MCAT cell biology is the study of life’s basic unit — how the membrane controls what enters and leaves, how organelles divide labor, and how cells grow and divide. Because every physiological system is built from cells, this topic underpins the entire biological sciences section, from metabolism to signaling.
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update Updated October 2026
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bolt The 60-second version
check_circleThe cell membrane is a fluid mosaic of phospholipids with embedded proteins; hydrophilic heads face water, hydrophobic tails face inward.
check_circlePassive transport moves substances down their gradient without energy; active transport moves them against it using ATP.
check_circleOrganelles compartmentalize functions — mitochondria make ATP, ribosomes build proteins, the ER and Golgi process and ship them.
check_circleThe cell cycle is interphase (G₁, S, G₂) followed by the mitotic (M) phase.
check_circleCheckpoints ensure the cell only divides when DNA is intact and conditions are right.
check_circleEukaryotes have membrane-bound organelles and a nucleus; prokaryotes do not.
target What you'll be able to do · aligned to AAMC content category 2A
2ADescribe the fluid-mosaic membrane and how its structure controls permeability.
2AIdentify major organelles and relate each to its function.
2ADistinguish passive transport from active transport and their energy requirements.
2AOutline the phases of the cell cycle and the purpose of checkpoints.
How the MCAT tests cell biology
Cell biology anchors AAMC content category 2A — assemblies of molecules, cells, and groups of cells within multicellular organisms. The MCAT emphasizes the membrane and transport, organelle function, and the cell cycle, and it connects them to physiology and disease. Understanding beats memorization here.
workspace_premium The organizing idea
The cell is defined by compartments. The membrane separates inside from outside, and organelles create specialized microenvironments — so function follows which compartment a molecule is in.
The cell membrane
The plasma membrane is a fluid mosaic: a phospholipid bilayer studded with proteins. Its selective permeability is what lets a cell maintain an internal environment different from its surroundings.
The phospholipid bilayer. Hydrophilic phosphate heads face the watery interior and exterior; hydrophobic tails cluster inside, forming a barrier that embedded proteins span.
Amphipathic phospholipids spontaneously form the bilayer, with tails shielded from water.
Small nonpolar molecules (O₂, CO₂) cross freely; large or charged molecules need protein channels or carriers.
Cholesterol buffers fluidity, and membrane proteins handle transport, signaling, and adhesion.
Organelles and their functions
Eukaryotic cells divide labor among membrane-bound organelles. Knowing each organelle’s job lets you predict how a cell behaves.
Organelle
Function
Nucleus
Houses DNA; site of transcription
Mitochondria
ATP production (cellular respiration)
Ribosomes
Protein synthesis (translation)
Rough ER / Golgi
Protein processing, modification, and shipping
Lysosomes
Digestion and recycling
Prokaryotes lack these membrane-bound organelles and a true nucleus.
Membrane transport
Moving substances across the membrane is either free (down a gradient) or costs energy (up a gradient) — a distinction the MCAT tests constantly.
Passive vs active transport. Passive transport follows the concentration gradient and needs no energy; active transport pushes against the gradient and consumes ATP.
Passive: simple diffusion, facilitated diffusion (via proteins), and osmosis (water) — all move down the gradient, no ATP.
Active: pumps like the Na⁺/K⁺-ATPase move ions against their gradient using ATP.
Bulk transport: endocytosis and exocytosis move large materials in membrane vesicles.
The cell cycle
Cells grow and divide in an ordered cycle, most of which is spent in interphase preparing for a brief mitotic division.
The cell cycle. Interphase (G₁ growth, S DNA replication, G₂ preparation) occupies most of the cycle; the M phase carries out mitosis and cytokinesis.
S phase replicates the DNA so each daughter cell gets a full copy.
Mitosis (prophase → metaphase → anaphase → telophase) separates the chromosomes; cytokinesis splits the cytoplasm.
Checkpoints (notably G₁/S and G₂/M) verify DNA integrity; their failure underlies cancer.
Cells in context
These fundamentals scale up: tissues are organized cells, and the membrane transport you learn here drives nerve impulses and kidney function.
Membrane potentials and transport reappear in the nervous system, and cell division connects to genetics.
How to study it — and the traps to avoid
Cell biology rewards linking structure to function. Learn what each part does, not just its name.
Anchor everything to the membrane’s amphipathic structure.
Classify transport by direction (gradient) and energy (ATP or not).
Match each organelle to its job.
Know the cell-cycle phases and why checkpoints matter.
error Highest-yield trap
Facilitated diffusion is still passive. Because it uses a protein, students often assume it needs ATP — but it moves substances down their gradient, so it requires no energy. Only transport against the gradient (active transport) costs ATP.
Straight from the Progress Note question bank, each with a cited explanation. No account needed.
123
A red blood cell is exposed to a hypertonic solution. Which physicochemical process drives water efflux?
Answer: C. Water movement across membranes occurs by osmosis down its chemical potential gradient. Aquaporins facilitate passive diffusion; they do not use ATP.
Sodium–potassium ATPase maintains the membrane potential in many cells. Which transport stoichiometry is correct for one catalytic cycle?
Answer: A. The Na+/K+-ATPase extrudes three Na+ and imports two K+ per ATP hydrolyzed, generating an electrogenic current that helps maintain resting membrane potential and ionic gradients.
Source: Kaplan, J. H. (2002). Biochemistry of Na,K-ATPase. Annual Review of Biochemistry, 71, 511–535; updated perspectives in: Clausen, T. (2013). The Na+,K+ pump in muscle. Physiological Reviews, 93(1), 179–228.
Intestinal cells absorb glucose against its concentration gradient by coupling its uptake to Na⁺ moving down its electrochemical gradient (via SGLT). What type of transport is this, and what ultimately powers it?
Answer: A. Coupling glucose uptake to Na⁺ flowing down its gradient is secondary active transport: the carrier itself does not hydrolyze ATP, but the Na⁺ gradient it exploits is created and maintained by the Na⁺/K⁺ ATPase, which does use ATP — so the process is ultimately ATP-powered. B and D are wrong because glucose moves against its own gradient (requiring energy), C is incorrect because the glucose carrier does not directly split ATP, and E misdescribes the mechanism as osmosis. The sodium gradient, maintained by the pump, is the energy source.
What is the fluid-mosaic model of the membrane?expand_more
The fluid-mosaic model describes the cell membrane as a phospholipid bilayer (a fluid) with a mosaic of proteins embedded in and moving through it. The phospholipids’ hydrophilic heads face the aqueous environments while their hydrophobic tails face inward, creating a selectively permeable barrier.
What is the difference between passive and active transport?expand_more
Passive transport moves substances down their concentration gradient without energy, including simple diffusion, facilitated diffusion, and osmosis. Active transport moves substances against their gradient and requires energy, usually ATP, as in the sodium–potassium pump.
Is facilitated diffusion active or passive?expand_more
Facilitated diffusion is passive. Although it uses a membrane protein (a channel or carrier) to help large or charged molecules cross, it still moves them down their concentration gradient and requires no energy. Only movement against the gradient counts as active transport.
What are the phases of the cell cycle?expand_more
The cell cycle consists of interphase — G₁ (growth), S (DNA replication), and G₂ (preparation for division) — followed by the M phase, which includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). Most of the cycle is spent in interphase.
Why are cell-cycle checkpoints important?expand_more
Checkpoints, such as the G₁/S and G₂/M checkpoints, verify that conditions are right and DNA is undamaged before the cell proceeds. They prevent the propagation of DNA errors; when checkpoints fail, uncontrolled division can occur, which is central to the development of cancer.
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