MCAT Practice Questions
This is a free 15-question sampler of MCAT science questions, written by the StudyPDF team. Every item here is a discrete question, the standalone kind that comes with no passage attached, drawn from all three science sections: biology and biochemistry, general and organic chemistry with physics, and psychology and sociology. Answer all 15, then check your score and read the explanation for every question. Reading comprehension is not tested here, that lives on the CARS page linked below.
A globular protein folds in water. Which amino acid side chain is most likely to end up buried in the hydrophobic core?
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All 15 questions at a glance
Prefer to read before you play? Every question in the sampler is listed below. Answers and explanations stay collapsed until you open them.
1. A globular protein folds in water. Which amino acid side chain is most likely to end up buried in the hydrophobic core?
- A. Aspartate
- B. Lysine
- C. Valine
- D. Serine
Show answer and explanation
Answer: C. Valine
Folding in water pushes nonpolar side chains inward and leaves charged and polar ones on the surface where they can interact with water. Valine has a branched hydrocarbon side chain with nothing to hydrogen bond, so the core is where it costs the least energy. Aspartate and lysine carry charges at physiological pH and serine has a hydroxyl group, so all three prefer the surface.
2. A competitive inhibitor is added to an enzyme assay. What happens to the apparent Km and the Vmax?
- A. Km increases and Vmax is unchanged
- B. Km decreases and Vmax is unchanged
- C. Km is unchanged and Vmax decreases
- D. Km increases and Vmax decreases
Show answer and explanation
Answer: A. Km increases and Vmax is unchanged
A competitive inhibitor binds the active site, so it fights substrate for the same spot. That means you need more substrate to reach half maximal velocity, which shows up as a higher apparent Km. Enough substrate still outcompetes the inhibitor completely, so Vmax is untouched. Pure noncompetitive inhibition is the mirror image: Vmax falls and Km stays put.
3. Complete the sentence.
In oxidative phosphorylation, the electron transport chain uses the energy released by electron transfer to pump protons into the _____. ATP synthase then makes ATP as those protons flow back down their gradient into the _____.
Options for blank 1: mitochondrial matrix, intermembrane space, cytosol
Options for blank 2: mitochondrial matrix, intermembrane space, cytosol
Show answer and explanation
Answer: intermembrane space, mitochondrial matrix
The electron transport chain is a series of redox reactions, and the energy it releases is stored as a proton gradient across the inner mitochondrial membrane rather than as ATP directly. ATP synthase is the only part that turns that gradient back into chemical energy. This is why an uncoupler that lets protons leak across the membrane keeps electron transport and oxygen consumption running while ATP output collapses.
4. A buffer is made from a weak acid and its conjugate base. Its buffering capacity is greatest when:
- A. The solution pH is two units below the pKa
- B. The solution pH is two units above the pKa
- C. The solution pH equals the pKa
- D. The weak acid is present in large excess over the conjugate base
Show answer and explanation
Answer: C. The solution pH equals the pKa
The Henderson-Hasselbalch equation says pH equals pKa plus the log of the base to acid ratio, so pH equals pKa exactly when that ratio is one. With equal amounts of both forms the buffer has the most room to absorb either added acid or added base before one form runs out. Practically, a buffer is useful roughly one pH unit on either side of its pKa, which is why bicarbonate and phosphate cover different ranges in the body.
5. Which statements about eukaryotic gene expression are correct?
Select all that apply.
- A. Introns are spliced out of the primary transcript before the mature mRNA leaves the nucleus
- B. The genetic code is degenerate, so several codons can specify the same amino acid
- C. RNA polymerase requires a primer to begin synthesis
- D. A missense mutation changes a codon so a different amino acid is inserted
- E. Ribosomes read the mRNA from the 3 prime end toward the 5 prime end
Show answer and explanation
Answer: A. Introns are spliced out of the primary transcript before the mature mRNA leaves the nucleus, B. The genetic code is degenerate, so several codons can specify the same amino acid, D. A missense mutation changes a codon so a different amino acid is inserted
Splicing, capping and polyadenylation all happen in the nucleus, and only the processed mRNA is exported. Degeneracy is why the third base of a codon often tolerates a change with no effect on the protein, which is the definition of a silent mutation, while a missense mutation swaps the amino acid and a nonsense mutation creates a stop codon. RNA polymerase can start a chain from scratch, unlike DNA polymerase, and ribosomes read mRNA 5 prime to 3 prime.
6. In the proximal tubule of the nephron, glucose crosses the apical membrane into the cell against its own concentration gradient by moving together with sodium. This is best described as:
- A. Simple diffusion
- B. Facilitated diffusion
- C. Primary active transport
- D. Secondary active transport
Show answer and explanation
Answer: D. Secondary active transport
Secondary active transport means the carrier itself burns no ATP. It rides an ion gradient that some other pump paid for. Here the sodium potassium ATPase on the basolateral side spends ATP to keep intracellular sodium low, and the sodium glucose cotransporter uses that inward sodium gradient to drag glucose uphill. Block the ATPase and the sodium gradient collapses, and glucose reabsorption stops even though the cotransporter is untouched.
