1,800+ MCAT terms, defined.

Fluid balance is the regulation of body water and solute concentration so blood volume, pressure and osmolarity stay in a narrow range, mainly through the kidney. Testable point: high osmolarity raises ADH (more water reabsorbed); low bl...

The fundus is the dome-shaped upper part of the stomach, above the esophageal opening. More generally, it is the part of a hollow organ farthest from its opening (the uterus has one too). Testable point: acid-secreting glands lie mainly...

Heterozygous describes a genotype with two different alleles of a gene, one on each homologous chromosome (for example Aa). The most testable point: with complete dominance the dominant allele sets the phenotype, but incomplete dominance...

Membrane transport is the movement of substances across a membrane, passively down a gradient (diffusion, facilitated diffusion, osmosis) or actively against it with energy. Most testable: active transport moves solute against its gradie...

The pylorus is the narrow distal end of the stomach, ending in the pyloric sphincter, a smooth-muscle ring that meters chyme into the duodenum. Most testable: it is the stomach's exit, and emptying through it slows when fat or acid reach...

Thrombocytes are platelets: anucleate cell fragments budded from bone marrow megakaryocytes. They stick to damaged vessel walls and aggregate into a plug, and they support the clotting cascade that makes fibrin. Impaired platelets or clo...

The cortical reaction is the egg's release of cortical granule contents by exocytosis after a sperm fuses with it. The enzymes modify the zona pellucida (vitelline layer in sea urchins) so no more sperm can bind. It is the slow block to...

Hypermutation (somatic hypermutation) is a very high rate of point mutations in the variable-region genes of antibodies in activated B cells. Selection of the B cells whose mutated antibodies bind antigen more tightly produces affinity m...

The plasma membrane is a fluid mosaic: a phospholipid bilayer with embedded and surface proteins, cholesterol, and carbohydrates on the outer face. Its hydrophobic core makes it selectively permeable, so small nonpolar molecules cross re...

Merkel cells are touch-sensing cells in the deepest layer of the epidermis (stratum basale) that contact sensory neurons. They detect sustained light touch and fine texture, and they are slowly adapting, unlike the rapidly adapting Meiss...

Interphase is the part of the cell cycle between divisions, made of G1, S and G2, when the cell grows, carries out its normal functions and replicates its DNA. Most testable point: DNA is copied in S phase, so DNA content doubles but chr...

A spermatid is a haploid (n), round, immature male germ cell produced when a secondary spermatocyte completes meiosis II. It has no tail yet and differentiates into a mature sperm cell through spermiogenesis, with no further division.

The venae cavae are the superior and inferior vena cava, the large systemic veins that return deoxygenated blood to the right atrium. Veins are defined by direction (toward the heart), not oxygen content: the pulmonary veins, not the ven...

Calcium homeostasis is the negative-feedback control of blood Ca²⁺ within a narrow range. Low Ca²⁺ triggers parathyroid hormone (PTH), which raises it through bone, kidney and (via vitamin D) gut; high Ca²⁺ triggers calcitonin from the t...

Chemical digestion is the enzyme-catalyzed hydrolysis of food macromolecules into small, absorbable units. Most of it occurs in the small intestine. The testable point is matching each enzyme to its site and pH, such as pepsin in the aci...

Passive diffusion is the net movement of a solute directly across the lipid bilayer, down its concentration gradient, with no transport protein and no ATP. Its rate rises linearly with the gradient and does not saturate, unlike facilitat...

Smooth muscle contraction is the slow, usually involuntary shortening of nonstriated muscle. Ca²⁺ binds calmodulin, which activates myosin light-chain kinase to phosphorylate myosin, so it lacks the troponin step used in skeletal muscle.

Blood glucose control is the hormonal regulation of blood glucose within a narrow range by negative feedback. Insulin from pancreatic β cells lowers glucose after a meal, and glucagon from α cells raises it during fasting. Know the direc...

Cell junctions are specialized structures that connect neighboring animal cells or link a cell to the extracellular matrix. The three main types are tight junctions (seal and block paracellular leakage), desmosomes (anchor cells mechanic...

Gene mapping places genes in order along a chromosome and estimates the distances between them. On the MCAT it uses recombination frequencies from crosses: the more often crossing over separates two linked genes, the farther apart they a...

K+ channels are membrane proteins that let K+ diffuse down its gradient, usually out of the cell. Leak channels hold the resting potential near EK, and voltage-gated K+ channels open after depolarization to repolarize the neuron, so bloc...

Lysogenic describes a viral life cycle in which a phage's DNA joins the host genome as a prophage and is copied with it, without making virions or lysing the cell at that time. Stress can induce the prophage to enter the lytic cycle.

The mitotic spindle is the microtubule array that attaches to chromosomes at their kinetochores and separates them during mitosis. Its microtubules typically grow from centrosomes at opposite poles, and in anaphase the kinetochore microt...

An allergic reaction is an overreaction of the immune system to a harmless antigen (an allergen). In the classic immediate form, a first exposure makes IgE that coats mast cells, and re-exposure triggers histamine release. Some delayed a...

Chromosomal abnormalities are changes in chromosome number (aneuploidy such as trisomy or monosomy, usually from nondisjunction) or in structure (deletion, duplication, inversion, translocation). Most testable: nondisjunction in meiosis...

