MCAT, Medical exams

Biological and Biochemical Foundations of Living Systems

This section covers the biology and biochemistry of living systems, from the amino acids that build proteins up through the integrated organ systems that hold an organism stable. Its content sits under Foundational Concepts 1, 2, and 3, spanning biomolecules, cellular and microbial biology, and the physiology of the major organ systems. Passages lean heavily on experimental data, so reasoning about study design and interpreting figures carry as much weight as recall.

Position: 3 of 4 sections
Questions: 59 questions
Time: 95 minutes
Topics covered: 10 topics in this guide

What is covered

Topics inside this section.

These topics describe the scope students most often review for this part of MCAT. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Amino acids, proteins, and enzymes: Amino acid structure and charge, protein folding, and enzyme kinetics and regulation. Know the side chains cold; a disproportionate share of biochemistry questions starts there.
Nucleic acids and gene expression: DNA and RNA structure, replication, transcription, translation, and the control of expression. Follow information flow end to end rather than memorizing each step in isolation.
Genetics, inheritance, and evolution: Mendelian and non-Mendelian inheritance, recombination, mutation, and the mechanisms that generate diversity. Practice pedigree and probability problems until the setup is automatic.
Bioenergetics and metabolism: Glycolysis, the citric acid cycle, oxidative phosphorylation, and fatty acid and amino acid metabolism. Map each pathway to its location, its regulators, and its net yield instead of every intermediate.
Cell structure, membranes, and transport: Organelles, cytoskeleton, membrane composition, and the transport mechanisms that move material across it. Tie each structure to the function a passage might disrupt experimentally.
Prokaryotes and viruses: Bacterial structure, growth, and genetics alongside viral life cycles and replication strategies. Contrast prokaryotic and eukaryotic machinery directly; the differences are frequent question targets.
Cell cycle, differentiation, and development: Mitosis, meiosis, apoptosis, stem-cell concepts, and the basics of specialization. Learn the checkpoints and what fails when each one is bypassed.
Nervous and endocrine control: Neuronal signaling, neurotransmission, and hormonal axes that coordinate the other organ systems. Draw each feedback loop from memory; loop logic answers many stems on its own.
Organ systems and homeostasis: Circulatory, respiratory, digestive, immune and lymphatic, excretory, muscular, skeletal, and reproductive physiology. Anchor every system to the variable it regulates and how it responds to a perturbation.
Research design and data interpretation: Reading experimental figures, judging controls, and reasoning about what a result does and does not show. This is a tested skill in its own right, so review it as content rather than as test-taking advice.

Study tips

How to practice Bio/Biochem.

Section-specific guidance that follows from how this material is usually tested.

Learn amino acid structures, charges, abbreviations, and pKa behavior early; they support far more questions than their share of study time suggests.
Read every experimental figure before the surrounding prose — an axis label and a trend often answer the question outright.
Use spaced repetition for physiology and pathway detail, which decays quickly, and reserve longer sessions for passage-based problem solving.
Practice explaining each metabolic pathway aloud in three sentences: where it happens, what it consumes, and what regulates it.

Where this fits

Biological and Biochemical Foundations of Living Systems is 3 of 4 sections in the MCAT guide. Use the full guide to balance it against everything else you have to review.

Back to the MCAT guide Official exam information

Sample Bio/Biochem questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
To which class of amino acids do lysine, arginine, and histidine belong based on their side chains?
1. ABasic amino acids
2. BAcidic amino acids
3. CNonpolar aliphatic amino acids
4. DAromatic amino acids
Show answer
Answer: A. Basic amino acids
Lysine, arginine, and histidine carry nitrogen-containing side chains that can accept a proton, which makes them the three basic amino acids. Acidic amino acids are the tempting distractor, but that group is aspartate and glutamate, whose carboxylate side chains donate protons and carry a negative charge at physiologic pH.
Key insight: Three basic side chains, two acidic ones: knowing those five explains most charge and pH reasoning in biochemistry passages.
Question 2Foundational
In a eukaryotic cell, in which compartment does the citric acid cycle take place?
1. AThe cytosol
2. BThe mitochondrial matrix
3. CThe inner mitochondrial membrane
4. DThe mitochondrial intermembrane space
Show answer
Answer: B. The mitochondrial matrix
The enzymes of the citric acid cycle are soluble in the mitochondrial matrix, with succinate dehydrogenase the single exception embedded in the inner membrane. The cytosol is the tempting distractor because glycolysis happens there, but pyruvate must first be transported into the mitochondrion and oxidized before the cycle can begin.
Key insight: Glycolysis in the cytosol, the citric acid cycle in the matrix, and the electron transport chain in the inner membrane.
Question 3Intermediate
Which type of reversible enzyme inhibition raises the apparent Michaelis constant while leaving the maximum velocity unchanged?
1. ACompetitive inhibition
2. BNoncompetitive inhibition
3. CUncompetitive inhibition
4. DMixed inhibition
Show answer
Answer: A. Competitive inhibition
A competitive inhibitor binds the active site and can be outcompeted, so more substrate is needed to reach half-maximal velocity while saturating substrate still reaches the original maximum velocity. Noncompetitive inhibition is the tempting distractor, but that inhibitor binds away from the active site where substrate cannot displace it, which lowers Vmax and leaves Km unchanged.
Key insight: Competitive inhibition can be overcome by adding substrate, which is why only the apparent Km moves.
Question 4Intermediate
During which phase of meiosis does crossing over between homologous chromosomes occur?
1. AProphase I
2. BMetaphase I
3. CProphase II
4. DAnaphase II
Show answer
Answer: A. Prophase I
Homologous chromosomes synapse and exchange segments at chiasmata during prophase I, which is the source of recombination between linked genes. Anaphase I is when those homologs separate, and by prophase II the homologs are already in different cells, so no crossing over can occur there.
Key insight: Recombination happens in prophase I; independent assortment happens at metaphase I.
Question 5Advanced
Which enzyme catalyzes the rate-limiting committed step of glycolysis?
1. APhosphofructokinase-1
2. BHexokinase
3. CPyruvate kinase
4. DGlyceraldehyde-3-phosphate dehydrogenase
Show answer
Answer: A. Phosphofructokinase-1
Phosphofructokinase-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate, the first step committed to glycolysis, and it is inhibited by ATP and citrate and activated by AMP and fructose-2,6-bisphosphate. Hexokinase is the tempting distractor because it acts first, but glucose-6-phosphate can still be diverted to glycogen synthesis or the pentose phosphate pathway, so that step is not committed.
Key insight: The committed step is the first one whose product has nowhere else to go, which is why regulation concentrates there.

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Biological and Biochemical Foundations of Living Systems