1,800+ MCAT terms, defined.

Inhibitory describes a synaptic input that makes the postsynaptic cell less likely to fire an action potential, typically by hyperpolarizing it (an IPSP) through Cl⁻ influx or K⁺ efflux. Whether a transmitter is inhibitory depends on the...

Cytokines are small secreted signaling proteins, made largely by immune cells, that bind receptors on target cells to change their behavior. The most testable point: they coordinate immune responses (interleukins, interferons, TNF), usua...

A genotype is the specific combination of alleles an individual carries for a gene, such as AA, Aa, or aa. It differs from the phenotype, the observable trait. Different genotypes can produce the same phenotype, as when AA and Aa both sh...

Viroids are infectious agents made of a short, circular, single-stranded RNA with no protein coat. They mainly infect plants and do not typically encode proteins. Key contrast: viroids are naked RNA, prions are infectious protein, and vi...

Tight junctions are belts of proteins (claudins, occludin) that seal neighboring epithelial cells near their apical surface. They restrict paracellular movement, so substances mostly cross through the cells instead of between them. The s...

Germ layers are the three embryonic tissue layers (ectoderm, mesoderm, endoderm) formed at gastrulation. Most testable: nervous system and epidermis come from ectoderm, muscle, bone and blood from mesoderm, and gut and respiratory lining...

Evolution is a change in the allele frequencies of a population over generations. Natural selection is one cause, alongside genetic drift, mutation, gene flow and nonrandom mating. Populations evolve; individuals do not.

Codominance is an inheritance pattern in which both alleles are fully expressed in a heterozygote, so both traits show distinctly rather than blending. The classic case is ABO type AB (IAIB). The testable contrast is with incomplete domi...

Binary fission is the asexual reproduction of prokaryotes: the cell replicates its circular chromosome, elongates, and splits into two genetically identical daughter cells. Because it makes clones, variation in bacteria comes mainly from...

Centrioles are short cylinders of nine microtubule triplets, found in pairs inside the centrosome of animal cells. The centrosome organizes the spindle during cell division, and centrioles also form the basal bodies of cilia and flagella...

Ligaments are bands of dense regular connective tissue, rich in collagen, that connect bone to bone across a joint and stabilize it. The classic MCAT contrast is that ligaments join bone to bone, while tendons join muscle to bone.

HCl is the strong acid secreted by parietal cells of the stomach, giving gastric juice a pH of about 1.5–2. It denatures proteins, kills many microbes, and activates pepsinogen to pepsin, an enzyme that works best at low pH.

A recessive allele's trait is masked when a dominant allele is present (under complete dominance), so it shows in homozygous recessive individuals (aa), or in males for X-linked genes. Testable: Aa × Aa carrier parents give 1/4 affected...

A steroid hormone is a lipid-soluble hormone made from cholesterol, such as cortisol, aldosterone, estrogen, progesterone, and testosterone. It typically crosses the cell membrane, binds an intracellular receptor, and changes gene transc...

Anaerobic metabolism makes ATP without oxygen, typically by glycolysis followed by fermentation (lactate in animal cells, ethanol and CO₂ in yeast). Fermentation regenerates NAD⁺ so glycolysis can continue, giving a net 2 ATP per glucose.

Anaphase is the stage of mitosis and meiosis II in which sister chromatids separate and are pulled to opposite poles of the cell. The key point is that in meiosis I, anaphase I separates homologous chromosomes while sister chromatids sta...

Leukocytes are white blood cells, nucleated cells made from stem cells in bone marrow that defend the body. The most testable point is sorting them: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monoc...

Blood cells are the cellular part of blood: red blood cells (erythrocytes) carry O₂, white blood cells (leukocytes) defend against pathogens, and platelets (cell fragments) aid clotting. A top testable point: mature mammalian red cells l...

Excitatory describes input that makes a postsynaptic neuron more likely to fire, usually by depolarizing it toward threshold (an EPSP). Whether it fires depends on summation of excitatory and inhibitory inputs at the axon hillock, and th...

Thermoregulation is the maintenance of a fairly stable core body temperature by negative feedback, with the hypothalamus as the integrating center. Most testable: heat is lost by skin vasodilation and sweating, and conserved by vasoconst...

Segregation is Mendel's first law: the two alleles of a gene separate during gamete formation, so each gamete receives one. Its physical basis is homologous chromosomes separating in meiosis I. Most testable: Aa × Aa gives 1:2:1 genotype...

Meissner's corpuscle is an encapsulated mechanoreceptor in the upper dermis of hairless (glabrous) skin, such as fingertips and lips. It detects light touch and low-frequency vibration. It adapts rapidly, so it signals changes in touch,...

Mendelian inheritance is the pattern of single-gene traits whose alleles segregate into separate gametes and, for unlinked genes, assort independently. Most testable: with complete dominance, Aa × Aa gives a 3:1 phenotype ratio and a 1:2...

A reporter gene encodes an easily detected product, such as GFP, luciferase, or β-galactosidase (lacZ). It is linked to a promoter or to a gene of interest to show when and where that gene is expressed. The signal reflects the activity o...

The skeletal system is the bones, cartilage, and joining ligaments that support the body, protect organs, give muscles levers for movement, store calcium and phosphate, and house marrow. Testable point: bone is living, remodeled tissue t...

