1,800+ MCAT terms, defined.

Microtubules are hollow, polar cytoskeletal tubes made of α- and β-tubulin dimers. They form the mitotic spindle, the core of cilia and flagella, and tracks for motor proteins. Most testable: kinesin usually walks toward the plus end, dy...

Absorption is the movement of digested nutrients, water, and ions from the gut lumen across the intestinal epithelium into blood or lymph, mostly in the small intestine. Testable point: sugars and amino acids go to capillaries, but most...

The CNS (central nervous system) is the brain and spinal cord, which integrate sensory input and generate motor output. Everything outside them is the peripheral nervous system (PNS). The autonomic and somatic systems are PNS divisions,...

The lymphatic system is a one-way network of vessels, nodes, and organs that returns leftover interstitial fluid (lymph) to the blood, carries dietary fat absorbed by intestinal lacteals, and houses lymphocytes. Testable point: blocked l...

Membrane potential is the voltage across a cell membrane (inside relative to outside), about −70 mV in a resting neuron. It comes from ion gradients plus selective permeability, mainly K⁺ leak channels. Testable: it shifts toward the equ...

Secretion is the export of a cell-made product, such as a hormone, enzyme, or mucus, usually by exocytosis. In the kidney, it means moving solutes from blood into the filtrate. Either way, material is added to a lumen or extracellular fl...

A virus is an acellular infectious particle: a DNA or RNA genome in a protein capsid, sometimes with a lipid envelope. It is an obligate intracellular parasite that typically needs host ribosomes and enzymes to replicate. Testable point:...

Na⁺ is the sodium ion, the main extracellular cation. The Na⁺/K⁺ ATPase keeps it high outside cells and low inside (3 Na⁺ out, 2 K⁺ in per cycle). Most testable: Na⁺ flowing in down this gradient through voltage-gated channels causes the...

O₂ is molecular oxygen, the final electron acceptor of the electron transport chain, where it is reduced to H₂O. Without it the chain stalls, ATP yield drops, and cells shift toward glycolysis and fermentation. Blood carries it on hemogl...

Genetic drift is a random change in allele frequencies from one generation to the next due to chance sampling of gametes and survivors. Its effects are strongest in small populations, and it tends to reduce genetic variation, potentially...

Skeletal muscle is striated, voluntary, multinucleated muscle attached to bones and controlled by somatic motor neurons. It contracts by the sliding filament mechanism: Ca²⁺ binds troponin, and myosin pulls actin, shortening the sarcomer...

A G-protein coupled receptor is a seven-transmembrane receptor that, when its ligand binds, activates an associated G protein by exchanging GDP for GTP on the α subunit. Testable point: Gα-GTP regulates an effector (adenylyl cyclase or p...

An antigen is a molecule, often a protein or polysaccharide on a pathogen or foreign cell, that is specifically bound by an antibody or a lymphocyte receptor and can trigger an adaptive immune response. Testable point: the antibody binds...

Natural selection is the differential survival and reproduction of individuals with heritable traits, so alleles that improve fitness become more common in a population over generations. Individuals do not evolve; populations do. Selecti...

Recombination frequency is the percentage of offspring with a new allele combination, not seen in either parent, produced by crossing over between two linked genes. The farther apart the genes, the higher the frequency, and it does not e...

A reflex arc is the neural pathway of a reflex: receptor, sensory (afferent) neuron, integration center, motor (efferent) neuron, effector. Spinal reflexes are integrated in the spinal cord, so the response can occur without input from t...

Reabsorption is the return of water and solutes from the nephron's filtrate to the blood through the peritubular capillaries. Most Na⁺ and water, and nearly all glucose, are reclaimed in the proximal tubule, while ADH and aldosterone fin...

Gastrulation is the embryonic stage in which the blastula's cells move and rearrange into three primary germ layers: ectoderm, mesoderm, and endoderm. It also forms the primitive gut (archenteron). The most testable point is which adult...

Bile is a fluid made by the liver, stored in the gallbladder, and released into the duodenum, where its bile salts emulsify fats. It increases the surface area for lipase but is not an enzyme and does not hydrolyze fats itself.

Cleavage is the series of rapid mitotic divisions of the zygote after fertilization, with little or no growth between divisions. Cell number rises while individual cells shrink, so the embryo's overall size stays roughly constant as it b...

The cell membrane (plasma membrane) is a selectively permeable phospholipid bilayer with embedded proteins that separates the cell from its surroundings. Small nonpolar molecules cross it by simple diffusion, while ions and large polar m...

A phenotype is an organism's observable or measurable trait, produced by its genotype together with the environment. Testable point: different genotypes can give the same phenotype (AA and Aa under complete dominance), so phenotype does...

Incomplete dominance is when a heterozygote shows a phenotype intermediate between the two homozygotes, because neither allele fully masks the other. The classic case is red × white flowers giving pink, with a 1:2:1 phenotypic ratio in t...

The peripheral nervous system (PNS) is all nervous tissue outside the brain and spinal cord (the CNS): the nerves and ganglia. Sensory neurons are afferent (toward the CNS) and motor neurons are efferent (away), and the motor part divide...

Pancreatic juice is the exocrine secretion of the pancreas, sent through the pancreatic duct into the duodenum. It contains bicarbonate, which neutralizes acidic chyme, and digestive enzymes (amylase, lipase, nucleases, proteases). Prote...