7. Complete the sentence.
In an infrared spectrum, a broad absorption centered near 3300 reciprocal centimeters points to an _____ bond, while a strong sharp band near 1700 reciprocal centimeters points to a _____.
Options for blank 1: O to H, C to H, C to C double
Options for blank 2: carbon carbon double bond, carbonyl group, nitrile group
Show answer and explanation
Answer: O to H, carbonyl group
Infrared spectroscopy reports bond vibrations, and the two most useful signals for a first pass are the broad hydroxyl stretch and the sharp carbonyl stretch. The hydroxyl band is broad because hydrogen bonding gives the bond a range of strengths across the sample. Seeing both a broad hydroxyl band and a carbonyl band in one spectrum is the classic hint that you are looking at a carboxylic acid.
8. Which carboxylic acid derivative is most reactive toward nucleophilic acyl substitution?
- A. An acyl chloride
- B. An ester
- C. An amide
- D. A carboxylate ion
Show answer and explanation
Answer: A. An acyl chloride
Reactivity in this family tracks two things: how good the leaving group is and how much the attached atom donates electron density into the carbonyl. Chloride is a weak base and a good leaving group, and it donates very little, so the acyl chloride sits at the top. Nitrogen donates strongly, which makes amides the most stable of the neutral derivatives, and a carboxylate is already deprotonated and negatively charged, so a nucleophile has little reason to attack it.
9. A reaction has a negative delta G under the given conditions. Which statements are correct?
Select all that apply.
- A. The reaction must proceed quickly
- B. The reaction is spontaneous as written under those conditions
- C. The reaction must be exothermic
- D. An enzyme could speed it up without changing the value of delta G
- E. Under standard conditions, a negative standard delta G means the equilibrium constant is greater than one
Show answer and explanation
Answer: B. The reaction is spontaneous as written under those conditions, D. An enzyme could speed it up without changing the value of delta G, E. Under standard conditions, a negative standard delta G means the equilibrium constant is greater than one
Delta G is about thermodynamics, not speed. A spontaneous reaction can take years if the activation energy is high, and a catalyst lowers that barrier without touching the free energy difference between reactants and products. Spontaneity also does not require heat release, since a positive entropy change at high enough temperature can make delta G negative for an endothermic reaction. Standard delta G is related to the equilibrium constant by delta G naught equals minus RT ln K, so a negative value puts K above one.
10. Circulating thyroid hormone rises because the thyroid gland itself is overproducing. In an intact feedback loop, what happens to TSH released by the anterior pituitary?
- A. It rises, because hormone levels and TSH move together
- B. It falls, because the high hormone level suppresses the pituitary and the hypothalamus
- C. It stays the same, because TSH is regulated only by the hypothalamus
- D. It rises first and then stays permanently elevated
Show answer and explanation
Answer: B. It falls, because the high hormone level suppresses the pituitary and the hypothalamus
The hypothalamus releases TRH, the anterior pituitary releases TSH, and the thyroid releases its hormones, which then feed back to shut down the two levels above. When the gland at the end of the chain is the source of the excess, the feedback signal is strong and TSH goes down. That inverse pattern is the point of a negative feedback loop, and the same logic runs the hypothalamus, pituitary and adrenal axis with cortisol.
11. A vessel behaves as a rigid tube with steady laminar flow. If its radius drops to half while the pressure difference across it stays the same, the flow rate becomes about:
- A. One half of the original
- B. One quarter of the original
- C. One eighth of the original
- D. One sixteenth of the original
Show answer and explanation
Answer: D. One sixteenth of the original
Poiseuille's law says flow rate is proportional to the fourth power of the radius for a given pressure difference, viscosity and length. Halving the radius therefore divides flow by two to the fourth, which is sixteen. That fourth power dependence is why radius changes dominate resistance in the circulation, and why small changes in vessel diameter matter far more than changes in length or viscosity.
12. A student has a headache, takes a painkiller, and the headache goes away. She now reaches for the painkiller sooner whenever a headache starts. In operant terms this is:
- A. Positive reinforcement
- B. Negative reinforcement
- C. Positive punishment
- D. Classical conditioning
Show answer and explanation
Answer: B. Negative reinforcement
In operant conditioning, reinforcement means the behavior becomes more frequent and punishment means it becomes less frequent. Positive and negative describe whether something was added or taken away, not whether it felt good. Here an unpleasant stimulus was removed and the behavior increased, which is negative reinforcement by definition. Classical conditioning would involve pairing two stimuli to produce an involuntary response, not strengthening a voluntary action by its consequence.
13. The smallest change in a stimulus that a person can detect half of the time is called the:
- A. Absolute threshold
- B. Signal detection criterion
- C. Difference threshold
- D. Point of sensory adaptation
Show answer and explanation
Answer: C. Difference threshold
The difference threshold, also called the just noticeable difference, is about telling two stimuli apart, while the absolute threshold is the smallest stimulus that can be detected at all half the time. Weber's law adds that the difference threshold is a constant proportion of the original stimulus rather than a fixed amount, so a heavier weight needs a bigger change before you notice it. The signal detection criterion is a separate idea about how willing someone is to say yes when they are unsure.
