IB Diploma Biology All terms (2025+)

Abiotic

the non-living components of an ecosystem.

Abiotic factor

a non-living, physical factor that can influence an organism or ecosystem, e.g. temperature, sunlight, pH, salinity or precipitation.

ABO system

a system of four basic blood types (A, AB, B and O), based on the presence or absence of specific inherited antigens.

Absorption spectrum

a graph showing the relative absorbance of different wavelengths of light by a pigment.

Accessory pigments

light-absorbing compounds that trap light energy and channel it to chlorophyll a, the primary pigment, which initiates the reactions of photosynthesis.

Accuracy

how close to the true value a result is.

Acetylcholine (ACh)

a neurotransmitter that functions in both the central and peripheral nervous systems.

Acrosome

sac at head of sperm containing protease enzymes.

Actin

a thin protein that forms (together with myosin) the contractile filaments of muscle cells; contains binding sites for the myosin heads.

Action potential

the potential difference produced across the plasma membrane of the nerve cell when stimulated, reversing the resting potential from about −70mV to about +40mV.

Action spectrum

range of wavelengths of light within which a process like photosynthesis takes place.

Activated B-cell

B-cells absorb an antigen and present pieces of it on their surface via a major histocompatibility complex (MHC); helper T-cells then recognize the antigens via the MHC and activate the B-cells, resulting in B-cell differentiation into memory B-cells or plasma cells.

Activation energy

energy required by a substrate molecule before it can undergo a chemical change.

Active site

region of an enzyme molecule where the substrate molecule binds and catalysis occurs.

Active transport

movement of particles from lower to higher concentration, using energy from ATP that has been created during respiration. Movement is through carrier proteins.

Adaptive immune system

responds in a specific way to particular pathogens and builds up a memory of pathogens encountered, so the immune response becomes more effective.

Adaptive radiation

the diversification of an ancestral species into new species, characterized by great ecological and morphological diversity, filling different ecological niches.

Adenine

a purine nitrogenous base, found in the coenzymes ATP and NADP, and in nucleic acids (DNA and RNA), that pairs with thymine.

Adenosine diphosphate (ADP)

a nucleotide, present in every living cell, made of adenosine and two phosphate groups bonded in a linear sequence; it is important in energy-transfer reactions of metabolism.

Adenosine triphosphate (ATP)

a nucleotide, present in every living cell, formed in photosynthesis and respiration from ADP and Pi; it functions in metabolism as a common intermediate between energy-requiring and energy-yielding reactions.

Adenylyl cyclase/adenylate cyclase

enzyme that catalyses the formation of cAMP from ATP.

Adhesion

the force by which individual molecules stick to surrounding materials and surfaces.

Adipose tissue

a tissue found beneath the skin layer, containing fat cells.

Aerobic respiration

respiration requiring oxygen, involving the oxidation of glucose to carbon dioxide and water.

Afforestation

the establishment of forests in an area where there was no previous tree cover.

Agglutination

process in which red blood cells are clumped together by an antibody.

Agrochemical

a chemical used in agriculture, such as a pesticide or a fertilizer.

AIDS (acquired immune deficiency syndrome)

the name used to describe a number of potentially life-threatening infections and illnesses that happen when the immune system has been severely damaged by the HIV virus.

Albedo

the fraction of solar radiation reflected by a surface or object, often expressed as a percentage.

Alien species

species that are introduced into an area by human activity.

Allele

different versions of the same gene occupying a specific locus on a chromosome.

Allelopathy

chemical inhibition of one plant (or other organism) by another, due to the release of chemicals (allelochemicals) that act as germination or growth inhibitors.

Allopatric speciation

speciation that occurs when two groups of organisms are spatially separated by a physical or geographic barrier, e.g. mountain ranges and large rivers.

Allosteric protein

a protein that can exist in multiple conformations (shapes) depending on the binding of a molecule (at a site other than the catalytic site).

Allosteric regulators

molecules that change the shape and activity of an enzyme by reversibly binding at a site on the enzyme.

Alpha (α) cell

glucagon-secreting cell of the islets of Langerhans in the pancreas.

Alternative hypothesis

there is a statistically significant difference between two variables.

Alternative splicing

process of splicing of exons to produce variants of a protein from a single gene.

Altruism

behaviour of an animal that benefits another at its own expense. Biologists call a behaviour pattern altruistic if it increases the number of offspring produced by the recipient and decreases that of the altruist.

Alveolus (plural, alveoli)

air sac in the lung.

Amphipathic

a molecule that has two different affinities - a polar end that is attracted to water (hydrophilic) and a non-polar end that is repelled by it (hydrophobic).

Anabolism

the synthesis of complex molecules from simpler molecules including the formation of macromolecules from monomers by condensation reactions.

Anaerobic respiration

respiration in the absence of oxygen, producing either lactic acid (humans) or ethanol and carbon dioxide (plants and yeast).

Analogous structure

a feature with a similar function and superficial structural similarity, but different fundamental structure and evolutionary origin.

Analyse

break down in order to bring out the essential elements or structure.

Anaphase

stage in nuclear division where chromosomes move away from one another to opposite poles of the cell.

Annotate

add brief notes to a diagram or graph.

Antagonistic muscle

a muscle that works as one of a pair: as one muscle contracts, the other muscle relaxes/lengthens. The muscle that is contracting is called the agonist and the muscle that is relaxing/lengthening is called the antagonist.