The thyroid is a neck endocrine gland that secretes T3 and T4, which raise basal metabolic rate, and calcitonin, which lowers blood calcium. Most testable: the TRH → TSH → T3/T4 negative feedback loop, where low T4 with high TSH means a...

The lagging strand is the new DNA strand made discontinuously during replication, as short Okazaki fragments. DNA polymerase extends only a 3′ end, so on this strand it works away from the fork. Each fragment needs its own RNA primer, an...

Lipase is a hydrolase that cleaves the ester bonds of triglycerides, releasing free fatty acids and monoglycerides or glycerol. Most testable: pancreatic lipase acts in the small intestine on fat droplets emulsified by bile salts, so blo...

The immune response is the body's coordinated defense against pathogens and other foreign antigens. It combines a fast, broad innate arm with a slower, antigen-specific adaptive arm. Most testable: memory cells make a second exposure to...

Venules are the smallest veins: thin-walled vessels that collect blood from capillaries and merge into larger veins. Pressure in them is low and they are very compliant, so venules and veins act as a blood reservoir. They are named by di...

A pathogen is an infectious agent, such as a bacterium, virus, fungus, protozoan, or parasitic worm, that causes disease in a host. The MCAT tests whether you can identify the type of pathogen and predict which immune response or treatme...

Ruffini endings are slowly adapting mechanoreceptors in the deep dermis that respond to skin stretch and sustained pressure. The key point is that they keep firing during a steady stimulus, unlike rapidly adapting Meissner and Pacinian c...

Intercalated discs are the junctions between end-to-end cardiac muscle cells, containing desmosomes for mechanical adhesion and gap junctions for ion flow. The gap junctions let depolarization spread quickly, so the heart contracts as a...

White blood cells (leukocytes) are nucleated blood cells that defend the body and arise from stem cells in bone marrow. Neutrophils, macrophages and other phagocytes act mainly in innate immunity, while B and T lymphocytes carry out adap...

Pacinian corpuscles are encapsulated mechanoreceptors deep in the skin that detect vibration and deep pressure. They are rapidly adapting, so they fire at the start and end of a stimulus but very little during steady pressure.

Intermediate filaments are cytoskeletal fibers made of proteins such as keratin, vimentin, neurofilaments and lamins. They give cells mechanical strength and anchor structures like the nucleus. Most testable: they are structural and comp...

A genome is the complete set of genetic material of a cell, organism or virus: genes plus noncoding DNA (RNA in some viruses). Most somatic cells of one individual share the same genome, so cell types differ mainly through differential g...

Chyme is the acidic, semi-liquid mixture of partially digested food and gastric juice that leaves the stomach through the pyloric sphincter. Its acid in the duodenum triggers secretin, so the pancreas releases bicarbonate to neutralize it.

Prophase is the first stage of mitosis, when chromatin condenses into visible chromosomes and the spindle begins to form. Texts differ on whether nuclear envelope breakdown ends prophase or starts prometaphase. Testable point: homologs p...

Inhalation is the movement of air into the lungs, driven by muscles that enlarge the chest cavity and lower alveolar pressure below atmospheric pressure. The MCAT tests it in Biology, not as a separate Psychology or Sociology concept. Ke...

The stratum basale is the deepest layer of the epidermis, where stem-like keratinocytes divide by mitosis to replace cells shed from the surface. It also contains melanocytes and Merkel cells. The testable point: it is the epidermal laye...

The stratum spinosum is the "spiny" layer of the epidermis, lying between the stratum basale and the stratum granulosum. Its living keratinocytes are linked by many desmosomes, which give the layer mechanical strength. The usual test poi...

Retroviruses (such as HIV) are enveloped RNA viruses that carry reverse transcriptase, which copies their RNA genome into DNA. That DNA integrates into the host chromosome as a provirus. The testable point is the reversed flow of informa...

Immunity is the body's resistance to pathogens and foreign material, built from fast, nonspecific innate defenses and slower, antigen-specific adaptive responses. The most testable point is that memory, meaning a faster and stronger seco...

Heritability is the share of variation in a trait, within a given population and environment, that is due to genetic differences (all genetic variance in broad-sense, additive only in narrow-sense). It is not how genetic a trait is in on...

A peptide hormone is a hormone made of amino acids, such as insulin, glucagon, or ADH. Being polar, it generally cannot cross the cell membrane, so it binds a surface receptor and acts through second messengers like cAMP. That contrasts...

Complete dominance means one allele fully masks the other in a heterozygote, so Aa looks the same as AA. The most testable point: an Aa × Aa cross typically gives a 3:1 dominant to recessive phenotype ratio and a 1:2:1 genotype ratio.

Cytokinesis is the division of the cytoplasm and plasma membrane that splits one cell into two daughter cells, usually beginning in late anaphase or telophase. Animal cells pinch in two with an actin–myosin contractile ring; plant cells...

Senescence is the permanent loss of a living cell's ability to divide, usually after a limited number of divisions as telomeres shorten (the Hayflick limit), or after DNA damage. Testable point: telomerase can delay it, and most cancer c...

The parathyroid glands are small endocrine glands, usually four, on the back of the thyroid. They release parathyroid hormone (PTH) when blood Ca²⁺ is low, and PTH raises blood calcium through bone, kidney, and (via vitamin D) intestine....