MCAT: Biological and Biochemical Foundations

The MCAT section titled Biological and Biochemical Foundations of Living Systems is often described as the most “science-forward” part of the exam, but its real challenge is integration. You are tested on biology and biochemistry as interconnected systems, then asked to reason through experimental passages under time pressure. Content knowledge matters, but it is not enough. High scores come from understanding mechanisms, predicting outcomes, and interpreting data the way a researcher or clinician would.

This section is commonly understood as an integration of biology (about 65%), biochemistry (about 25%), and general chemistry (about 10%), all filtered through passage-based analysis. The best preparation mirrors that structure: you learn the foundations, then practice applying them to unfamiliar scenarios.

What This Section Actually Tests

At its core, this section evaluates whether you can connect molecular events to cellular behavior, tissue function, and organism-level physiology. That means you are rarely rewarded for memorizing isolated facts. Instead, you are expected to:

• Trace cause and effect across biological scales (gene → protein → pathway → phenotype)
• Interpret graphs, tables, blots, enzyme kinetics, or experimental controls
• Use basic chemistry to explain biological behavior (charge, polarity, pH, bonding, energetics)
• Evaluate hypotheses and alternative explanations based on evidence

A typical passage might introduce a protein mutation, show a change in enzymatic activity, and then ask what will happen to a pathway product, a cell type, or an organ function. Success depends on linking the data to the underlying mechanism.

Biology (65%): From Cells to Systems, With Mechanisms

Because biology dominates the section, your foundation must be strong across cellular biology, genetics, and physiology. The key is to learn biology as a set of interacting systems rather than a list of topics.

Cell structure and function

Expect frequent testing of membranes, transport, organelles, and signaling. You should be comfortable reasoning about:

• How membrane composition affects permeability and transport
• Why electrochemical gradients drive movement (especially ions)
• How receptors convert extracellular signals into intracellular responses

A common reasoning pattern is to infer what happens when a transporter is inhibited, when a receptor is overexpressed, or when a signaling pathway is disrupted. If you understand directionality and regulation, many questions become straightforward.

Genetics and gene expression

You will see questions that connect inheritance and molecular biology, often framed through experiments. Focus on:

• DNA replication, repair, and mutation consequences
• Transcription and translation regulation
• How changes in genotype can alter protein function and phenotype

Rather than memorizing every enzyme, emphasize what each step accomplishes and what goes wrong when it fails. For example, a frameshift mutation is not just a definition. It is a predictable disruption of downstream amino acid sequence and often protein function.

Physiology and homeostasis

Physiology questions reward conceptual clarity. If you can explain homeostatic feedback loops, you can reason through unfamiliar variants. Key themes include:

• Endocrine signaling and negative feedback
• Organ system roles in maintaining pH, osmolarity, and blood pressure
• Tissue-specific function tied to cellular specialization

Even when a passage focuses on molecular data, the questions may end by asking about organism-level impact, such as altered oxygen delivery or hormone response.

Biochemistry (25%): Molecular Function, Regulation, and Experimental Thinking

Biochemistry in this section is less about reciting pathways and more about understanding why molecules behave the way they do in biological contexts.

Proteins: structure determines function

Protein structure questions are high-yield because they unify multiple concepts. You should be able to reason about:

• How primary sequence influences folding and active site formation
• Effects of pH, temperature, and ionic strength on stability
• How mutations alter binding, catalysis, or regulation

If a passage describes a point mutation near an active site, the likely consequence is not “protein is changed,” but a specific shift in binding affinity, catalytic rate, or regulation.

Enzymes and kinetics

Enzyme questions often include graphs or described trends. Know the meaning of core kinetic parameters and how inhibitors change them. The most tested relationships include the Michaelis-Menten model:

$$v=\frac{V_{\max }[S]}{K_m+[S]}$$

You do not need to derive equations, but you do need to interpret them. $K_m$ is commonly treated as a proxy for substrate concentration at half-max velocity, and shifts in $K_m$ or $V_{\max }$ indicate different types of inhibition or altered enzyme performance.