Anther

produces pollen, which contains the male gamete.

Anthropogenic

relating to human activity.

Antibiotics

chemicals that block processes occurring in bacteria but not in eukaryotic cells.

Antibody

a protein produced by blood plasma cells derived from B lymphocytes when in the presence of a specific antigen, which then binds with the antigen, aiding its destruction.

Anticodon

set of three consecutive nucleotides in a tRNA molecule that recognizes, through base pairing, the three-nucleotide codon on a messenger RNA molecule.

Antigen

a substance (usually glycoproteins or other protein) capable of binding specifically to an antibody. It is recognized by the body as foreign (non-self) and stimulates an immune response.

Antigenic drift

small changes (caused by mutations) in viral genes that can lead to changes in the surface proteins of a virus, HA (haemagglutinin) and NA (neuraminidase).

Antigenic shift

an abrupt, major change in an influenza A virus, resulting in new HA (haemagglutinin) and/or new HA and NA (neuraminidase) proteins. Happens when recombination occurs.

Antigen presentation

process where an antigen is taken into a white blood cell by receptor-mediated endocytosis, digested, complexed with MHC II molecules and then presented on the cell surface membrane for interaction with T-cells.

Antigen-specific B-cell

lymphocytes that form either antibody-secreting cells or memory B-cells after infection or vaccination.

Aorta

main artery that carries blood away from the heart to the rest of the body.

Apoplast pathway

the pathway (e.g. of water) through the non-living part of a cell, e.g. cell walls and spaces between cells.

Apoptosis

a form of programmed cell death that allows cells that are unneeded or unwanted to be eliminated from an adult or developing organism.

Aquaporin

a water channel pore (protein) in a membrane.

Arrested succession (plagioclimax)

an area where human activity has prevented the ecosystem developing a climax community.

Arteriole

a very small artery.

Artificial classification

classifying organisms on the basis of few, self-evident features.

Artificial selection

selection in breeding, carried out deliberately by humans to alter populations.

Asexual reproduction

reproduction not involving gametes and fertilization.

Assimilation

uptake of nutrients into cells and the utilization of this material to provide energy and to synthesize new biological molecules.

Atherosclerosis

deposition of plaque (cholesterol derivative) in the inner wall of blood vessels.

ATPase

a group of enzymes that catalyze the hydrolysis of a phosphate bond in adenosine triphosphate (ATP) to form adenosine diphosphate (ADP).

ATP synthase (ATPase)

a complex of integral proteins located in the mitochondrial inner membrane where it catalyses the synthesis of ATP from ADP and phosphate, driven by a flow of protons.

Atrioventricular node

mass of specialized cardiac muscle cells in the wall of the right atrium.

Atrioventricular valve

heart valve that opens to allow the passage of blood into a ventricle; it closes to prevent the backflow of blood into the atrium.

Atrium (plural, atria)

upper chambers of the heart from which blood is passed to the ventricles.

Autoinducers

a signalling molecule produced and used by bacteria participating in quorum sensing.

Autonomic nervous system (ANS)

the involuntary nervous system.

Autophosphorylation

the phosphorylation of a kinase by itself.

Autosomal gene

gene located on one of the numbered, or non-sex, chromosomes.

Autotrophic

using external energy sources to synthesize glucose from simple inorganic substances.

Auxin

plant growth substance, indoleacetic acid, which generally stimulates cell elongation.

Axon

fibre carrying impulses away from the cell body of a neuron.

Bacteriophage

a virus that parasitizes bacteria (also known as a phage).

Baroreceptor

a sensory receptor responding to stretch in the walls of blood vessels to monitor blood pressure.

Belt transect

all organisms within a band, usually between 0.5m and 1m, are sampled along an environmental gradient.

Benign tumour

a tumour that tends to grow slowly and does not spread to other parts of the body.

Beta (β) cell

insulin-secreting cells of the islets of Langerhans in the pancreas.

Bicuspid valve

atrioventricular valve on the left side of the heart.

Binary fission

when a cell divides into two daughter cells, typically in asexual reproduction of prokaryotes.

Binomial system

double names for organisms, in Latin, with the generic name preceding the specific name.

Bioaccumulation

the build-up of non-biodegradable or slowly biodegradable chemicals in the body.

Biochemical oxygen demand (BOD)

a measure of the amount of dissolved oxygen required to break down the organic material in a given volume of water through aerobic biological activity.

Biodiversity

the amount of biological or living diversity per unit area. It includes the concepts of species diversity, habitat diversity and genetic diversity.

Biological species concept

a species is a group of organisms that can breed and produce fertile offspring.

Biomagnification

the process by which chemical substances become concentrated in the tissues of organisms at higher trophic levels.

Biome

groups of ecosystems with similar abiotic conditions and communities, defined by their climate and dominant plant species.

Biotic factor

a living part of an ecosystem (i.e. part of the community) that can influence an organism or ecosystem.

Bivalent

a pair of duplicated homologous chromosomes, held together by chiasmata during meiosis.

Blastocyst

embryo as fluid-filled ball of cells, at the stage of implantation.

B-lymphocytes (B-cells)

lymphocytes that produce antibodies and are responsible for presenting antigens to T-cells. Once activated, they can mature into plasma cells or memory B-lymphocytes.

Bohr effect

the decrease in the oxygen affinity of haemoglobin in response to decreased blood pH, resulting from increased carbon dioxide concentration in the blood.