IB Biology HL key terms

The principle that all living organisms are made of one or more cells, that the cell is the basic unit of structure and function, and that new cells arise only from existing cells by division.

A single-celled organism lacking a membrane-bound nucleus and other membrane-bound organelles; its DNA lies free in the cytoplasm as a circular chromosome, and it typically has 70S ribosomes and a cell wall.

An organism whose cells contain a nucleus enclosing the chromosomes and membrane-bound organelles such as mitochondria and the endoplasmic reticulum; includes protists, fungi, plants and animals.

The division of a eukaryotic cell into membrane-bound regions with distinct chemical conditions, allowing incompatible reactions such as protein synthesis and digestion to occur simultaneously and increasing the efficiency of metabolism.

A small, circular, extrachromosomal DNA molecule found in many bacteria, carrying a few non-essential genes such as antibiotic resistance, and widely used as a vector in genetic engineering.

A double-membraned organelle in which aerobic respiration produces ATP; its inner membrane is folded into cristae bearing the electron transport chain, and it contains its own circular DNA and 70S ribosomes.

A membrane-bound vesicle containing hydrolytic enzymes at low pH, used to digest material taken in by endocytosis and to break down worn-out organelles during autophagy.

A stack of flattened membrane sacs that modifies proteins and lipids received from the endoplasmic reticulum, then packages them into vesicles for secretion or delivery to other organelles.

An interconnected network of membrane tubules and sacs continuous with the outer nuclear membrane; rough ER studded with ribosomes folds and transports proteins, while smooth ER synthesises lipids and detoxifies substances.

The movement of substances within cells by membrane-bound sacs that bud from one compartment and fuse with another, carrying cargo such as proteins between the endoplasmic reticulum, Golgi apparatus and plasma membrane.

The theory that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by a host cell, supported by their own circular DNA, 70S ribosomes and double membranes.

A double-membraned plant organelle containing thylakoid stacks and stroma, where light energy drives photosynthesis to fix carbon dioxide into carbohydrate; it holds its own DNA and 70S ribosomes.

A flattened membrane sac inside a chloroplast whose membrane carries photosystems, electron transport proteins and ATP synthase; stacked into grana, it is the site of the light-dependent reactions.

ATP synthesis driven by the flow of protons down their concentration gradient through ATP synthase, following a proton gradient built up across a membrane by electron transport.

A series of membrane-embedded protein complexes that pass electrons from carrier to carrier, releasing energy used to pump protons and establish the gradient that drives chemiosmosis.

The description of the plasma membrane as a phospholipid bilayer in which proteins and other molecules are embedded and can move laterally, giving the membrane a dynamic, patchy structure.

A protein with attached carbohydrate chains, found projecting from the outer surface of the plasma membrane, where the sugar groups act in cell recognition, adhesion and receptor binding.

The net movement of water across a partially permeable membrane from a region of higher water potential to one of lower water potential, driven by differences in solute concentration.

The potential energy of water per unit volume, measured in kPa; pure water at atmospheric pressure is zero, and adding solutes lowers it, so water moves from higher to lower values.

Movement of a substance across a membrane against its concentration gradient using energy from ATP hydrolysis, typically through a pump protein that changes shape as it binds and releases ions.

Passive movement of polar or charged substances across a membrane through channel or carrier proteins, down their concentration gradient and without expenditure of metabolic energy.

An ATP-driven carrier protein that moves three sodium ions out of and two potassium ions into the cell per cycle, maintaining resting membrane potential and driving secondary active transport such as glucose uptake.

A rapid, self-propagating reversal of membrane potential along an axon, produced by voltage-gated sodium channels opening and then potassium channels restoring the resting state.

The junction between a neuron and another cell, where an arriving action potential triggers release of neurotransmitter into a narrow cleft, generating a new signal in the postsynaptic cell.

The process by which a cell detects an external chemical message, transduces it through a receptor and intracellular pathway, and produces a specific response such as gene expression or secretion.

