Biology

Biology Glossary

Every term used across all 51 Biology tracks, defined and alphabetized. Search it, or jump by letter.

424 terms

2 5 A B C D E F G H I K L M N O P Q R S T U V W X

2

2^n gamete combinations: The formula describing how many genetically distinct gametes an organism can produce through independent assortment alone, where n is the number of chromosome pairs. Humans, with 23 pairs, can produce 2^23 (over 8 million) genetically distinct gametes from independent assortment alone, before crossing over adds even more variation.

5

5' cap: A modified nucleotide added to the front (5') end of a new mRNA transcript during processing, which protects the mRNA from degradation and helps it get recognized by the ribosome later.

A

Abiotic synthesis: The formation of simple organic molecules through ordinary chemical reactions, without the involvement of living organisms, thought to have occurred on early Earth and contributed to the origin of life.
Acetyl-CoA: The two-carbon molecule formed when pyruvate is oxidized and loses one carbon as CO2, just before entering the Krebs cycle.
Acid: A substance that increases the H+ concentration of a solution by donating hydrogen ions, lowering the pH.
Actin: The thin filament protein in a sarcomere that myosin heads bind to and pull on during muscle contraction.
Action potential: A rapid, all-or-nothing reversal of a neuron's membrane potential (from negative to positive and back) that occurs once a stimulus depolarizes the membrane past a threshold, allowing a nerve signal to travel down the axon.
Activation energy: The minimum amount of energy required to get a chemical reaction started, essentially the energy 'hill' that reactants must climb before they can be converted into products.
Active site: The specific region on an enzyme's surface where the substrate binds and the chemical reaction is catalyzed.
Active transport: The movement of a substance across a membrane against its concentration gradient (from low to high concentration), which requires the cell to spend ATP energy, since it goes against the natural direction of diffusion.
Adaptive immunity: The immune system's slower, highly specific response, which learns to recognize a particular pathogen and, unlike innate immunity, creates lasting memory that allows a faster response on re-exposure.
Adaptive radiation: A rapid burst of diversification in which many new species evolve from a common ancestor to fill a wide range of newly available ecological niches, often following a mass extinction.
ADH (antidiuretic hormone): A hormone that increases the permeability of the kidney's collecting duct to water, allowing more water to be reabsorbed back into the blood, which conserves water and produces more concentrated urine when the body is dehydrated.
Adhesion: The tendency of water molecules to stick to other polar surfaces (not just to each other), which allows water to climb upward through narrow tubes against gravity, like in a plant's xylem.
Aldosterone: A hormone that increases sodium reabsorption in the kidney's tubules (with water following osmotically), helping regulate overall blood volume and pressure.
Allele: One of two or more alternate versions of a gene that can exist at a particular location on a chromosome, producing different versions of a trait.
Allele frequency: The proportion of a specific allele among all the alleles for a gene present in a population's gene pool.
Allele frequency equation (p + q = 1): The equation stating that, for a gene with two alleles, the frequency of the dominant allele (p) and the frequency of the recessive allele (q) must together account for the entire population's alleles at that gene.
Allopatric speciation: Speciation that occurs when a physical/geographic barrier separates a population into two groups that can no longer interbreed, allowing each to evolve independently.
Altruism: A behavior in which an individual helps another individual at some cost to itself (in terms of survival or reproductive success).
Alveoli: Tiny, thin-walled, balloon-like air sacs in the lungs where gas exchange occurs — oxygen diffuses into the blood and carbon dioxide diffuses out to be exhaled.
Amino acid: The building-block monomer of proteins, consisting of a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable side chain (R group) that gives each of the 20 amino acids its unique chemical properties.
Amino group (-NH2): A functional group that acts as a base, capable of accepting a hydrogen ion; found in amino acids, where it gives them part of their acid-base behavior.
Amylase: A digestive enzyme, present in saliva and produced by the pancreas, that begins breaking down starch into smaller sugar units.
Analogous structure (convergent evolution): An anatomical structure that is similar in function between two species but did not arise from a shared common ancestor -- instead evolving independently in each lineage because both faced similar environmental pressures (convergent evolution). A bird's wing and an insect's wing are analogous, not homologous.
Anaphase: The phase of mitosis in which sister chromatids are pulled apart and move toward opposite poles of the cell.
Angiosperm: A group of seed plants, also called flowering plants, whose seeds are enclosed within a fruit that develops from the flower's ovary.
Antagonistic hormone pair: Two hormones that have opposite effects on the same physiological variable, working together through negative feedback to keep that variable within a stable range — insulin/glucagon (blood glucose) and parathyroid hormone/calcitonin (blood calcium) are classic examples.
Antibody: A Y-shaped protein produced by plasma cells (differentiated B cells) that binds specifically to a particular antigen, tagging it for destruction or directly neutralizing it.
Anticodon: The three-nucleotide sequence on a tRNA molecule that is complementary to, and base-pairs with, a specific mRNA codon, ensuring the correct amino acid is added at each step of translation.
Antiparallel: Describes the orientation of DNA's two strands, which run in opposite directions relative to each other (one 5' to 3', the other 3' to 5') — a structural feature that has direct consequences for how replication has to proceed.
Apoptosis: Programmed cell death -- an orderly, controlled process by which a cell actively dismantles itself, often triggered as a cellular response to irreparable damage or as a normal part of development. It's a common downstream outcome of a signal transduction pathway, and its failure is a key step in cancer development (Track 22).
Archaea: One of the three domains of life, made up of prokaryotic (no-nucleus) organisms that are biochemically distinct from bacteria, including differences in membrane lipid structure; many archaea are extremophiles that thrive in extreme heat, salt, or acidity.
ATP synthase: The membrane-embedded enzyme that generates ATP by allowing protons to flow back down their gradient through it, using that flow like a turbine to physically drive the synthesis of ATP from ADP and phosphate.
Autocrine signaling: A form of cell signaling in which a cell releases a signaling molecule that binds to receptors on that same cell, affecting its own behavior.
Autosomal: Describes a gene located on one of the non-sex chromosomes (autosomes), meaning its inheritance pattern doesn't depend on an individual's sex.
Auxin: A plant hormone that promotes cell elongation and is a key driver of tropisms, particularly phototropism, by accumulating more on the shaded side of a stem and causing it to elongate faster, bending the plant toward light.

B

B cell: A type of adaptive immune cell that, once activated by a matching antigen, proliferates via clonal selection and differentiates into plasma cells that mass-produce antibodies specific to that antigen.
Bacteriophage: A virus that specifically infects bacteria, often used as a model system for studying the lytic and lysogenic cycles.
Base: A substance that decreases the H+ concentration of a solution, either by accepting hydrogen ions or by donating hydroxide ions (OH-), raising the pH.
Bile: A fluid produced by the liver (and stored in the gallbladder) that breaks large fat droplets into smaller ones, increasing their surface area so digestive enzymes can act on them more effectively.
Binary fission: The method of asexual reproduction used by prokaryotes (bacteria and archaea), in which a single cell copies its DNA and divides into two genetically identical daughter cells.
Biodiversity: The variety of life in a given area, encompassing genetic diversity within species, diversity of species, and diversity of ecosystems.
Biogeochemical cycle: The pathway by which a chemical element or molecule (like nitrogen, carbon, or phosphorus) moves through the living and non-living parts of an ecosystem.
Blastula: An early stage of animal embryonic development consisting of a hollow ball of cells, produced by cleavage, immediately preceding gastrulation.
Bohr effect: The phenomenon in which more acidic, warmer, and higher-CO2 conditions (as found in actively metabolizing tissue) cause hemoglobin to release oxygen more readily — ensuring that oxygen is delivered preferentially to the tissues that need it most.
Bottleneck effect: A dramatic, sudden reduction in a population's size (due to a disaster, disease, or other event) that leaves the survivors' allele frequencies a matter of chance rather than a representative sample of the original population -- a specific form of genetic drift with an especially strong effect because so few individuals remain.
Buffer: A chemical system that resists changes in pH by absorbing excess H+ when acid is added or releasing H+ when base is added, keeping the solution's pH relatively stable. The bicarbonate buffer system keeps human blood pH steady around 7.4.