Metabolism as regulated flow, not memorized maps

Metabolism appears frequently, but top performance comes from understanding regulation and energetic logic. Instead of brute memorization, learn:

• Which pathways generate reducing equivalents and ATP
• How cells switch fuel use in fed versus fasted states
• Where regulation points typically occur (rate-limiting steps, hormonal control)

If a passage shows decreased ATP in muscle or altered NADH levels, you should be able to propose plausible bottlenecks, not just name pathways.

Chemistry (10%): The “Quiet” Driver of Biological Behavior

Although chemistry is a smaller fraction here than in the dedicated chemical section, it still underpins many questions. The chemistry tested is usually the type that directly explains biological function.

Acid-base and pH in living systems

Expect questions where pH affects charge, binding, or activity. You should be comfortable with:

• Ionizable groups and how protonation changes interactions
• Buffering logic and why small pH shifts matter for proteins
• How environment influences solubility and membrane crossing

Many passage-based questions hinge on whether a molecule is neutral or charged at a given pH, which impacts diffusion, binding, and catalysis.

Noncovalent interactions and energetics

Hydrogen bonding, hydrophobic effects, and electrostatics are central to protein folding, DNA base pairing, and ligand binding. If you can explain what stabilizes a complex and what would disrupt it, you can often solve questions without heavy memorization.

Passage-Based Reasoning: Where Scores Are Won

The section is called “Foundations,” but the exam is designed to reward scientific reasoning. Many questions can be answered by carefully reading the passage and applying a small set of principles.

How to read like a scientist

Effective passage reading is selective, not exhaustive. Prioritize:

• The research question and hypothesis
• What was manipulated versus measured
• Controls and comparison groups
• Trends in figures (direction, magnitude, saturation, outliers)

If a figure shows diminishing returns with increasing substrate, that suggests saturation behavior. If a knockout animal shows a phenotype only under stress, that points to compensatory mechanisms.

Common experimental setups

Even without memorizing niche techniques, you should recognize what typical results imply. Questions often test whether you understand what data can and cannot conclude. For example, correlation does not establish mechanism, and a change in expression does not automatically mean causation unless the design supports it.

Practical Preparation Strategy That Matches the Section

To prepare efficiently, align your study time with the tested distribution and the section’s reasoning style.

Build a mechanism-first content base

For biology and biochemistry, prioritize concepts that generalize:

• Flow of information (DNA → RNA → protein)
• Energy and regulation (ATP, redox, rate-limiting steps)
• Structure-function relationships (proteins, membranes, nucleic acids)
• Homeostasis (feedback loops across systems)

When you learn a fact, attach it to a “why.” That reduces memorization load and improves transfer to new passages.

Train with passages early and often

Because high-yield content is embedded in experiments, you should practice passage sets alongside review. After each set, ask:

• What was the core mechanism?
• Which details were distractors?
• What did the figures prove, and what did they not prove?

This habit builds the ability to reason under uncertainty, which is the defining skill of this section.

Use errors to target weak links

Missed questions usually trace back to one of three gaps:

1. A missing foundational concept (for example, enzyme inhibition logic)
2. Misreading data (axis confusion, control group misunderstanding)
3. Overreaching beyond evidence (assuming causation without support)

Categorizing your mistakes this way helps you fix the real problem instead of just redoing questions.

What “High-Yield” Really Means Here

“High-yield” in Biological and Biochemical Foundations does not mean a short list of facts. It means a set of principles that appear repeatedly in different disguises. If you can explain how molecules interact, how cells regulate pathways, and how systems maintain balance, you can handle the majority of passages even when the topic feels unfamiliar.

This section rewards the mindset of applied science: learn the foundations, then practice reasoning until interpretation becomes routine. That is how content knowledge turns into points.