 12.2 Bernoulli’s Equation - College Physics 2e | OpenStax — https://openstax.org/books/college-physics-2e/pages/12-2-bernoullis-equation; 12.4 Viscosity and Laminar Flow; Poiseuille’s Law - College Physics 2e | OpenStax — https://openstax.org/books/college-physics-2e/pages/12-4-viscosity-and-laminar-flow-poiseuilles-law
Electrochemistry: from cell potential to free energy and equilibrium
Standard reduction potentials give the cell potential, which connects through free energy to the equilibrium constant; a concentration cell shows why actual conditions, not just standard values, decide behavior.
The content list for this category includes galvanic and electrolytic cells, half-reactions, reduction potentials and cell potential, concentration cells, and batteries. These connect through a standard chain of general chemistry relationships: a positive cell potential corresponds to a negative Gibbs free energy change, and the standard free energy relates to the equilibrium constant, so any problem naming one of E, ΔG, or K shows you the path to the other two. The linked OpenStax chapter derives the relationships ΔG = −nFE and ΔG° = −RT ln K and explains their signs.
Worked reasoning for a concentration cell. A passage states that the concentrations on the two sides differ and asks about spontaneity. A plausible mistake is to see a zero standard cell potential, conclude no driving force exists, and pick an answer denying spontaneity. The correct reading is that a concentration cell's standard potential is zero by definition, so the entire driving force comes from the concentration difference expressed through the reaction quotient; the cell does work as the concentrations equalize. Standard values describe standard conditions, and the passage's stated conditions determine actual behavior — conflating the two produces a confidently wrong answer.
Sources: Chemical and Physical Foundations of Biological Systems (AAMC content download) — Official section content PDF, © 2020 AAMC; 17.4 Potential, Free Energy, and Equilibrium - Chemistry 2e | OpenStax — https://openstax.org/books/chemistry-2e/pages/17-4-potential-free-energy-and-equilibrium
Structure and reactivity of biologically relevant molecules
Review acid–base equilibria, buffers, amino acids and proteins, functional groups, and the reactivity topics listed under AAMC’s foundational concept 5.
Make a checklist from the actual content categories: conjugate acids and bases, Ka and Kb, pKa and pKb, buffers, isoelectric point, carbonyl reactivity, substituent effects and steric hindrance. For each item, pair your notes with a practice question that requires a calculation, comparison or explanation.
When reviewing an organic chemistry question, label the functional groups, identify the requested property or transformation, and check your explanation against the relevant course material. Keep a separate error log for structure recognition and reaction reasoning so your next session addresses the right gap.
A preparation sequence and next steps
Work in four passes: rebuild equation chains per content category, drill selection on mixed sets, run timed passage sets, then audit errors by cause.
First, build one page per category — fluids, circuits, electrochemistry, gases, atomic structure, acid–base, thermodynamics — listing each relationship, its output units, and its assumptions. Second, work mixed problem sets where consecutive problems need different relationships, writing the chosen equation plus a one-line reason before solving. Third, work full passage-style sets under the official time budget, still writing brief justifications. Fourth, audit: sort every miss into assumption errors, selection errors, conversion stalls, or reading errors, and target the largest category in the next pass. Adjust how long each pass lasts based on your audit results rather than a fixed calendar.
For official coverage and administration, the AAMC's What's on the MCAT Exam? pages and the downloadable section content PDF are the authoritative references; this guide addresses study method, not exam policies. For further practice, use the site's free CPBS practice questions and other MCAT study guides.
- Confirm the current content outline and section logistics in the AAMC's official materials before finalizing your plan.
- Build the per-topic equation pages first, then drill selection before timing yourself.
- Keep a running error audit by cause; a low score tells you which habit to drill, not whether you are ready.
Sources
Facts checked:
- Chemical and Physical Foundations of Biological Systems Section: Overview | Students & Residents
- Chemical and Physical Foundations of Biological Systems (AAMC content download)
- 12.1 Flow Rate and Its Relation to Velocity - College Physics 2e | OpenStax
- 12.2 Bernoulli’s Equation - College Physics 2e | OpenStax
- 12.4 Viscosity and Laminar Flow; Poiseuille’s Law - College Physics 2e | OpenStax
- 17.4 Potential, Free Energy, and Equilibrium - Chemistry 2e | OpenStax