C

C4 and CAM photosynthesis: Two adaptations that reduce photorespiration in hot or dry climates. C4 plants (like corn) spatially separate initial carbon fixation from the Calvin cycle, concentrating CO2 before it reaches RuBisCO. CAM plants (like cacti) separate the two steps by time instead, fixing carbon at night when stomata can open without losing much water, then running the Calvin cycle during the day with stomata closed.
Calvin cycle: The light-independent reactions of photosynthesis, occurring in the chloroplast's stroma, in which CO2 is fixed (captured) and built into organic sugar molecules using the ATP and NADPH generated by the light-dependent reactions.
Capillary: The smallest type of blood vessel, with walls thin enough for direct diffusion of gases, nutrients, and waste between the blood and surrounding tissue.
Capsid: The protein shell that encases and protects a virus's genetic material, built from repeating protein subunits.
Carbon cycle: The biogeochemical cycle by which carbon moves between the atmosphere (as CO2), living organisms (via photosynthesis and respiration), and long-term storage reservoirs like fossil fuels.
Carbon fixation: The process of incorporating inorganic carbon (from atmospheric CO2) into an organic molecule, the first step of the Calvin cycle.
Carbon skeleton: The chain, branch, or ring structure formed by covalently bonded carbon atoms that serves as the structural backbone of organic molecules. Carbon's ability to form four bonds is what makes such a huge diversity of biological molecules possible.
Carbonyl group (C=O): A functional group consisting of a carbon double-bonded to an oxygen. When it appears at the end of a carbon chain it's called an aldehyde, and when it appears in the middle it's called a ketone -- this distinction is exactly what separates aldose sugars (like glucose) from ketose sugars (like fructose).
Carboxyl group (-COOH): A functional group that acts as an acid, releasing a hydrogen ion (H+) in solution; found in amino acids and fatty acids.
Carrier: An individual who is heterozygous for a recessive allele -- possessing one copy of it without displaying the associated recessive trait or condition themselves -- but who can still pass that allele on to offspring.
Carrying capacity (K): The maximum population size that a given environment can sustainably support long-term, based on the availability of resources like food, water, and space.
Cell cycle: The ordered sequence of growth, DNA replication, and division that a cell goes through, consisting of interphase (G1, S, G2) followed by mitosis and cytokinesis.
Cell cycle checkpoint: A control point in the cell cycle (most importantly at the G1/S and G2/M transitions) where the cell checks for DNA damage or incomplete replication before allowing the cycle to proceed. Failure of these checkpoints is central to how cancer develops.
Cell theory: The foundational principle of biology stating that all living things are composed of cells, the cell is the basic structural and functional unit of life, and all cells arise from pre-existing cells.
Cell-mediated immunity: The branch of adaptive immunity carried out by T cells, effective against pathogens hiding inside a host's own cells (like viruses) since it directly targets and destroys infected cells.
Central dogma: The fundamental flow of genetic information in a cell: DNA is transcribed into RNA, and RNA is translated into protein. Transcription (Track 17) and translation (this track) together are the two mechanistic steps that carry out the central dogma.
Central vacuole: A large, fluid-filled organelle found in plant cells that stores water, ions, and waste products, and whose internal pressure against the cell wall (turgor pressure) helps maintain the plant's structural rigidity.
Chemiosmosis: The process of generating ATP using the energy stored in a proton gradient built up across the inner mitochondrial membrane; protons flow back through ATP synthase, and that flow drives ATP production.
Chemoreceptor: A sensory receptor cell that undergoes transduction in response to a specific chemical molecule binding to it — the basis of taste and smell.
Chitin: A structural polysaccharide that makes up the cell walls of fungi (and the exoskeletons of arthropods), distinct from the cellulose that makes up plant cell walls.
Chlorophyll: The main light-absorbing pigment in chloroplasts, primarily absorbing red and blue light while reflecting green (which is why plants appear green), and initiating the light-dependent reactions of photosynthesis.
Chloroplast: The organelle in plant cells (and algae) where photosynthesis occurs, converting light energy into chemical energy; like mitochondria, it has a double membrane and its own genome from its endosymbiotic bacterial origin.
Chromosomal theory of inheritance: The theory, established by Sutton and Boveri, that genes are physically located on chromosomes, and that the behavior of chromosomes during meiosis (segregating and independently assorting) is the physical mechanism underlying Mendel's abstract inheritance rules.
Clade: A group on a phylogenetic tree consisting of a common ancestor and all of its descendants -- a complete, self-contained branch of the evolutionary tree.
Cleavage: The rapid series of cell divisions that a fertilized egg undergoes early in development, increasing cell number without significant increase in overall size.
Clonal selection: The process by which an antigen 'selects' the one lymphocyte (from a huge pre-existing pool with different receptor specificities) whose receptor happens to match it; that single cell is then triggered to rapidly proliferate into a large clone.
Codominance: An inheritance pattern in which both alleles in a heterozygote are fully and simultaneously expressed, rather than blending — AB blood type, where both A and B antigens are present, is the classic example.
Codon: A three-nucleotide sequence on mRNA that specifies either a single amino acid to be added to the growing protein or a stop signal to end translation.
Coenzyme: An organic (carbon-based) cofactor, often derived from a vitamin, that assists an enzyme in catalysis, sometimes by temporarily carrying a chemical group or electrons.
Coevolution: A process in which two species exert reciprocal selective pressure on each other over evolutionary time, each adapting in response to changes in the other — as seen in predator-prey arms races or specialized plant-pollinator relationships.
Cofactor: A non-protein helper, often a metal ion (like zinc or iron), that some enzymes require to be able to function properly.
Cohesion: The tendency of water molecules to stick to each other via hydrogen bonding, which is responsible for properties like surface tension.
Collagen: The most abundant structural protein in the extracellular matrix of animals, providing tensile strength to skin, tendons, and connective tissue.
Commensalism: A symbiotic relationship in which one species benefits from the interaction while the other species is neither helped nor harmed.
Common ancestor / node: A node on a phylogenetic tree represents a common ancestor species, the point where two lineages diverged from each other and began evolving independently.
Competition: An interaction in which two or more species (or individuals) use the same limited resource, generally reducing the fitness of both/all parties involved.
Competitive inhibitor: A molecule that resembles the substrate closely enough to bind directly to the enzyme's active site, physically competing with the real substrate and reducing the enzyme's activity — which can be overcome by adding more substrate.
Complementary base pairing: The specific pairing rule that lets nucleic acid strands bind to each other: adenine always pairs with thymine (or uracil in RNA), and guanine always pairs with cytosine, held together by hydrogen bonds.
Concentration gradient: The difference in concentration of a substance between two regions, such as the inside and outside of a cell. Substances tend to move down their gradient (high to low) unless the cell spends energy to move them against it.
Countercurrent multiplier: The mechanism by which fluid flowing in opposite directions through adjacent segments of the loop of Henle progressively builds up a strong salt concentration gradient in the kidney's medulla, allowing the kidney to concentrate urine efficiently.
Covalent bond: A chemical bond formed when two atoms share a pair of electrons rather than transferring them outright. Covalent bonds hold together most of the molecules of life, including carbon-based biomolecules.
CRISPR-Cas9: A precise gene-editing tool adapted from a natural bacterial immune defense. A short guide RNA directs the Cas9 enzyme to a specific matching DNA sequence, where Cas9 cuts both strands; the cell's own repair machinery then fixes the cut, allowing genes to be disabled or precisely edited.
Cross-bridge cycle: The repeating cycle of myosin binding actin, pulling it via a power stroke, releasing upon binding fresh ATP, and re-cocking using energy from ATP hydrolysis — the fundamental mechanical cycle underlying muscle contraction.
Crossing over (recombination): The exchange of DNA segments between homologous chromosomes during meiosis, which can separate genes that would otherwise be linked, and is the physical basis for genetic mapping — genes that are farther apart are more likely to be separated by a crossover.
Cytokinesis: The physical division of a cell's cytoplasm into two separate daughter cells, which typically occurs right after mitosis is complete.
Cytoplasmic (mitochondrial) inheritance: An inheritance pattern for genes located in mitochondrial DNA rather than nuclear DNA. Because mitochondria are passed down almost exclusively through the mother's egg cytoplasm (sperm contribute essentially none), these traits are inherited only from the mother, regardless of the father's genotype.
Cytoskeleton: An internal network of protein filaments (microtubules, microfilaments, and intermediate filaments) that gives a cell its shape, holds organelles in place, and enables movement.
Cytotoxic T cell: A type of adaptive immune cell that directly recognizes and kills cells that are infected with viruses or are cancerous, by detecting abnormal peptides displayed on that cell's MHC.

D

Denaturation: The loss of a protein's normal folded shape, caused by heat, extreme pH, or other stress, which typically destroys the protein's function even though the amino acid sequence itself is unchanged.
Density-dependent limiting factor: An environmental factor whose effect on population growth intensifies as population density increases, such as competition for resources, predation, or disease spread.
Density-independent limiting factor: An environmental factor that limits population growth regardless of population density, such as a natural disaster or extreme weather event.
Depolarization: A shift in a neuron's membrane potential toward a more positive value, caused by sodium ions flowing into the cell — the first phase of an action potential.
Desmosome: A type of cell junction that acts like a spot-weld or rivet, anchoring adjacent cells firmly together to resist mechanical stress, without sealing the space between them the way a tight junction does.
Diastole: The phase of the cardiac cycle in which the heart's chambers relax and fill with blood.
Differential reproductive success: The core mechanism of natural selection — the idea that individuals with different traits don't reproduce at equal rates; some traits lead to more surviving offspring than others.
Differentiation: The process by which cells become specialized for a specific function during development, driven by differences in gene expression even though all cells in the body carry the same genome.
Dihybrid cross: A genetic cross that tracks the inheritance of two different traits (two genes) simultaneously between two parents.
Diploid: Having two complete sets of chromosomes (one from each parent), as most body cells do — represented as '2n.'
Directional selection: A mode of natural selection that favors one extreme phenotype over the population average, shifting the population's trait distribution in that direction over time.
Disruptive selection: A mode of natural selection that favors both extreme phenotypes over the average, splitting the population's trait distribution toward the two ends and away from the middle over time.
DNA ligase: The enzyme that seals the remaining gaps between Okazaki fragments on the lagging strand (and joins any other DNA nicks), stitching the sugar-phosphate backbone into one continuous strand.
DNA methylation: The addition of a methyl chemical group to DNA, typically at a gene's promoter region, which generally suppresses (silences) transcription of that gene without changing its underlying sequence.
DNA polymerase: The enzyme responsible for adding new nucleotides to a growing DNA strand, always in the 5' to 3' direction, using the original strand as a template. It also proofreads its work, correcting most errors as it goes.
DNA repair: The set of cellular mechanisms that detect and correct damage or errors in DNA, helping to prevent mutations from becoming permanent.
Dominant allele: An allele whose trait is expressed in the phenotype whenever at least one copy is present, masking the effect of a recessive allele if one is also present.
Double helix: The twisted-ladder structure of DNA, formed by two antiparallel strands of nucleotides wound around each other, with the nitrogenous bases paired in the middle held by hydrogen bonds.