14. Researchers record the diets of 2,000 healthy adults, then follow the same people for 20 years and record who develops heart disease. Which statements about this study are correct?
Select all that apply.
- A. It is a prospective cohort study
- B. Participants were randomly assigned to a diet
- C. Participants dropping out over the 20 years can bias the result
- D. It can measure how many new cases of heart disease appear in each diet group
- E. It shows that diet causes heart disease
Show answer and explanation
Answer: A. It is a prospective cohort study, C. Participants dropping out over the 20 years can bias the result, D. It can measure how many new cases of heart disease appear in each diet group
The exposure was measured before the outcome appeared and the group was followed forward, which makes this a prospective cohort study. That design can measure incidence, meaning new cases over time, which case control studies cannot do directly. What it cannot do is establish causation, because nobody assigned the exposure and people who eat differently may differ in many other ways. Attrition matters because the people who leave a long study are often not a random sample of those who stay.
15. Complete the sentence.
Blaming a stranger's rudeness on their personality while ignoring what is going on around them is the _____. Explaining your own good grade by your ability but your own bad grade by an unfair test is the _____.
Options for blank 1: fundamental attribution error, self-serving bias, just world hypothesis
Options for blank 2: actor observer bias, self-serving bias, halo effect
Show answer and explanation
Answer: fundamental attribution error, self-serving bias
Attribution questions turn on two axes: whether the cause is placed inside the person or in the situation, and whether the person being judged is you or somebody else. The fundamental attribution error is over-weighting personality when explaining other people. The self-serving bias protects your own self-image by taking internal credit for success and external blame for failure. Both are on the MCAT because they shape how patients and clinicians read each other.
What the three science sections actually ask
The MCAT has four sections. Three of them are science: Chemical and Physical Foundations of Biological Systems, Biological and Biochemical Foundations of Living Systems, and Psychological, Social, and Biological Foundations of Behavior. The fourth is Critical Analysis and Reasoning Skills, which is pure reading and has its own page. Each science section currently runs 59 questions in 95 minutes, and roughly 15 of those 59 are discrete questions that stand on their own with no passage. Section scores run from 118 to 132, and the four sections add up to a total between 472 and 528, with 500 sitting at the middle of the scale. The AAMC can change the format, so confirm the current numbers on aamc.org before you build a schedule around them.
The content itself comes from the content categories the AAMC publishes. The surprising part for most people is how much biochemistry appears, and that it appears in two different sections. Amino acids, enzyme kinetics and metabolism can be tested in the biology section and again in the chemistry and physics section. The physics is narrow and applied, mostly fluids and pressure, thermodynamics, optics and circuits, and it is nearly always framed inside a living system rather than a block sliding down a ramp.
The psychology and sociology section is a real content section with its own vocabulary, not a common sense section, and that catches people out. Conditioning, memory systems, perceptual thresholds, attribution theory and research design all have precise definitions, and the questions turn on those definitions. Across all three sections most items are not simple recall. They hand you a scenario, a graph or a set of results and ask what follows from it, so knowing a term is only half of what you need.
How to use this sampler
Every question below is a discrete question on purpose. Discretes are the cleanest test of whether you actually know the content, because there is no passage sitting next to the question quietly handing you context. If you can answer discretes cold, passages get much easier. If you cannot, passage practice will hide the gap rather than fix it. The two sibling quizzes linked below cover the mixed passage style and the reading section, so run all three and compare where you drop points.
Take it in one sitting without notes, at about 90 seconds a question, then read the explanation for every item including the ones you got right. Guessing correctly and knowing are not the same thing, and only one of them survives test day. Treat each miss as a topic rather than a fact. If you missed the buffer question and the thermodynamics question, review acid base and equilibrium together, because they are the same reasoning applied to two different quantities.
A note on what these questions are. The StudyPDF team wrote all 15 from the content categories the AAMC publishes. They are not AAMC items, they are not copied from any test bank or prep book, and StudyPDF is not affiliated with the AAMC. They are exam practice, not medical guidance, and nothing on this page should be used to make a decision about a real patient.
How StudyPDF builds full practice tests from your own materials
Fifteen questions can show you which content category is soft. They cannot carry a whole MCAT prep plan, and a generic question bank rarely matches the emphasis of the review books and class notes you are actually studying from.
StudyPDF works from your own material instead. You upload your prep book chapter, your biochemistry lecture slides or the pathway summary you built yourself, and Bo, the study agent, writes full-length practice tests from those exact pages. Every question is grounded in your material and every explanation cites where the answer came from, so when something looks wrong you can go straight to the source and check it. You can regenerate fresh tests as often as you want, narrow them to one area such as enzyme kinetics or research design, and track which concepts keep coming back wrong.
You do not need a file to start. If you have nothing to upload, name a topic instead, for example the citric acid cycle or operant conditioning, and Bo writes a practice set from that. Starting is free.
Written by the StudyPDF team. Last updated 2026-08-19.