MCAT Biology & Biochemistry Guide

The official name is Biological and Biochemical Foundations of Living Systems. It's the third section on test day: 59 questions in 95 minutes, about 1 minute 37 seconds per question. Like the other two science sections, it has 10 passage sets (4–6 questions each) built around an experiment, a data table, or a short explainer, plus 15 stand-alone (discrete) questions.

AAMC's content outline breaks the section down two ways. By discipline:

And by foundational concept:

Takeaway: more than half the section is Foundational Concept 1—proteins, molecular genetics, and metabolism. If your biochem is shaky, that's where to start.

Every science question is also tagged with one of four Scientific Inquiry and Reasoning Skills. The AAMC's targets for each science section are roughly:

• Knowledge of scientific concepts and principles — about 35%. Can you recognize and apply the science?
• Scientific reasoning and problem-solving — about 45%. Can you use that science to predict, explain, or evaluate?
• Reasoning about the design and execution of research — about 10%. Controls, variables, confounds, ethics.
• Data-based and statistical reasoning — about 10%. Graphs, tables, error bars, correlation vs. causation.

Translation: roughly two-thirds of the section expects you to do something with the science. Flashcards get you the first 35%. Passages get you the rest.

This list follows AAMC's content categories and flags the topics that sit underneath the most other material—the ones that make passages readable. It's a study-priority guide, not a frequency count.

Here's an original mini-passage in the MCAT's style, followed by how to attack it.

Researchers compared a wild-type hexokinase (WT) with a variant (V) carrying one amino acid substitution near the glucose-binding pocket. Both enzymes were assayed at the same enzyme concentration.

Enzyme	Km for glucose (mM)	Vmax (µmol/min/mg)
WT	0.10	100
V	1.0	100

Question: At a glucose concentration of 0.10 mM, the rate of the variant is closest to what percentage of the wild-type rate?

A) 9% · B) 18% · C) 50% · D) 100%

Step 1 — Read the table before the math. V_max is identical, so the substitution didn't change catalysis at saturation. K_m went up tenfold, so the variant needs much more glucose to reach half speed—it binds glucose less well.

Step 2 — Plug into Michaelis–Menten. v = V_max[S]/(K_m + [S]).

• WT: 100 × 0.10/(0.10 + 0.10) = 50 µmol/min/mg
• V: 100 × 0.10/(1.0 + 0.10) ≈ 9.1 µmol/min/mg

Step 3 — Answer the question asked. 9.1/50 ≈ 0.18, so the variant runs at about 18% of the wild-type rate. The answer is B.

Why the traps work: 9% is the variant's rate as a fraction of V_max, not of the WT rate. 50% is WT's fraction of V_max. 100% is what you'd pick if you only looked at V_max. Every wrong answer is a number you actually calculated along the way—which is exactly how real MCAT distractors are built.

• Read passages for the story, not the details. What question were the researchers asking, what did they change, and what did they measure? Skim methods; come back for specifics when a question needs them.
• Label variables as you go. Independent variable, dependent variable, control. Many passage questions get much easier once you know what each experiment is actually testing.
• Look at figures before the answer choices. Check axes, units, and which condition is the control. Many misses are misread graphs, not missing knowledge.
• Answer from the passage when it conflicts with your memory. Passages can describe unusual organisms or hypothetical drugs. The passage wins.
• Do discretes fast. The stand-alone questions test straight content. Bank the time for data-heavy passages.

• Conditional probability in genetics. "Probability that a son is affected" and "probability that a child is an affected son" are different questions (1/2 vs. 1/4 for a carrier mother).
• Correlation dressed as causation. If the passage only shows two things moving together, an answer that says one causes the other is usually wrong.
• True but irrelevant. An answer choice can be a correct biology fact that doesn't answer the question. Ask: does this explain these results?
• Reversed regulation. Insulin vs. glucagon, sympathetic vs. parasympathetic, ADH vs. aldosterone. Know which direction each pushes.
• Smooth vs. skeletal muscle. Troponin in skeletal muscle; calmodulin and myosin light-chain kinase in smooth muscle. It's a favorite swap.

This assumes you're studying the other sections in parallel and can give Bio/Biochem about 8–10 hours a week.

Then test yourself on our full-length MCAT practice test, whose Bio/Biochem section runs 59 questions in 95 minutes, like the real one.