E

Ecosystem services: The benefits that humans obtain from healthy, functioning ecosystems, such as water filtration, pollination, climate regulation, and soil formation — often difficult or impossible to fully replace artificially.
Ectoderm, mesoderm, endoderm: The three primary germ layers established during gastrulation, each of which gives rise to different body structures: ectoderm forms skin and the nervous system; mesoderm forms muscle, bone, and the circulatory system; and endoderm forms the lining of the digestive and respiratory tracts.
Ectotherm: An organism that relies primarily on external heat sources to regulate its body temperature, rather than generating significant heat through its own metabolism.
Electron: A negatively charged subatomic particle that occupies shells (energy levels) around the nucleus. The electrons in the outermost shell, called valence electrons, determine how an atom will bond with other atoms.
Electron transport chain (ETC): A series of protein complexes embedded in the inner mitochondrial membrane that pass electrons (delivered by NADH and FADH2) down an energy gradient, using the released energy to pump protons across the membrane and build a proton gradient.
Endergonic reaction: A chemical reaction with a positive ΔG that requires an input of energy to proceed — described as 'uphill' because the products have more free energy than the reactants.
Endocrine signaling: A form of cell signaling in which a signaling molecule (a hormone) is released into the bloodstream and travels to affect distant target cells throughout the body -- the basis of the endocrine system covered in Track 45.
Endocytosis: The process by which a cell brings large molecules or particles into itself by having the plasma membrane engulf the material and pinch off into an internal vesicle.
Endosymbiotic theory: The theory that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by an ancestral eukaryotic cell and, over evolutionary time, became permanent, mutually beneficial residents. Evidence includes their own circular genome, double membrane, and independent (binary-fission-like) division.
Endotherm: An organism that generates and regulates its own internal body heat primarily through its own metabolism, largely independent of external temperature.
Energy coupling: The strategy cells use to drive an endergonic (energy-requiring) reaction by pairing it with an exergonic (energy-releasing) reaction, most commonly ATP hydrolysis, so the overall combined reaction is favorable.
Energy pyramid: A diagram representing the amount of energy available at each successive trophic level in an ecosystem, which narrows dramatically at higher levels due to the ten percent rule.
Entropy: A measure of disorder or randomness in a system. Physical and chemical processes tend to proceed in the direction that increases the total entropy of the universe.
Enzyme: A protein that acts as a biological catalyst, speeding up a specific chemical reaction by lowering its activation energy, without being permanently changed or consumed in the process.
Epigenetics: Changes in gene expression that do not involve altering the underlying DNA sequence itself, but instead involve chemical modifications (like DNA methylation) that influence whether a gene is accessible for transcription.
Epistasis: An interaction in which one gene's expression masks or modifies the phenotypic expression of a different gene entirely, rather than the two genes contributing independently.
Eukaryote: An organism (protist, fungus, plant, or animal) whose cells contain a true, membrane-bound nucleus housing the DNA, along with other membrane-bound organelles.
Excitation-contraction coupling: The overall process linking a nerve-triggered electrical signal (action potential) to the mechanical contraction of a muscle fiber, via calcium release from the sarcoplasmic reticulum and its effect on troponin and tropomyosin.
Exergonic reaction: A chemical reaction with a negative ΔG that releases energy and can proceed spontaneously — described as 'downhill' because the products have less free energy than the reactants.
Exocytosis: The process by which a cell releases large molecules or particles to the outside by fusing an internal vesicle with the plasma membrane, dumping its contents out.
Exon: A coding section of a pre-mRNA transcript that remains after splicing and is joined together with other exons to form the mature mRNA that will actually be translated into protein.
Exponential growth: Population growth that occurs at an accelerating rate under ideal, unlimited conditions, producing a J-shaped growth curve, since the larger the population gets, the faster it grows in absolute numbers.
Extracellular matrix (ECM): The mesh of proteins (such as collagen) and other molecules located outside of cells that provides structural support to tissues and helps cells communicate with their surrounding environment.

F

Facilitated diffusion: The passive movement of larger or polar/charged molecules (like glucose or ions) across the membrane through a specific channel or carrier protein, still moving down the concentration gradient and requiring no cellular energy.
FAD/FADH2: FAD is an electron carrier molecule, similar to NAD+, that becomes reduced to FADH2 when it picks up high-energy electrons during the Krebs cycle, later delivering them to the electron transport chain.
Feedback inhibition: A regulatory mechanism in which the final product of a metabolic pathway inhibits an enzyme earlier in that same pathway, preventing the cell from wastefully overproducing a substance it already has enough of.
Fermentation: A metabolic pathway that regenerates NAD+ (needed to keep glycolysis running) without an electron transport chain, by transferring electrons from NADH to an organic molecule like pyruvate. It produces far less ATP than full aerobic respiration but lets a cell keep generating energy without oxygen.
Fertilization: The fusion of a sperm and an egg, combining their haploid genetic material to form a diploid zygote.
Filtration: The first step of nephron function, in which blood pressure pushes water and small solutes out of the glomerulus and into Bowman's capsule, while blood cells and large proteins remain in the bloodstream.
First law of thermodynamics: The principle that energy cannot be created or destroyed, only converted from one form to another — also called the law of conservation of energy.
Fluid mosaic model: The modern model of the plasma membrane describing it as a flexible phospholipid bilayer with a shifting 'mosaic' of proteins, cholesterol, and other molecules embedded within and floating through it.
Founder effect: A change in allele frequencies that occurs when a new population is established by a small number of individuals from a larger original population, so the new population's gene pool reflects only whatever alleles that small founding group happened to carry -- a specific form of genetic drift.
Frameshift mutation: A mutation caused by inserting or deleting a number of DNA bases not divisible by three, which shifts the reading frame for every codon downstream of the mutation, typically scrambling the entire protein sequence from that point on.
Functional group: A specific small group of atoms attached to a carbon skeleton that gives the molecule its characteristic chemical properties and reactivity — the same carbon backbone with different functional groups can behave very differently.
Fungi: A kingdom of mostly decomposer eukaryotic organisms that absorb nutrients from their environment rather than ingesting food, have cell walls made of chitin, and often have a body made of thread-like filaments called hyphae.

G

G0 phase: A resting state outside the active cell cycle that a cell can enter from G1, in which it is not preparing to divide. Many mature, fully differentiated cells (like most neurons and muscle cells) spend most or all of their life in G0; re-entry into the cell cycle from G0 is tightly regulated, and its loss of control is relevant to cancer.
Gamete: A haploid reproductive cell (sperm or egg) produced through meiosis, which combines with another gamete during fertilization to restore the full diploid chromosome number.
Gap junction: A type of cell junction that forms a direct channel connecting the cytoplasm of two adjacent cells, allowing ions and small molecules to pass directly between them — essential for the coordinated, synchronized contraction of heart muscle cells.
Gas exchange: The process, occurring in the lungs' alveoli, of oxygen diffusing from inhaled air into the blood and carbon dioxide diffusing from the blood into the air to be exhaled.
Gastrulation: The developmental stage in which the embryo reorganizes into distinct germ layers, setting up the basic body plan that will give rise to different tissues and organs.
Gel electrophoresis: A laboratory technique that separates DNA (or protein) fragments by size, using an electric current to pull negatively charged DNA through a gel matrix; smaller fragments move through the gel faster and travel farther than larger fragments.
Gene flow: The movement of alleles into or out of a population, typically caused by migration of individuals between populations.
Genetic code degeneracy: The property of the genetic code in which most amino acids are specified by more than one codon, meaning multiple different codons can code for the same amino acid.
Genetic drift: Random, chance-based fluctuations in allele frequencies within a population from one generation to the next, which have a much larger effect in small populations than in large ones.
Genetic linkage: The tendency of two genes located close together on the same chromosome to be inherited together, rather than assorting independently as Mendel's law would otherwise predict.
Genotype: The specific combination of alleles an individual has for a gene, written out as letters (e.g., Bb).
Gibbs free energy (ΔG): A measure of the energy available in a chemical reaction to do useful work, and the value that determines whether a reaction will proceed spontaneously. A negative ΔG means the reaction releases energy and can proceed on its own; a positive ΔG means the reaction requires an energy input to proceed.
Glomerulus: A cluster of tiny blood vessels within the nephron where blood pressure forces small molecules out of the blood and into Bowman's capsule, beginning the filtration process.
Glucagon: A hormone released by the pancreas that raises blood glucose levels by promoting the release of stored glucose back into the bloodstream — functioning as insulin's antagonistic opposite.
Glycolysis: The metabolic pathway, occurring in the cytoplasm, that splits one molecule of glucose into two molecules of pyruvate. It nets a small amount of ATP directly and produces NADH, and does not require oxygen.
Glycosidic linkage (alpha vs. beta): The type of covalent bond joining monosaccharides together into a larger carbohydrate. An alpha linkage (as in starch and glycogen) creates a bond shape human digestive enzymes can break down; a beta linkage (as in cellulose) creates a different bond shape those same enzymes cannot break, which is why humans can digest starch but not cellulose (fiber).
Golgi apparatus: A stack of flattened membrane sacs that receives proteins and lipids from the ER, modifies and packages them, and tags them for delivery to their correct destination in or outside the cell.
Gymnosperm: A group of seed plants whose seeds are 'naked' — not enclosed within a fruit — such as conifers.