GPCRs (G protein-coupled receptors) are seven-transmembrane-helix receptors that, when a ligand binds, activate an associated G protein by swapping GDP for GTP. The G protein then changes an effector such as adenylyl cyclase or phospholi...

Contraction is force generation in muscle by the sliding filament mechanism: myosin heads bind actin, perform a power stroke, and pull thin filaments toward the sarcomere center. Ca²⁺ turns it on and ATP is needed to detach myosin. Filam...

The plasma membrane is the phospholipid bilayer, with embedded proteins and cholesterol, that surrounds a cell and is selectively permeable. Small nonpolar molecules cross it by simple diffusion, while ions and large polar molecules usua...

A synapse is the junction where a neuron passes a signal to another neuron, a muscle cell, or a gland cell. At a chemical synapse, an action potential triggers Ca²⁺ entry, vesicle release of neurotransmitter, and receptor binding on the...

Gap junctions are protein channels that join the cytoplasm of adjacent animal cells, letting ions and small molecules pass directly between them. The testable point is electrical coupling: they let depolarization spread cell to cell, as...

The axon is a neuron's long projection that conducts action potentials away from the cell body toward the axon terminals. Myelin speeds conduction because the signal regenerates at the nodes of Ranvier (saltatory conduction), and a wider...

A dominant allele is expressed in the phenotype of a heterozygote, masking the recessive allele, so Aa looks like AA. Dominance describes expression, not how common or fit an allele is. Aa × Aa gives a 3:1 dominant to recessive phenotype...

A species is a group of organisms that can interbreed in nature and produce viable, fertile offspring, and that is reproductively isolated from other such groups (the biological species concept). Key test point: a mule shows that mating...

The pharynx is the muscular throat tube behind the nasal and oral cavities, shared by the respiratory and digestive tracts. Air passes on to the larynx and trachea, and food passes to the esophagus. The testable point is how swallowing k...

The centromere is the constricted region of a chromosome where sister chromatids are joined and where the kinetochore forms for spindle attachment. Chromosomes are counted by centromeres, so sisters become separate chromosomes only when...

Endocrine signaling is when ductless glands release hormones into the blood, which carry them to distant target cells that have the matching receptor. Most testable point: endocrine signals travel through the bloodstream, unlike exocrine...

A gene is a segment of DNA carrying the information for a functional product, usually a protein but sometimes a functional RNA such as tRNA or rRNA. Most testable: a gene is transcribed into RNA, and in eukaryotes introns are spliced out...

The immune system is the network of cells, tissues, and molecules that detects and destroys pathogens and abnormal cells. It has a fast, nonspecific innate arm and a slower, specific adaptive arm; adaptive memory cells make a second expo...

Somatic describes the body's non-reproductive cells, as opposed to germ-line cells that give rise to gametes. Most somatic cells are diploid (2n) and divide by mitosis, so a somatic mutation is not passed to offspring through gametes.

The sympathetic nervous system is the autonomic division that drives fight-or-flight responses, such as faster heart rate and slower digestion. Its fibers have short preganglionic and long postganglionic neurons, and most postganglionic...

Metabolism is the sum of all enzyme-catalyzed reactions in a cell or organism. Catabolism breaks molecules down and captures energy as ATP and reduced carriers; anabolism builds molecules and uses that energy. Pathways are linked by ATP...

A sperm is the mature male gamete: a haploid (n) cell with a head (nucleus and acrosome), a mitochondria-rich midpiece and a flagellum. It is made by spermatogenesis in the seminiferous tubules, where one primary spermatocyte (2n) yields...

Gas exchange is the passive diffusion of O₂ into the blood and CO₂ out of it across a thin, moist respiratory surface such as alveoli, gills or tracheae. Each gas moves down its own partial pressure gradient, and the rate rises with surf...

An allele is one alternative version of a gene, found at the same locus on homologous chromosomes. A diploid individual has two alleles per gene, and the pair (genotype), together with dominance relationships, determines the phenotype.

A filament is a long, thin polymer of protein subunits. The MCAT uses it for cytoskeletal microfilaments (actin) and intermediate filaments (keratin, lamins), and for the thin (actin) and thick (myosin) filaments of muscle, which slide p...

Cilia are short, hair-like projections of eukaryotic cells with a core of microtubules in a 9+2 pattern. Dynein arms slide the microtubules to bend the cilium, so motile cilia beat to move fluid or mucus. Many cells also have a nonmotile...

Red blood cells (erythrocytes) carry O₂ on hemoglobin. Mature mammalian RBCs have no nucleus or mitochondria, so they make ATP by glycolysis alone and do not use the O₂ they carry. Low O₂ delivery raises erythropoietin, which increases R...

Organelles are specialized compartments inside a cell, each with a specific job. Most eukaryotic organelles are membrane-bound, which lets incompatible reactions run in separate conditions. Most testable: match each organelle to its func...

Cell differentiation is the process by which a less specialized cell becomes a specialized cell type with its own structure and function. It results from differential gene expression, and in most cells the DNA sequence itself stays the s...

The sarcolemma is the plasma membrane of a muscle fiber. It carries the action potential across the cell surface and into the T-tubules, which leads the sarcoplasmic reticulum to release Ca²⁺ and the fiber to contract.