European Baccalaureate Biology key terms

The maintenance of a stable internal environment, such as temperature, blood pH or glucose concentration, despite changes outside the organism. It depends on receptors, a control centre and effectors working as negative or positive feedback loops.

A control mechanism in which a deviation from a set point triggers a response that reduces that deviation, bringing the system back towards its normal value. Most physiological regulation, including thermoregulation and blood glucose control, works this way.

A control mechanism in which a change is amplified rather than reversed, pushing the system further from its starting state. It is usually short-lived and self-limiting, as in oxytocin-driven uterine contractions during childbirth or the spread of an action potential.

The control of water and dissolved solute concentrations in body fluids so that cells neither swell nor shrink excessively. In mammals it involves the kidneys, antidiuretic hormone and the hypothalamus detecting plasma water potential.

A protein, or occasionally an RNA molecule, that speeds up a specific biochemical reaction by lowering its activation energy. Enzymes are not consumed and are sensitive to temperature, pH and substrate concentration.

The small region of an enzyme where the substrate binds and catalysis occurs. Its shape and chemical properties are complementary to the substrate, which explains enzyme specificity and competitive inhibition.

The loss of a protein's three-dimensional shape, and therefore its function, caused by heat, extreme pH or chemical agents that break hydrogen and ionic bonds. The primary sequence of amino acids remains unchanged.

A form of enzyme inhibition in which a molecule resembling the substrate binds to the active site and blocks access. Raising substrate concentration can overcome it, so maximum reaction rate is unchanged but affinity appears lower.

The first stage of cellular respiration, occurring in the cytoplasm, in which glucose is split and oxidised to pyruvate with a net gain of two ATP and two reduced NAD. It requires no oxygen and is common to aerobic and anaerobic pathways.

A sequence of reactions in the mitochondrial matrix in which acetyl groups from pyruvate are oxidised to carbon dioxide, generating reduced NAD and FAD and a small amount of ATP. It also supplies intermediates for other metabolic pathways.

ATP production driven by a proton gradient across the inner mitochondrial membrane. Electrons passed along the electron transport chain pump protons, and their return through ATP synthase generates most of the ATP from aerobic respiration.

Adenosine triphosphate, the universal energy currency of cells. Hydrolysis of its terminal phosphate bond releases usable energy for processes such as active transport, muscle contraction and synthesis, and it is regenerated during respiration.

Nuclear division producing two genetically identical daughter nuclei with the same chromosome number as the parent cell. It underlies growth, repair and asexual reproduction and is followed by cytokinesis.

A reduction division producing four haploid gametes from one diploid cell. Crossing over in prophase I and independent assortment in metaphase I create genetic variation among the products.

The exchange of corresponding segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis. It creates new combinations of alleles on a chromosome and contributes to genetic variation.

The random orientation of each homologous pair or chromosome at metaphase, so that maternal and paternal chromosomes are distributed to gametes independently of one another. It generates a huge number of possible allele combinations.

One of the alternative versions of a gene at a given locus, which may differ slightly in DNA sequence and produce different phenotypes. An individual may carry two identical or two different alleles for a gene.

The particular combination of alleles an organism carries for one or more genes, whether or not those alleles are expressed. It is often written as pairs of letters such as AA, Aa or aa.

The observable characteristics of an organism resulting from the interaction between its genotype and the environment. It includes features such as eye colour, blood group or enzyme activity.

The relationship in which one allele of a heterozygous pair determines the phenotype, masking the effect of the other. Incomplete dominance and codominance are variants in which both alleles influence the visible trait.

The process by which individuals with heritable traits that improve survival and reproduction leave more offspring, so those alleles increase in frequency over generations. It requires variation, inheritance and differential reproductive success.

The formation of new species when populations become reproductively isolated and diverge genetically, for example through geographic separation. Over time the populations accumulate differences that prevent interbreeding.

The variety of life at genetic, species and ecosystem levels. It is measured by species richness and evenness and is influenced by habitat loss, climate and human activity.

A community of interacting organisms together with the non-living components of their environment, such as light, water and soil. Energy flows through it while nutrients are recycled.

A feeding position in a food chain, such as producer, primary consumer or secondary consumer. Energy and biomass generally decrease at higher levels because much is lost as heat and in waste.

The net movement of particles from a region of higher concentration to one of lower concentration, driven by random molecular motion. It is passive and continues until equilibrium is reached.

The diffusion of water across a partially permeable membrane from a region of higher water potential to one of lower water potential. It explains water uptake by plant roots and cell behaviour in different solutions.

Movement of substances across a membrane against their concentration gradient using energy from ATP and carrier proteins. It allows cells to accumulate ions or nutrients at concentrations above those outside.

A chemical messenger secreted by an endocrine gland into the blood, which travels to target cells bearing specific receptors and alters their activity. Insulin and adrenaline are examples.

A hormone released by the beta cells of the pancreas when blood glucose rises. It increases glucose uptake by cells and promotes glycogen formation in the liver, lowering blood glucose concentration.

A rapid reversal of electrical charge across a neuron's membrane, caused by ion channels opening and ions moving. It propagates along the axon and is the basis of nerve signalling.

The junction between two neurons or between a neuron and a muscle, across which a signal passes by release of neurotransmitter. Transmission is unidirectional and involves a short delay.

The ability of an organism to resist infection through physical barriers and immune responses. Active immunity follows infection or vaccination and produces memory cells; passive immunity involves transferred antibodies and is short-lived.

A protein produced by plasma cells that binds specifically to an antigen, marking pathogens for destruction or neutralising them. Its variable region determines which antigen it recognises.

The administration of antigens, often weakened or inactivated pathogens, to stimulate an immune response and memory cells without causing disease. It provides active immunity and can protect populations through herd immunity.

A testable, falsifiable statement proposing an explanation for an observation. In experimental work it predicts the effect of an independent variable on a dependent variable and guides the design of controls.

The factor deliberately changed or selected by the investigator in an experiment, whose effect on the dependent variable is measured. All other relevant factors are kept constant.

The factor measured or observed in an experiment to assess the effect of the independent variable. It should be quantified with appropriate units and repeated to reduce random error.

An investigation in which all variables except the independent variable are held constant, often including a control group receiving no treatment. This allows observed effects to be attributed to the variable tested.

The extent to which repeated measurements under the same conditions give consistent results. It is improved by repeating trials, taking multiple readings and removing obvious anomalies.

The extent to which an experiment measures what it is intended to measure and the conclusion follows from the data. Good control of variables and appropriate measurement techniques increase validity.

A statistical relationship in which two variables change together, either positively or negatively. Correlation does not by itself establish causation, because a third factor may influence both.

The likelihood that an observed result is not due to chance alone, usually expressed by a probability value such as p < 0.05. It supports rejecting a null hypothesis but does not measure effect size.

The study of moral questions raised by biology and medicine, such as genetic testing, cloning, organ donation or animal experimentation. It weighs benefits against harms, rights, autonomy and social justice.

Agreement to take part in research or treatment given freely after receiving understandable information about purpose, procedures, risks and alternatives. It is a central requirement of ethical research involving humans.

The idea that action to prevent serious or irreversible harm should not be postponed because of incomplete scientific certainty. It is often invoked in environmental and biotechnology policy debates.