H

Habitat fragmentation: The breaking up of a large, continuous habitat into smaller, isolated patches, typically due to human development, which reduces population sizes and increases isolation, making species more vulnerable to local extinction.
Haploid: Having a single set of chromosomes (half the normal number), as gametes do — represented as 'n.'
Hardy-Weinberg equation: The equation p² + 2pq + q² = 1, which relates the frequencies of the two alleles (p and q) for a gene to the frequencies of the three possible genotypes (homozygous dominant, heterozygous, and homozygous recessive) in a population.
Hardy-Weinberg principle: A mathematical model describing a theoretical population in which allele and genotype frequencies remain constant across generations, provided that no mutation, gene flow, genetic drift, or natural selection is occurring, and mating is random.
Heart valves (AV and semilunar): One-way valves that keep blood flowing in a single direction through the heart. The atrioventricular (AV) valves sit between the atria and ventricles and close to produce the 'lub' sound; the semilunar valves sit at the exits of the ventricles (to the aorta and pulmonary artery) and close to produce the 'dub' sound.
Helicase: The enzyme that unwinds and separates the two strands of the DNA double helix at the replication fork, breaking the hydrogen bonds between base pairs.
Helper T cell: A type of adaptive immune cell that reads antigen presented on MHC by other immune cells and coordinates the overall immune response by releasing signaling molecules that activate B cells, cytotoxic T cells, and macrophages.
Hemoglobin: The oxygen-carrying protein found in red blood cells, made of four subunits, each capable of binding one oxygen molecule.
Heterozygous: Having two different alleles for a given gene (one dominant, one recessive); the dominant allele's trait will typically be the one expressed.
Homeostasis: The body's ability to maintain a relatively stable internal environment (temperature, pH, glucose levels, etc.) despite changes in the external environment.
Homologous chromosomes: A matched pair of chromosomes, one inherited from each parent, that carry the same genes (though possibly different alleles) at the same locations.
Homologous structure: An anatomical structure shared by different species due to common ancestry, even if the structure has been modified for different functions — such as the similar bone arrangement in a human arm, a whale flipper, and a bat wing.
Homozygous: Having two identical alleles for a given gene (either two dominant or two recessive).
Horizontal gene transfer: The movement of genetic material between organisms other than from parent to offspring, which is how bacteria can rapidly spread traits like antibiotic resistance through a population. It occurs by three main mechanisms: conjugation (direct cell-to-cell transfer through a physical connection), transformation (taking up free DNA from the environment), and transduction (a virus accidentally carrying bacterial DNA from one cell to another).
Hormone: A chemical signaling molecule released by an endocrine gland into the bloodstream, which travels to and affects distant target cells throughout the body.
Humoral immunity: The branch of adaptive immunity carried out by B cells and the antibodies they produce, most effective against extracellular (free-floating) pathogens like bacteria and toxins.
Hydrogen bond: A relatively weak attraction between a hydrogen atom that is partially positively charged (because it's bonded to an electronegative atom like oxygen or nitrogen) and a nearby atom that is partially negatively charged. Individually weak, hydrogen bonds are collectively powerful — they hold DNA's two strands together and give water its unique properties.
Hydroxyl group (-OH): A functional group consisting of an oxygen bonded to a hydrogen, which is polar and increases a molecule's solubility in water; found in alcohols and sugars.
Hyphae: The thread-like filaments that make up the body (mycelium) of most fungi, through which the fungus absorbs nutrients from its environment.
Hypothalamus: A region of the brain that acts as the master regulator of the endocrine system, signaling the pituitary gland to release hormones that in turn control other endocrine glands.

I

Incomplete dominance: An inheritance pattern in which heterozygous offspring show a blended, intermediate phenotype rather than either parent's phenotype fully — a red flower crossed with a white flower producing pink offspring is the classic example.
Independent assortment: The random orientation of homologous chromosome pairs during Meiosis I, meaning which chromosome of each pair ends up in a given gamete is independent of how the other pairs sorted — a major source of genetic variation.
Induced fit: The model describing how an enzyme's active site is not a rigid, fixed shape but instead subtly changes shape as it binds the substrate, molding around it to grip it more snugly — like a handshake rather than a lock and key.
Inducible operon: An operon that is normally off (repressed) and gets switched on when a specific molecule (an inducer) is present — the lac operon, switched on by lactose, is the classic example.
Inflammation: The body's protective innate response to injury or infection, producing redness, heat, swelling, and pain, driven by increased blood flow and vessel permeability that brings immune cells to the affected area.
Innate behavior: A behavior that an organism is born with, performed correctly without prior learning or experience, and generally consistent across the whole species.
Innate immunity: The immune system's fast, non-specific first line of defense, present from birth, that responds broadly to a wide range of threats without needing prior exposure and does not create lasting memory. Includes physical barriers, phagocytes, and inflammation.
Insulin: A hormone released by the pancreas that lowers blood glucose levels by promoting the uptake and storage of glucose by cells.
Intermediate filament: A rope-like cytoskeletal filament that provides mechanical strength and structural stability to a cell, acting like cables holding the cell together.
Interphase: The longest portion of the cell cycle, during which the cell grows and copies its DNA, made up of the G1, S, and G2 phases; the cell is not yet dividing during this time.
Intron: A non-coding section of a pre-mRNA transcript that is removed (spliced out) during RNA processing and does not end up in the mature mRNA.
Invasive species: A non-native species introduced (often by human activity) into a new environment, where it can spread aggressively and outcompete native species that have no evolved defenses against it.
Invertebrate: An animal that lacks a backbone (vertebral column), a category that includes the vast majority of animal species, such as sponges, cnidarians, mollusks, and arthropods.
Ionic bond: A chemical bond formed when one atom completely transfers an electron to another, creating two oppositely charged ions that are then attracted to each other. Table salt (Na+ and Cl-) is a classic example.

K

Kin selection: An evolutionary explanation for altruistic behavior, proposing that helping a genetic relative survive and reproduce can still propagate an individual's shared genes, even at a cost to the helper's own direct survival or reproduction.
Krebs cycle: Also called the citric acid cycle — a series of reactions occurring in the mitochondrial matrix that fully oxidizes acetyl-CoA, releasing CO2 and generating the electron carriers NADH and FADH2.

L

Lagging strand: The DNA strand that must be synthesized discontinuously, in short backward-facing segments (Okazaki fragments), because it runs in the opposite orientation to the direction the replication fork is opening.
Law of independent assortment: Mendel's principle that the alleles for different genes separate into gametes independently of one another, meaning the inheritance of one trait doesn't influence the inheritance of a different trait — as long as the genes are on different chromosomes (or far apart on the same one).
Law of segregation: Mendel's principle that the two alleles an individual carries for a gene separate from each other during gamete formation, so each gamete receives only one of the two alleles.
Leading strand: The DNA strand that can be synthesized continuously in the same direction that the replication fork is opening, because DNA polymerase naturally builds in the 5' to 3' direction on this strand.
Learned behavior: A behavior that an organism acquires or refines through experience, observation, or practice, rather than being present automatically from birth.
Ligand: A signaling molecule (such as a hormone or neurotransmitter) that binds specifically to a receptor to trigger a response in the target cell.
Light-dependent reactions: The first stage of photosynthesis, occurring in the thylakoid membrane, in which light energy is absorbed by chlorophyll, water is split (releasing O2), and the energy is used to generate ATP and NADPH.
Limiting nutrient: The nutrient in shortest supply relative to an organism's or ecosystem's needs, which therefore restricts growth even if all other nutrients are abundant. Phosphorus is often the limiting nutrient in freshwater ecosystems, while nitrogen is more frequently the limiting nutrient in marine and many terrestrial ecosystems.
Logarithmic scale: A scale in which each single-unit step represents a tenfold (10x) change in the underlying quantity. Because pH is logarithmic, a solution at pH 4 has ten times more H+ than a solution at pH 5, not just slightly more.
Logistic growth: Population growth that slows as the population approaches the environment's carrying capacity, producing an S-shaped growth curve — a more realistic model than exponential growth for populations facing limited resources.
Loop of Henle: The hairpin-shaped segment of the nephron that plunges into the kidney's medulla, creating a countercurrent multiplier that builds a strong salt gradient used to concentrate the urine.
Lysogenic cycle: A viral replication pathway in which the virus's DNA integrates into the host chromosome as a dormant prophage, being passively copied along with the host's own DNA at every division, until a trigger causes it to switch into the lytic cycle.
Lysosome: A membrane-bound organelle containing digestive enzymes used to break down waste material, damaged organelles, and material taken in from outside the cell.
Lytic cycle: The viral replication pathway in which a virus immediately redirects the host cell's machinery to assemble many new viral particles, then lyses (bursts) the cell to release them.