An endocrine gland is a ductless gland that secretes hormones into the surrounding fluid and blood, which carry them to distant target cells. Only cells with the matching receptor respond, so target specificity depends on receptors, not...

Gram staining is a differential stain that sorts bacteria by cell wall structure. Gram-positive cells have a thick peptidoglycan wall and stay purple; gram-negative cells have a thin wall plus an outer membrane and appear pink-red after...

A multipotent cell is a stem cell that can differentiate into several cell types, but generally only those of one tissue or lineage. The classic example is the hematopoietic stem cell, which gives rise to the blood cell types. Its potenc...

Vesicles are small membrane-bound sacs that carry cargo within a cell, into it or out of it. They bud from one membrane and fuse with another, so the bilayer is conserved and the vesicle interior stays equivalent to the extracellular spa...

An exocrine gland releases its secretion through a duct onto an epithelial surface or into a lumen, such as the skin or the gut, rather than into the blood. Most testable point: the pancreas is both. Acinar cells are exocrine (digestive...

G1 (Gap 1) is the first phase of interphase, after mitosis and before S phase. The cell grows and makes proteins and organelles. The testable point is the G1 checkpoint (restriction point): p53 and Rb normally block entry into S phase if...

Creatine phosphate is a high-energy phosphate store in muscle cells that rapidly regenerates ATP by giving its phosphate group to ADP. It is the fastest ATP source and needs no oxygen, but it runs out within seconds, so glycolysis and ae...

Mechanical digestion is the physical breakdown of food into smaller pieces without breaking covalent bonds. It includes chewing, stomach churning and segmentation in the small intestine. The testable point is that it increases surface ar...

Allele frequency is the fraction of all copies of a gene in a population that are a given allele. The MCAT usually finds it with Hardy-Weinberg (p + q = 1), starting from the recessive phenotype: q = √q². This holds only if the populatio...

The autonomic nervous system controls involuntary effectors (smooth muscle, cardiac muscle, glands) through the sympathetic (fight or flight) and parasympathetic (rest and digest) divisions. Most organs get both, and the two usually have...

Bacterial genetics covers how bacteria store, express, and exchange genes: usually one circular chromosome plus optional plasmids. Variation comes from mutation and horizontal transfer (transformation, transduction, conjugation), not mei...

A bacterial growth curve plots the log of viable cell number against time in a closed batch culture. It has four phases: lag, exponential (log), stationary, and death. Most testable: in exponential phase, binary fission gives a constant...

Bone remodelling is the continual turnover of bone: osteoclasts resorb old matrix and osteoblasts deposit new matrix. It repairs bone, adapts it to load and exchanges calcium with blood. PTH raises blood Ca²⁺ by promoting resorption; cal...

Capillary exchange is the movement of gases, nutrients, wastes and fluid between capillary blood and interstitial fluid. Solutes diffuse down gradients, while fluid shifts with the balance of hydrostatic pressure (out) and oncotic pressu...

Cell signalling is how a cell detects an extracellular signal (a ligand) and turns it into a response: reception, transduction, response. Most testable: water-soluble ligands typically bind surface receptors and use second messengers lik...

A demyelinating disease damages the myelin sheath around axons, so action potentials conduct more slowly or fail to propagate. Multiple sclerosis (loss of CNS myelin made by oligodendrocytes) is the classic example. The testable point is...

A dihybrid test cross mates an AaBb individual with a homozygous recessive (aabb) to reveal the gametes the AaBb parent makes. Independent assortment gives about 1:1:1:1 offspring; mostly parental types signal linked genes, and the recom...

Early embryogenesis is the sequence from fertilization through cleavage, morula, blastocyst and gastrulation, ending in three germ layers (ectoderm, mesoderm, endoderm). Most testable: cleavage makes smaller cells without growth, and eac...

The endomembrane system is the network of eukaryotic membrane compartments (nuclear envelope, ER, Golgi, lysosomes, vesicles, often the plasma membrane) linked by continuity or vesicle traffic. Most testable: secreted proteins move rough...

Evolutionary fitness is an individual's or genotype's reproductive success: the number of offspring that survive and reproduce, relative to others in the population. It is not strength or lifespan. Inclusive fitness also counts relatives...

Gene flow is the transfer of alleles between populations when migrants (or gametes, such as pollen) breed in the new population. It changes allele frequencies and tends to make populations more alike. Reduced gene flow lets populations d...

The Hardy-Weinberg principle describes a population that is not evolving: with a large population, random mating, and no mutation, migration or selection, allele and genotype frequencies stay constant. Its equations are p + q = 1 and p²...

Host range is the set of species, and often the cell types, that a particular virus can infect. It depends mainly on whether viral surface proteins can bind receptors on the host cell, so a change in either one can widen or narrow the ra...

Hypothalamic-pituitary axes are hormone cascades: the hypothalamus releases hormones that control the anterior pituitary, which releases tropic hormones that act on a target gland. The most testable point is that the target gland's hormo...

A kinase cascade is a signaling pathway in which each kinase phosphorylates and activates the next, so one signal is amplified into a large response. Testable point: blocking one kinase stops the steps downstream of it, while a constitut...