Biology key terms

The net movement of particles from an area of higher concentration to an area of lower concentration, driven by random molecular motion and requiring no added energy.

The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration, until equilibrium or opposing pressure halts the flow.

Movement of ions or molecules across a membrane against their concentration gradient, powered by ATP hydrolysis or by coupling to another gradient such as sodium.

A protein or RNA catalyst that speeds a biochemical reaction by lowering its activation energy, typically binding substrates at a specific active site without being consumed.

Adenosine triphosphate, the cell's main energy currency; hydrolysis of its terminal phosphate bond releases usable free energy that drives endergonic reactions and transport.

The oxygen-dependent or anaerobic breakdown of organic molecules, chiefly glucose, into carbon dioxide and water, capturing released energy as ATP through glycolysis, the Krebs cycle, and oxidative phosphorylation.

The initial stage of respiration in which one glucose molecule is split in the cytoplasm into two pyruvate molecules, yielding a small net gain of ATP and reduced electron carriers.

An anaerobic pathway that regenerates oxidized electron carriers by transferring electrons to organic products such as lactate or ethanol, allowing glycolysis to continue without oxygen.

The light-driven conversion of carbon dioxide and water into organic compounds and oxygen, carried out in chloroplasts using pigments that absorb solar energy.

A green pigment in chloroplast thylakoid membranes that absorbs mainly red and blue light and donates excited electrons to the photosynthetic electron transport chain.

Nuclear division producing two genetically identical daughter nuclei with the same chromosome number as the parent cell, used for growth, repair, and asexual reproduction.

Two successive nuclear divisions that halve the chromosome number and generate genetically varied haploid gametes through crossing over and independent assortment of homologous chromosomes.

The ordered sequence of interphase growth and DNA synthesis followed by mitotic division, controlled by checkpoints that verify readiness before each major transition.

A lipid bilayer studded with proteins that permits some substances, such as small nonpolar molecules and water, to cross while restricting ions and large polar molecules.

A difference in the concentration of a dissolved substance between two adjacent regions, which stores potential energy that drives diffusion and powers secondary transport.

The relative ability of an external solution to cause water to enter or leave a cell, described as hypertonic, hypotonic, or isotonic relative to the cytoplasm.

The minimum energy input required for reactant molecules to reach a reactive transition state and proceed to product formation.

Loss of a protein's functional three-dimensional shape caused by heat, extreme pH, or chemical agents that disrupt hydrogen bonds and other stabilizing interactions.

One of two or more alternative versions of a gene at a given locus, which may produce different observable traits or different molecular variants of a product.

The specific combination of alleles an organism carries at one or more loci, which together with environmental influences helps determine its observable characteristics.

The observable or measurable characteristics of an organism, resulting from the interaction between its genotype and the environment it experiences.

Describes an allele whose effect is expressed in the phenotype even when only one copy is present, masking the observable effect of a recessive partner allele.

Exchange of corresponding chromosome segments between homologous chromosomes during prophase I of meiosis, creating new combinations of alleles on a single chromosome.

The random orientation of homologous chromosome pairs at meiosis I, so that alleles of genes on different chromosomes are distributed to gametes independently of one another.

A heritable change in the nucleotide sequence of DNA, ranging from a single base substitution to large deletions or insertions, and the ultimate source of new alleles.

Differential survival and reproduction of individuals whose heritable traits make them better suited to current environmental conditions, shifting allele frequencies across generations.

Random change in allele frequencies across generations, especially pronounced in small populations, independent of any effect of the alleles on survival or reproduction.

Transfer of alleles between populations through migration of individuals or dispersal of gametes, which tends to reduce genetic differences among populations.

The relative contribution an individual makes to the next generation's gene pool, often estimated as survival and reproductive success compared with other phenotypes.

The evolutionary splitting of one lineage into two or more reproductively isolated species, often initiated by geographic separation or by chromosomal and behavioral barriers.

Maintenance of relatively stable internal conditions, such as temperature, pH, and blood glucose, through negative feedback loops that detect and correct deviations.

A regulatory loop in which a change in a controlled variable triggers a response that counteracts the change, returning the system toward its set point.

A signaling molecule secreted by endocrine cells into the bloodstream, which acts on target cells bearing matching receptors to adjust their activity.

The relationship between an organism's or cell's outer exchange surface and its internal volume; as size increases, the ratio falls, limiting diffusion-based exchange.

The maximum population size an environment can sustain indefinitely given the available resources, beyond which growth slows or reverses.

Population increase at a constant per-capita rate, producing a J-shaped curve while resources remain abundant and no density-dependent limits apply.

A resource or condition, such as light, nitrogen, or predation, that is in shortest supply and therefore restricts growth, reproduction, or population size.

A feeding position in a food web, such as producer, primary consumer, or secondary consumer, through which energy and matter pass along a chain.

A diagram showing the decline in usable energy at each successive trophic level, typically retaining only about ten percent of energy from one level to the next.

The comparison group in an experiment that receives no treatment or a neutral treatment, establishing the baseline against which the experimental effect is judged.

The factor deliberately manipulated or selected by the investigator, whose effect on the measured outcome is being tested.

The outcome that is measured or observed in an experiment and is expected to respond to changes in the manipulated factor.

Any factor other than the independent variable that is held constant across treatment groups so it cannot confound the observed result.

A testable, falsifiable proposed explanation for an observation, stated precisely enough that predicted results can be compared with actual measurements.

The consistency of a measurement or experiment when repeated, reflecting how little random error affects the results rather than whether the results are correct.

The extent to which an experiment or measurement actually tests or represents the intended concept, free of confounding variables and systematic bias.

A statistic expressing the typical spread of data values around their mean, used to describe variability and to compare the overlap of treatment results.

The arithmetic average of a data set, calculated by summing all values and dividing by the number of observations; sensitive to extreme values.

The middle value of a data set when observations are ranked in order, a measure of central tendency that is resistant to extreme outliers.

The factor by which an image is enlarged relative to the actual specimen, calculated as image size divided by actual size.

The smallest distance at which two nearby points can still be distinguished as separate, determined mainly by the wavelength of illumination and the lens quality.

A stepwise series of dilutions in which each new solution is made from the previous one, producing a known concentration gradient for standards or plating.

Repeating an experiment or measurement multiple times, or across independent samples, to reduce the influence of random variation on conclusions.