M

Macroevolution: Large-scale evolutionary change and patterns occurring above the species level and across long timescales, including the origin of major new groups and mass extinction events.
Mass extinction: An event in which a very large proportion of Earth's species go extinct within a relatively short geological time period. Five such mass extinctions are recognized in the fossil record.
Meiosis I: The first division of meiosis, in which homologous chromosome pairs are separated from each other; this is the stage where crossing over and independent assortment occur, generating genetic variation.
Meiosis II: The second division of meiosis, in which sister chromatids are separated from each other, mechanically similar to mitosis but starting from haploid cells.
Memory cell: A long-lived B or T cell generated during an immune response that persists after the infection clears, allowing a much faster and stronger response if the same pathogen is encountered again — the basis of both natural immunity after infection and vaccine-induced immunity.
Metaphase: The phase of mitosis in which condensed chromosomes line up along the cell's center (the metaphase plate), attached to spindle fibers from opposite poles.
Metastasis: The process by which cancer cells break away from their original (primary) tumor site and spread through the blood or lymphatic system to establish new tumors elsewhere in the body.
MHC (major histocompatibility complex): A set of cell-surface molecules that display peptide fragments so T cells can inspect them, allowing the immune system to distinguish healthy self cells from infected or abnormal ones.
Microfilament: The thinnest type of cytoskeletal filament, built from actin protein, involved in maintaining cell shape and enabling cell movement (like crawling) — the same actin filaments involved in muscle contraction, discussed in Track 44.
Microtubule: The thickest type of cytoskeletal filament, built from tubulin protein, serving as a track along which motor proteins carry cargo through the cell and forming the spindle fibers used during cell division.
Mismatch repair: A DNA repair pathway that scans newly replicated DNA for bases that were mismatched (incorrectly paired) during replication and were missed by DNA polymerase's own proofreading, correcting them before they become permanent mutations.
Missense mutation: A point mutation that changes the resulting codon so it now codes for a different amino acid, which may or may not significantly affect the protein's function depending on how similar the new amino acid is.
Mitochondria: The 'powerhouse' organelle where aerobic respiration takes place, generating most of the cell's ATP; has a double membrane with folded inner cristae, and its own small genome — a legacy of its origin as a free-living bacterium.
Mitosis: The process by which a cell's duplicated chromosomes are divided equally into two genetically identical nuclei, proceeding through prophase, metaphase, anaphase, and telophase.
Monohybrid cross: A genetic cross that tracks the inheritance of a single trait (one gene) between two parents.
Monosaccharide: The simplest form of carbohydrate — a single sugar unit, like glucose or fructose, that serves as a basic building block for larger carbohydrates.
Morphogenesis: The developmental process by which cells and tissues take on their specific shapes and organized spatial arrangement, transforming a simple ball of cells into a structured body plan.
Motor protein: A protein (such as kinesin or dynein) that converts chemical energy from ATP into mechanical movement, walking along microtubules to transport cargo through the cell.
Multi-hit hypothesis: The concept that cancer does not result from a single mutation but develops through the gradual accumulation of multiple mutations in growth-control genes (proto-oncogenes and tumor suppressors) over time.
Multiple alleles: A situation in which more than two different alleles exist for a single gene across a population, even though any individual can still only carry two of them. Human ABO blood type is the classic example, with three possible alleles (IA, IB, and i) -- and it's worth noting that ABO blood type actually demonstrates two distinct genetic concepts at once: codominance (between IA and IB) and multiple allelism (three alleles existing in the population).
Mutagen: Any physical or chemical agent that increases the rate of mutations in DNA, including UV light (causing thymine dimers), certain chemicals, and radiation.
Mutualism: A symbiotic relationship in which both species involved benefit from the interaction.
Myelin / saltatory conduction: Myelin is a fatty insulating sheath (produced by glial cells) that wraps around an axon in segments, with small unwrapped gaps called nodes of Ranvier in between. Because the action potential can effectively 'jump' from node to node rather than regenerating continuously along the entire membrane, myelinated axons conduct signals much faster than unmyelinated ones -- this jumping pattern is called saltatory conduction.
Myosin: The thick filament protein in a sarcomere whose heads bind to actin, undergo a power stroke, and pull the actin filament inward, generating the force of muscle contraction.

N

NAD+/NADH: NAD+ is an electron carrier molecule that becomes reduced to NADH when it picks up high-energy electrons during glycolysis and the Krebs cycle. NADH then carries those electrons to the electron transport chain to help generate ATP.
NADPH: An electron carrier, similar to NADH, produced during the light-dependent reactions of photosynthesis and used to power the Calvin cycle's sugar-building reactions.
Natural selection: The process by which individuals with heritable traits better suited to their environment tend to survive and reproduce more successfully than others, causing those advantageous traits to become more common in a population over generations.
Negative feedback: A regulatory mechanism in which a change away from a set point triggers a response that reverses the change, bringing the system back toward its normal range — the most common feedback mechanism used to maintain homeostasis.
Negative-sense RNA: A viral RNA genome that is the complementary mirror-image of mRNA, so it cannot be directly translated; the virus must carry its own RNA polymerase to first transcribe it into a readable, positive-sense copy.
Nephron: The functional unit of the kidney, responsible for filtering blood and producing urine through the sequential processes of filtration, reabsorption, and secretion.
Neutron: An uncharged subatomic particle found in the atomic nucleus alongside protons. Atoms of the same element can have different numbers of neutrons, which are called isotopes, without changing what element they are.
Nitrogen fixation: The conversion of atmospheric nitrogen gas (N2), which most organisms cannot use directly, into ammonia — a biologically usable form. Only certain prokaryotes (like Rhizobium, using the enzyme nitrogenase) can perform this reaction.
Nitrogenous base: The nitrogen-containing ring structure attached to a nucleotide's sugar that gives each nucleotide its identity and pairing behavior — adenine (A), thymine (T, DNA only), uracil (U, RNA only), guanine (G), and cytosine (C).
Nitrogenous waste: Toxic waste product (ammonia, urea, or uric acid) generated when protein is broken down and its nitrogen must be excreted from the body.
Noncompetitive (allosteric) inhibitor: A molecule that binds to a site on the enzyme other than the active site, changing the enzyme's overall shape (including the active site) and reducing its activity — this type of inhibition cannot be overcome just by adding more substrate.
Nonsense mutation: A point mutation that changes a codon into a premature stop codon, causing translation to end early and typically producing a truncated, non-functional protein.
Nucleolus: A dense region inside the nucleus where ribosomal RNA is synthesized and ribosome subunits are assembled before being exported to the cytoplasm.
Nucleotide: The building-block monomer of DNA and RNA, made of three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base.
Nucleotide excision repair (NER): A DNA repair pathway that removes a damaged section of DNA (such as a thymine dimer caused by UV light) along with some surrounding healthy nucleotides, then fills the resulting gap using the undamaged complementary strand as a template. A defect in this pathway causes the disorder xeroderma pigmentosum, which leaves patients extremely vulnerable to UV-induced skin cancer.
Nucleus: The membrane-bound organelle in eukaryotic cells that houses the cell's DNA, physically separating transcription (which happens inside) from translation (which happens outside in the cytoplasm).

O

Okazaki fragment: A short, discontinuous segment of newly synthesized DNA on the lagging strand, produced because DNA polymerase can only build in the 5' to 3' direction; these fragments are later joined together by DNA ligase.
Oncogene: A mutated, overactive version of a proto-oncogene that is stuck in the 'on' position, continuously signaling the cell to divide even without the normal cues that would trigger division.
Operator: A DNA sequence within an operon where a repressor protein can bind, physically blocking RNA polymerase from transcribing the downstream genes.
Operon: A cluster of genes in bacteria transcribed together as a single mRNA molecule, under the shared control of one promoter and operator — allowing coordinated on/off regulation of genes that function together.
Organ: A structure made of two or more different tissue types working together to perform a specific function, the level of organization between tissues and organ systems.
Origin of replication: The specific location on a DNA molecule where the double helix begins to unwind and DNA replication starts.
Osmosis: The passive diffusion of water across a selectively permeable membrane, moving toward the side with a higher solute concentration (and therefore lower water concentration) until equilibrium is reached.
Oxidative phosphorylation: The production of ATP using energy released by the electron transport chain and chemiosmosis, which requires oxygen as the final electron acceptor — this is where the large majority of ATP from respiration is actually generated.
Oxygen-hemoglobin dissociation curve: A graph describing the relationship between oxygen availability and how saturated hemoglobin becomes with oxygen, showing that hemoglobin binds oxygen cooperatively — becoming progressively easier to load or unload as conditions change.