A globular protein that speeds up a specific reaction by lowering activation energy, binding its substrate at an active site whose shape complements the substrate.

The region of an enzyme, formed by a few amino acids brought together by folding, that binds the substrate and catalyses its conversion to product.

Loss of a protein's three-dimensional shape, and therefore its function, caused by heat, extremes of pH or other agents breaking hydrogen and ionic bonds without breaking peptide bonds.

The organisation of a polypeptide into primary sequence, secondary helices and sheets, tertiary three-dimensional folding and, for multi-subunit proteins, quaternary assembly of chains.

A molecule that reduces enzyme activity, either competing with substrate at the active site (competitive) or binding elsewhere and altering active-site shape (non-competitive), sometimes reversibly.

Nuclear division producing two genetically identical daughter nuclei with the same chromosome number, used in growth, repair and asexual reproduction; preceded by DNA replication in interphase.

Two successive nuclear divisions following one round of DNA replication that halve the chromosome number and generate genetic variation through crossing over and independent assortment.

Exchange of corresponding segments between non-sister chromatids of homologous chromosomes during prophase I, producing recombinant chromosomes and increasing genetic variation in gametes.

The random orientation of each homologous pair at metaphase I, so maternal and paternal chromosomes of different pairs are distributed to gametes independently, generating new allele combinations.

The introduction of a gene from one organism into another, for example by plasmids or viruses, producing a genetically modified organism that expresses the new trait.

A bacterial enzyme that cuts DNA at a specific recognition sequence, leaving sticky or blunt ends; it is used to insert genes into vectors and to analyse DNA fragments.

An in vitro technique that repeatedly denatures DNA, anneals primers and extends them with a heat-stable polymerase, doubling the target sequence each cycle to amplify it exponentially.

A technique separating DNA fragments or proteins by size and charge as they migrate through a gel under an electric field, with smaller fragments moving faster towards the positive electrode.

The process by which individuals with inherited traits giving higher survival and reproduction leave more offspring, so allele frequencies in a population change over generations.

The proportion of all copies of a gene in a population that are of one particular allele, calculated from genotype counts and used to measure evolutionary change.

The condition in which allele and genotype frequencies stay constant across generations in an idealised population with no selection, mutation, migration, genetic drift or non-random mating.

The formation of new species when populations become reproductively isolated, for example by geographical separation, so that genetic differences accumulate until interbreeding no longer yields fertile offspring.

A method of classifying organisms by shared derived characteristics to reconstruct evolutionary relationships, grouping species into clades that each contain an ancestor and all its descendants.

The two-part naming system for species, giving a genus name followed by a species epithet, both italicised with the genus capitalised, used internationally to avoid ambiguity.

The maximum population size of a species that an ecosystem can sustain indefinitely, set by limiting resources such as food, space and nesting sites.

A feeding position in a food chain, such as producer, primary consumer or secondary consumer, defined by the number of energy-transfer steps from the original light capture.

The passage of energy between trophic levels, of which only about ten per cent is passed on, the rest being lost as heat through respiration, in uneaten parts and in excreta.

Warming of the lower atmosphere as gases such as carbon dioxide and methane absorb and re-emit outgoing infrared radiation, a natural process intensified by human emissions.

A factor deliberately kept constant across experimental groups so that any measured difference can be attributed to the independent variable rather than to a confounding influence.

The factor deliberately manipulated or selected by the investigator in an experiment, whose effect on the measured dependent variable is being tested.

A statement that any apparent pattern or difference in data arises from chance alone, with no real relationship between the variables; statistical tests estimate the probability of obtaining the data if it is true.

The consistency of repeated measurements, improved by increasing sample size, repeating the whole experiment and using precise instruments, so that results can be reproduced by others.

A statistical association in which two variables change together, either positively or negatively; it does not by itself establish that one variable causes the other.