P

p53: A tumor suppressor protein often called the 'guardian of the genome' — it checks for DNA damage and, if damage is found, halts the cell cycle to allow repair or triggers programmed cell death (apoptosis) if the damage is too severe.
Paracrine signaling: A form of cell signaling in which a cell releases a signaling molecule that diffuses a short distance to affect nearby cells, without entering the bloodstream.
Parasitism: A symbiotic relationship in which one species (the parasite) benefits at the direct expense of the other species (the host).
Pedigree: A chart that tracks the inheritance of a trait through a family tree across multiple generations, used to determine a trait's inheritance pattern (dominant/recessive, autosomal/sex-linked).
Peptide bond: The covalent bond that links the carboxyl group of one amino acid to the amino group of the next, forming a chain of amino acids (a polypeptide).
Peptide hormone: A water-soluble hormone that cannot cross the plasma membrane directly; it binds a receptor on the target cell's surface and typically triggers a second-messenger signal transduction pathway inside the cell.
Peroxisome: A membrane-bound organelle containing enzymes that break down fatty acids and detoxify harmful substances, producing hydrogen peroxide as a byproduct, which is then safely broken down by the enzyme catalase within the same organelle.
pH scale: A logarithmic scale from 0 to 14 that measures the concentration of hydrogen ions (H+) in a solution. Lower numbers mean more H+ (more acidic), higher numbers mean less H+ (more basic), and 7 is neutral.
Phagocyte: An innate immune cell (such as a macrophage or neutrophil) specialized in engulfing and destroying pathogens, cellular debris, and other foreign material.
Phenotype: The observable physical trait that results from an individual's genotype (e.g., brown eyes).
Phloem: The plant vascular tissue responsible for transporting sugars produced during photosynthesis from source tissues (like leaves) to sink tissues (like roots or developing fruit) where they are needed or stored.
Phosphate group (-PO4): A functional group built around a phosphorus atom bonded to oxygens, carrying a negative charge and capable of storing energy in its bonds. Phosphate groups form the backbone of DNA/RNA and are the energy-carrying part of ATP.
Phospholipid: A lipid with a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. Phospholipids spontaneously arrange into a bilayer in water, which is the basic structure of every cell membrane.
Phosphorus cycle: The biogeochemical cycle by which phosphorus moves through ecosystems, distinguished by having no significant atmospheric gas phase — phosphorus instead cycles slowly by weathering out of rock, which is why it is often the limiting nutrient in ecosystems.
Photolysis: The light-driven splitting of a water molecule during the light-dependent reactions, which releases electrons (to replace those lost by chlorophyll), protons, and oxygen gas as a byproduct.
Photoreceptor: A sensory receptor cell in the eye that undergoes transduction in response to light, converting it into a nerve signal — the basis of vision.
Photorespiration: A wasteful side reaction that occurs when RuBisCO binds oxygen instead of CO2, especially on hot, dry days when a plant's stomata close to conserve water (which also traps oxygen and depletes CO2 inside the leaf). It consumes energy without producing sugar, reducing photosynthetic efficiency in standard (C3) plants.
Photosystem I and II: Two distinct clusters of chlorophyll and other pigments embedded in the thylakoid membrane that absorb light energy during the light-dependent reactions. Photosystem II acts first, splitting water and passing energized electrons down an electron transport chain to Photosystem I, which re-energizes them with more light before they're used to help generate NADPH.
Phylogenetic tree: A branching diagram that represents the evolutionary relationships and estimated common ancestry between different species or groups, typically built from shared physical characteristics or molecular (DNA) similarity.
Pituitary gland: An endocrine gland that receives signals from the hypothalamus and releases hormones that control the activity of other endocrine glands, such as the thyroid, adrenal glands, and gonads.
Pleiotropy: A phenomenon in which a single gene influences multiple, seemingly unrelated phenotypic traits.
Point mutation: A mutation that changes a single DNA base, which can be silent (no amino acid change), missense (changes one amino acid), or nonsense (creates a premature stop codon).
Polarity: An uneven distribution of electrical charge across a molecule, with one region slightly positive and another slightly negative. Water is polar because oxygen pulls shared electrons more strongly than hydrogen does, giving the molecule a partial negative charge near the oxygen and partial positive charges near the hydrogens.
Poly-A tail: A long chain of adenine nucleotides added to the back (3') end of a new mRNA transcript during processing, which protects the mRNA from degradation and helps it exit the nucleus.
Polygenic trait: A trait influenced by the combined effect of many different genes, producing a continuous range of phenotypes (like height or skin color) rather than distinct either/or categories.
Polymerase chain reaction (PCR): A laboratory technique that exponentially amplifies a specific segment of DNA through repeated cycles of heating and cooling, using a heat-stable DNA polymerase to make millions of copies from a tiny starting sample.
Polysaccharide: A long chain of many monosaccharides linked together, used for energy storage (starch in plants, glycogen in animals) or structural support (cellulose in plant cell walls, chitin in fungal cell walls and arthropod exoskeletons).
Population genetics: The study of how allele and genotype frequencies within a population change, or stay stable, over time.
Positive feedback: A regulatory mechanism in which a change triggers a response that amplifies the change further rather than reversing it, typically used to rapidly drive a process to completion rather than to maintain stability.
Positive-sense RNA: A viral RNA genome with the same sequence as mRNA, meaning host ribosomes can translate it directly into protein immediately upon entry, with no extra enzyme needed first.
Postzygotic isolation: A reproductive barrier that acts after fertilization has already occurred, such as a hybrid offspring having reduced viability or being sterile (like a mule), which still prevents successful gene flow between the two populations even though mating happened.
Predation: An interaction in which one species (the predator) consumes another species (the prey) as a food source.
Prezygotic isolation: A reproductive barrier that prevents mating or fertilization from happening at all between two populations, such as differences in habitat, timing, mating behavior, or physical incompatibility.
Primary consumer: An organism that eats producers directly, occupying the second trophic level of an ecosystem (herbivores).
Primary structure: The specific linear sequence of amino acids in a protein, determined directly by the mRNA sequence that was translated. This sequence dictates every higher level of folding that follows.
Primase: The enzyme that synthesizes a short RNA primer on the DNA template, giving DNA polymerase a starting point to begin adding new nucleotides, since DNA polymerase cannot start a strand from scratch.
Producer: An organism (typically a photosynthetic plant, alga, or cyanobacterium) that captures energy from an external source (usually sunlight) to make its own organic food, forming the base of an ecosystem's energy flow.
Prokaryote: An organism (bacteria or archaea) whose cells lack a nucleus and membrane-bound organelles; its DNA sits free in the cytoplasm in a region called the nucleoid.
Promoter: A specific DNA sequence located at the start of a gene that RNA polymerase binds to, marking where transcription of that gene should begin.
Prophage: Viral DNA that has integrated into and become part of a host bacterium's chromosome during the lysogenic cycle, remaining dormant until triggered to re-enter the lytic cycle.
Prophage induction: The process by which a dormant prophage is triggered to exit the lysogenic cycle and switch into the lytic cycle, typically in response to a stress signal like DNA damage or UV exposure. This same 'dormant, then reactivated later' pattern explains viral latency in eukaryotic viruses too, such as a herpesvirus reactivating to cause a cold sore, or HIV reactivating from a latent reservoir.
Prophase: The first phase of mitosis, in which chromosomes condense into visible structures and the mitotic spindle begins to form.
Protist: A highly diverse group of mostly single-celled eukaryotic organisms that don't fit cleanly into the plant, fungus, or animal kingdoms — some are plant-like (algae), some animal-like (protozoa), and some fungus-like.
Proto-oncogene: A normal gene that promotes cell growth and division as part of healthy cell cycle regulation. If mutated into an oncogene, it can drive uncontrolled cell division.
Proton: A positively charged subatomic particle found in the atomic nucleus. The number of protons defines which element an atom is — changing the proton count changes the element itself.
Provirus: The DNA form of a retrovirus's genome once it has been integrated into a host cell's chromosome by reverse transcriptase, becoming a permanent part of that cell's DNA.
Punnett square: A grid-based tool used to predict the probability of specific genotype and phenotype combinations in offspring, by systematically combining every possible allele from each parent's gametes.
Pyruvate: The three-carbon molecule produced as the end product of glycolysis. In the presence of oxygen, it is further oxidized and enters the mitochondria; in its absence, it is instead converted during fermentation.

Q

Quaternary structure: The structure formed when two or more separate polypeptide chains (subunits) join together to form one functional protein complex, such as hemoglobin's four subunits.

R

Rb (retinoblastoma protein): A tumor suppressor protein that enforces a critical cell cycle checkpoint, normally holding the cell back from entering S phase (DNA replication) until conditions are appropriate.
Reabsorption: The process in the nephron's tubule by which useful substances (glucose, salt, water) that were filtered out of the blood are reclaimed and transported back into the blood.
Receptor: A protein that binds to a specific ligand and initiates a response inside the cell. Receptors can be located on the cell surface (for ligands that cannot cross the membrane) or inside the cell (for ligands, like steroid hormones, that can cross the membrane directly).
Recessive allele: An allele whose trait is only expressed in the phenotype when two copies are present (no dominant allele masking it).
Recombinant DNA: DNA that has been artificially combined from two or more different sources, typically by using restriction enzymes to cut DNA fragments and then joining them together.
Redox reaction: A chemical reaction in which electrons are transferred from one molecule to another. Oxidation is the loss of electrons and reduction is the gain of electrons — remembered with the mnemonic OIL RIG (Oxidation Is Losing, Reduction Is Gaining).
Refractory period: A brief period immediately following an action potential during which a neuron is temporarily unable (or less able) to fire another action potential, ensuring signals travel in one direction and setting a limit on firing frequency.
Replication fork: The Y-shaped region where the DNA double helix is actively being unwound and copied during replication.
Repolarization: The return of a neuron's membrane potential back toward its negative resting value after depolarization, caused by potassium ions flowing out of the cell.
Repressible operon: An operon that is normally on (actively transcribed) and gets switched off once its end product becomes abundant, through negative feedback — the trp operon, shut off by tryptophan, is the classic example.
Repressor: A regulatory protein that binds to an operator sequence and blocks transcription of the associated gene or operon, turning it off.
Reproductive isolation: Any barrier that prevents two populations from successfully interbreeding and exchanging genes, allowing them to evolve independently into separate species.
Resting membrane potential: The stable electrical charge difference (around -70mV, negative inside relative to outside) maintained across a neuron's membrane when it is not actively firing. It is set up primarily by resting potassium permeability (K+ leak channels, which make the membrane far more permeable to K+ than to other ions at rest, pulling the resting potential close to potassium's own equilibrium potential), with the sodium-potassium pump maintaining the underlying ion gradients that make this possible.
Restriction enzyme: A bacterial enzyme that recognizes a specific DNA sequence and cuts the DNA at or near that site. Naturally used by bacteria as a defense against viral DNA, but also a foundational tool in genetic engineering for cutting and splicing DNA.
Retrovirus: A virus (like HIV) whose genome is RNA but which carries the enzyme reverse transcriptase, allowing it to convert its RNA into DNA and integrate that DNA into the host cell's chromosome.
Reverse transcriptase: An enzyme, carried by retroviruses, that converts single-stranded RNA into double-stranded DNA — reversing the normal DNA-to-RNA direction of the central dogma.
Ribosomal A, P, E sites: The three binding positions on a ribosome as it translates mRNA. A new aminoacyl-tRNA first enters the A (aminoacyl) site; the growing polypeptide chain is held at the P (peptidyl) site while the peptide bond forms; and the now-empty tRNA moves to the E (exit) site before leaving the ribosome, as the whole ribosome shifts (translocates) one codon down the mRNA.
Ribosome: The molecular machine (made of rRNA and protein) that reads mRNA and builds proteins during translation. Ribosomes can float free in the cytoplasm or be attached to the rough ER.
RNA polymerase: The enzyme that synthesizes RNA by reading a DNA template strand and adding complementary RNA nucleotides in the 5' to 3' direction, starting at the promoter.
RNA splicing: The process of removing introns from a pre-mRNA transcript and joining the remaining exons together to produce mature mRNA.
RNA world hypothesis: The hypothesis that early life was based on RNA rather than DNA and protein, since RNA molecules can both store genetic information (like DNA) and catalyze chemical reactions (like an enzyme) — potentially explaining how the first self-replicating systems could have arisen.
Rough endoplasmic reticulum (rough ER): A network of membranes studded with ribosomes, where proteins destined for secretion or for membranes are synthesized and initially folded.
RuBisCO: The enzyme that catalyzes the first step of the Calvin cycle, attaching (fixing) atmospheric CO2 onto an existing organic molecule. It's the key enzyme of carbon fixation and is thought to be the most abundant protein on Earth.

S

S phase: The stage of interphase during which a cell's entire DNA content is replicated, so that each chromosome now consists of two identical sister chromatids.
SA node: The sinoatrial node, a small cluster of specialized cells in the heart that generates the electrical signal initiating each heartbeat, acting as the heart's natural pacemaker.
Sarcomere: The basic repeating structural and functional unit of a skeletal muscle fiber, bounded by Z-lines and containing the overlapping actin and myosin filaments responsible for contraction.
Sarcoplasmic reticulum (SR): A specialized form of smooth ER inside muscle cells that stores calcium ions and releases them in response to an action potential, triggering muscle contraction; it also actively pumps calcium back in to allow the muscle to relax.
Saturated fat: A fat whose fatty acid tails have no carbon-carbon double bonds, allowing them to pack tightly together — which is why saturated fats are typically solid at room temperature.
Second law of thermodynamics: The principle that every energy conversion increases the total entropy (disorder) of the universe. A cell can build local order (like an organized molecule), but only by releasing enough disorder (usually as heat) elsewhere to keep the universe's overall entropy increasing.
Second messenger: A small, diffusible molecule (such as cyclic AMP) generated inside a cell in response to receptor activation, which spreads and amplifies the original signal to trigger downstream effects.
Secondary active transport (cotransport): A form of transport that moves one substance against its gradient by coupling that movement to a second substance moving down its own gradient, using the energy stored in that second gradient rather than spending ATP directly. The sodium-glucose cotransporter, which uses the sodium gradient (built by the sodium-potassium pump) to pull glucose into a cell against its own gradient, is a classic example -- and the same transporter reappears in the kidney's reabsorption step in Track 48.
Secondary structure: Local, repeating folding patterns within a polypeptide chain — alpha helices (coiled) and beta sheets (pleated) — held together by hydrogen bonds between atoms in the polypeptide's backbone.
Secretion (renal): The process in the nephron's tubule by which additional waste products and excess ions are actively transported from the blood into the tubule fluid, to be excreted in the urine.
Selective permeability: The property of the plasma membrane that allows some substances to cross freely while restricting or blocking others, letting the cell control what enters and leaves.
Sensory transduction: The process by which a sensory receptor converts a specific form of physical stimulus energy (light, sound-related pressure, or a chemical) into an electrical nerve signal that can be interpreted by the nervous system.
Set point: The target value that a homeostatic system works to maintain, around which negative feedback keeps a variable (like body temperature) stable.
Sex-linked: Describes a gene located on a sex chromosome (typically the X chromosome), which causes its inheritance pattern to differ between males and females.
Sexual selection: A form of natural selection in which certain traits evolve specifically because they improve an individual's ability to attract mates, even if the trait provides no direct survival advantage (or is even a survival disadvantage) — a peacock's tail is a classic example.
Shared derived characters: Traits that evolved in a common ancestor and are shared by all of its descendants, used to determine which species are most closely related when building a phylogenetic tree.
Signal amplification: The process by which a small number of ligand-receptor binding events can trigger a much larger cellular response, because each step of a signal transduction pathway can activate multiple downstream molecules.
Signal transduction pathway: A cascade of molecular relay steps inside a cell, triggered by a ligand binding to its receptor, that converts and often amplifies the original signal into a specific cellular response.
Silent mutation: A point mutation that does not change the resulting amino acid, because the genetic code's degeneracy means the new codon still codes for the same amino acid as the original.
Simple diffusion: The passive movement of small, nonpolar molecules (like O2 and CO2) directly across the phospholipid bilayer, from an area of higher concentration to lower concentration, requiring no energy or transport protein.
Single-strand binding protein: A protein that attaches to separated single strands of DNA during replication, preventing them from re-forming the double helix before they can be copied.
Sink (plant): A plant tissue, such as a root or developing fruit, that consumes or stores sugar delivered to it via phloem from source tissues.
Sister chromatid: One of two identical copies of a chromosome, produced during S phase and joined together at the centromere, that will be separated into two different daughter cells during mitosis.
Sliding filament model: The model describing muscle contraction as the actin and myosin filaments sliding past each other (rather than the filaments themselves shortening), which shortens the overall sarcomere and the muscle fiber.
Smooth endoplasmic reticulum (smooth ER): A network of membranes without ribosomes, responsible for synthesizing lipids and detoxifying harmful substances.
Sodium-potassium pump: An active transport protein that uses ATP to pump three sodium ions out of the cell and two potassium ions into the cell, maintaining the concentration gradients that are essential for processes like the nerve action potential (Track 43).
Solvent: A substance capable of dissolving other substances (solutes) within it. Water is called the 'universal solvent' of biology because its polarity lets it dissolve most of the charged and polar molecules that reactions in the cell depend on.
Source (plant): A plant tissue, such as a photosynthesizing leaf, that produces more sugar than it needs and exports the surplus via phloem.
Speciation: The evolutionary process by which one ancestral species splits into two or more distinct species, generally driven by reproductive isolation between diverging populations.
Specific heat: The amount of energy required to raise the temperature of a substance. Water has an unusually high specific heat because so much energy goes into breaking hydrogen bonds before the temperature can rise, which is why water resists rapid temperature swings and helps organisms maintain a stable internal temperature.
Spontaneous vs. induced mutation: A spontaneous mutation arises on its own from natural DNA replication errors or normal chemical instability, with no outside trigger. An induced mutation is caused by exposure to an outside mutagen, such as UV light, radiation, or a mutagenic chemical.
Stabilizing selection: A mode of natural selection that favors the average (intermediate) phenotype and selects against both extremes, narrowing the population's trait distribution around the middle over time.
Start codon: The codon (AUG) that signals the ribosome to begin translation, and which always codes for the amino acid methionine as the first amino acid of a new protein.
Steroid: A lipid built from four fused carbon rings, including cholesterol (a component of cell membranes) and steroid hormones (like estrogen and testosterone) that regulate gene expression.
Steroid hormone: A lipid-soluble hormone that can diffuse directly through the plasma membrane; it binds a receptor inside the target cell and directly influences gene transcription.
Stomata: Small pores on a leaf's surface, each flanked by a pair of guard cells that open and close it, allowing gas exchange (CO2 in, O2 out) and being the primary site of water loss through transpiration.
Stop codon: One of three specific mRNA codons that do not code for an amino acid but instead signal the ribosome to end translation and release the completed polypeptide.
Stroma: The fluid-filled space inside the chloroplast, outside the thylakoids, where the Calvin cycle takes place.
Substrate: The specific molecule that an enzyme acts upon, binding to the enzyme's active site to be converted into product.
Substrate-level phosphorylation: The direct transfer of a phosphate group from a substrate molecule to ADP to form ATP, without involving the electron transport chain — this is how glycolysis and the Krebs cycle each generate a small amount of ATP directly.
Sulfhydryl group (-SH): A functional group consisting of a sulfur bonded to a hydrogen, found in the amino acid cysteine. Two sulfhydryl groups on nearby cysteines can react to form a disulfide bridge, a strong covalent bond that helps stabilize a protein's tertiary and quaternary structure.
Surface-area-to-volume ratio: The relationship between a cell's outer surface area (which limits how much material can be exchanged with the environment) and its internal volume (which determines how much material the cell needs). As a cell grows larger, volume increases faster than surface area, which is a key constraint limiting how big a single cell can get.
Sympatric speciation: Speciation that occurs without any geographic separation, through mechanisms like polyploidy or strong behavioral divergence within the same physical location.
Synapse: The junction between two neurons (or a neuron and another cell) where a signal is transmitted, typically via the release of a chemical neurotransmitter from the sending cell to receptors on the receiving cell.
System integration: The way multiple organ systems (nervous, endocrine, immune, circulatory, etc.) coordinate together, rather than functioning in isolation, to maintain the overall homeostasis and function of the whole organism.
Systole: The phase of the cardiac cycle in which the heart's chambers contract, pumping blood out.

T

T-tubule: An inward extension of a muscle fiber's plasma membrane that carries the electrical action potential deep into the interior of the cell, allowing it to trigger calcium release from the sarcoplasmic reticulum quickly and simultaneously throughout the fiber.
Taq polymerase: A heat-stable DNA polymerase enzyme, originally isolated from a heat-loving bacterium, that can withstand the repeated high-temperature steps of PCR without being destroyed — a discovery that made automated PCR practical.
Telomerase: An enzyme that can rebuild and lengthen telomeres, counteracting the shortening that normally occurs with each cell division. It's active in germ cells and stem cells but largely inactive in most adult somatic cells -- and its abnormal reactivation is a common feature of cancer cells, allowing them to divide indefinitely.
Telomere: A repetitive, non-coding DNA sequence found at the very ends of a chromosome, acting as a protective cap. Because DNA polymerase cannot fully replicate the very end of a linear chromosome (the end-replication problem), telomeres shorten a little with each round of cell division, which is one proposed mechanism behind cellular aging.
Telophase: The final phase of mitosis, in which two new nuclear envelopes re-form around the separated sets of chromosomes at each pole, completing the formation of two genetically identical nuclei.
Template strand: The DNA strand that RNA polymerase reads (in the 3' to 5' direction) in order to synthesize a complementary RNA strand.
Ten percent rule: The general observation that only about 10% of the energy available at one trophic level is transferred to and available for the next trophic level, with the rest lost primarily as heat — which is why energy pyramids narrow sharply at higher trophic levels.
Terminator: A specific DNA sequence marking the end of a gene, signaling RNA polymerase to stop transcription and release the finished RNA transcript.
Tertiary structure: The overall three-dimensional shape that a single polypeptide chain folds into, driven by interactions (hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges) between the amino acid side chains.
Test cross: A genetic cross between an individual with an unknown genotype for a dominant trait and an individual that is homozygous recessive for that trait, used to determine whether the unknown individual is homozygous dominant or heterozygous based on the ratio of offspring phenotypes produced.
Threshold: The minimum level of membrane depolarization a stimulus must reach to trigger a full action potential; below threshold, no action potential occurs at all, which is why the action potential is described as 'all-or-nothing.'
Thylakoid: The membrane-bound, disc-shaped compartment inside the chloroplast where the light-dependent reactions of photosynthesis take place.
Thymine dimer: A type of DNA damage caused by ultraviolet light, in which two adjacent thymine bases on the same DNA strand become abnormally fused together, distorting the DNA helix and blocking normal replication and transcription unless repaired.
Tight junction: A type of cell junction that seals the space between adjacent cells so tightly that it forms a watertight barrier, preventing substances from leaking through the gaps between cells — important in epithelial linings like the gut and skin.
Tissue: A group of similar cells working together to perform a specific function, the level of organization between individual cells and organs.
Tonicity: A description of a solution's solute concentration relative to a cell, which determines the net direction water will move by osmosis. A hypertonic solution has more solute than the cell (water leaves, cell shrinks); a hypotonic solution has less solute than the cell (water enters, cell swells); an isotonic solution has equal solute concentration (no net water movement).
Topoisomerase: An enzyme that relieves the mechanical strain (supercoiling) that builds up in the DNA helix just ahead of the replication fork as helicase unwinds it, cutting and resealing the DNA strands to release that tension so replication can keep proceeding smoothly.
Trans fat: An unsaturated fat whose double bond has been chemically altered (often through partial hydrogenation) so its fatty acid tail is straight rather than kinked, letting it pack tightly and behave like a saturated fat despite technically being unsaturated -- associated with worse cardiovascular health outcomes than either natural saturated or unsaturated fats.
Transfer RNA (tRNA): A small RNA molecule that carries a specific amino acid on one end and has an anticodon on the other end, physically linking the mRNA code to the correct amino acid during translation.
Transpiration: The evaporation of water from a plant's leaf surfaces, which creates a pulling force that draws more water up through the xylem from the roots, relying on water's cohesive properties.
Triglyceride: A lipid made of a glycerol backbone attached to three fatty acid chains, used primarily for long-term energy storage in the body.
Trophic level: A feeding position in a food chain, defined by how many energy transfers separate an organism from the original energy source (producers).
Tropism: A directional growth response of a plant toward or away from an external stimulus. Phototropism is growth in response to light (typically toward it); gravitropism is growth in response to gravity (roots grow with gravity, shoots grow against it).
Tropomyosin: A regulatory protein that, at rest, covers the myosin-binding sites on actin, physically blocking the cross-bridge cycle from occurring until it is shifted out of the way by calcium-bound troponin.
Troponin: A regulatory protein bound to tropomyosin on the actin filament that binds calcium ions; when calcium binds troponin, it shifts tropomyosin's position to expose actin's myosin-binding sites.
Tumor suppressor gene: A gene that normally acts as a brake on cell division, halting the cell cycle or triggering repair/cell death when problems are detected. p53 and Rb are the two classic examples; losing both copies of a tumor suppressor gene removes an important safeguard against cancer.
Turnover: The property of an enzyme being able to catalyze a reaction and then release its product unchanged, allowing the same enzyme molecule to be reused repeatedly rather than being consumed.

U

Unsaturated fat: A fat with one or more carbon-carbon double bonds in its fatty acid tails, which create a kink that prevents tight packing — which is why unsaturated fats are typically liquid at room temperature.
Urea: The primary nitrogenous waste product excreted by mammals, produced in the liver from ammonia; less toxic than ammonia and requires less water to excrete safely.

V

Valence electron: An electron in an atom's outermost shell. The number of valence electrons determines an atom's bonding behavior — how many bonds it can form and with what kind of atoms.
Vascular tissue: Specialized plant tissue (xylem and phloem) that transports water, nutrients, and sugars throughout the plant body, allowing vascular plants to grow larger and live in a wider range of environments than non-vascular plants.
Vasoconstriction: The narrowing of blood vessels, which decreases blood flow near the skin's surface and helps the body conserve heat.
Vasodilation: The widening of blood vessels, which increases blood flow near the skin's surface and helps the body release excess heat.
Vertebrate: An animal that possesses a backbone (vertebral column), including fish, amphibians, reptiles, birds, and mammals.
Vestigial structure: An anatomical structure that has lost most or all of its original function over evolutionary time, but persists as a reduced remnant -- evidence of an organism's evolutionary history, such as the human appendix or the pelvic bones of some whales.
Villi: Tiny, finger-like projections lining the wall of the small intestine that dramatically increase its surface area, maximizing the absorption of digested nutrients into the bloodstream.
Viral envelope: A lipid membrane, derived from a host cell's own membrane, that surrounds the capsid of some viruses. Enveloped viruses are generally more fragile outside a host than non-enveloped viruses.

W

Wobble: The relaxed, non-strict base-pairing allowed at the third position of a codon-anticodon match, which lets a single tRNA recognize more than one codon, reducing the total number of distinct tRNAs a cell needs.

X

X-linked recessive: An inheritance pattern for a recessive gene located on the X chromosome. Because males have only one X chromosome, a single copy of an X-linked recessive allele is enough to cause the condition in a male, which is why conditions like color blindness and hemophilia appear far more often in sons than daughters; female carriers with one copy are typically unaffected.
Xylem: The plant vascular tissue responsible for transporting water and dissolved minerals upward from the roots to the rest of the plant, driven largely by transpiration pulling water up through cohesion.

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