IB Diploma Biology HL glossary

120 key terms from the IB Diploma Biology HL notes, A to Z. Each definition links to the note that teaches it.

A

Absorption spectrum: A graph showing the percentage of light absorbed by a pigment (or pigments) at each wavelength.Revise it: Absorption of light, absorption spectra and action spectra
Action spectrum: A graph showing the rate of photosynthesis at each wavelength of light.Revise it: Absorption of light, absorption spectra and action spectra
Adhesion: The attraction of water molecules to materials that are polar or charged, by hydrogen bonds or attraction to charges.Revise it: Cohesion and adhesion
Allele: One of the alternative forms of a gene, differing in base sequence by one or a few bases.Revise it: Genotype, phenotype, dominance and phenotypic plasticity
Allele frequency HL only: The proportion of all copies of a gene in a population that are a particular allele, given as a value from 0 to 1.Revise it: Gene pools and allele frequencies
Allelopathy: Release by a plant of chemicals that inhibit germination or growth of nearby plants, e.g. juglone from black walnut.Revise it: Allelopathy and secretion of antibiotics
Amphipathic: Having both a hydrophilic region and a hydrophobic region in the same molecule.Revise it: Phospholipid bilayers and steroids
Anabolism: Reactions that build macromolecules from monomers, for example by condensation.Revise it: Enzymes as catalysts and their role in metabolism
Antigenic drift HL only: Gradual mutations change influenza's H and N antigens, so existing antibodies bind less well; the vaccine is updated yearly.Revise it: Origins and rapid evolution of viruses
Aquaporins: Integral channel proteins that let water molecules cross the membrane rapidly, in single file.Revise it: Osmosis, channel proteins and pump proteins
Autotroph: An organism that uses an external energy source to synthesise carbon compounds from simple inorganic substances.Revise it: Autotrophs and heterotrophs
Autotroph (IB wording): An organism that uses an external energy source to synthesise carbon compounds from simple inorganic substances.Revise it: Autotrophs and heterotrophs

B

Biochemical oxygen demand (BOD): The amount of dissolved oxygen used by microorganisms in a water sample over a set time while breaking down organic matter. A high BOD shows heavy organic pollution.Revise it: Eutrophication
Biological species concept: A species is a group of organisms that can breed and produce fertile offspring.Revise it: The biological species concept and speciation
Bivalent: A pair of homologous chromosomes held together during meiosis I.Revise it: Meiosis, non-disjunction and variation

C

CAMs HL only: Cell-adhesion molecules: membrane proteins that bind cells together to form tissues.Revise it: Adhesion of cells to form tissues
Capillary action: Movement of water through narrow spaces, against gravity, caused by adhesion to the walls together with cohesion.Revise it: Cohesion and adhesion
Carrying capacity: The maximum population size of a species that an environment can support.Revise it: Carrying capacity and density-dependent factors
Catabolism: Reactions that break down macromolecules into monomers, for example by hydrolysis, or oxidise substrates as in respiration.Revise it: Enzymes as catalysts and their role in metabolism
Centromere: The region where the two sister chromatids are held together; its position along the chromosome is used to identify chromosomes.Revise it: Chromosome numbers and karyograms
Clade HL only: A group of organisms with common ancestry and shared characteristics: a common ancestor and all of its descendants.Revise it: Clades and the molecular clock
Cladogram HL only: A tree diagram that shows the most probable sequence of divergence in clades.Revise it: Constructing and analysing cladograms
Cohesion: The attraction of water molecules to each other, due to hydrogen bonding.Revise it: Cohesion and adhesion
Community: All of the interacting organisms in an ecosystem: all the populations in an area, including plants, animals, fungi and bacteria.Revise it: Intraspecific and interspecific relationships
Condensation reaction: A reaction that links two monomers by a covalent bond, releasing a water molecule.Revise it: Condensation and hydrolysis reactions
Conformation HL only: The three-dimensional shape of a protein.Revise it: R-groups and primary structure
Conjugated protein HL only: A protein with one or more non-polypeptide components (prosthetic groups) as well as polypeptides.Revise it: Quaternary structure: non-conjugated and conjugated proteins
Conserved sequence HL only: A base sequence (or amino acid sequence) that is identical or similar across a species or a group of species.Revise it: Conserved and highly conserved sequences
Covalent bond: A bond formed when two atoms share a pair of electrons. Covalent bonds are strong, so molecules built from them are stable.Revise it: Carbon atoms and the diversity of carbon compounds

D

Degeneracy: More than one codon can code for the same amino acid. For example, GAA and GAG both code for glutamic acid. Of the 64 codons, 61 code for amino acids and 3 are stop codons.Revise it: The genetic code
Denaturation: A change to the three-dimensional structure (conformation) of a protein so that it can no longer carry out its function. It is usually permanent.Revise it: Effect of pH and temperature on protein structure
Density-dependent factor: A factor whose effect on the population increases as population density increases, such as competition, predation or disease.Revise it: Carrying capacity and density-dependent factors
Density-independent factor: A factor that affects a population regardless of its density, such as a flood, fire, drought or very cold winter.Revise it: Carrying capacity and density-dependent factors
Differentiation: The development of unspecialized cells into specialized cells by expressing different genes.Revise it: Differentiation and stem cells
Diploid (2n): A nucleus with two of each type of chromosome, one from each parent (two full sets). Human body cells have 46 chromosomes.Revise it: Meiosis, non-disjunction and variation
DNA polymerase: Assembles a new strand: it brings free DNA nucleotides into position opposite their complementary bases on the template strand and links them together with covalent (phosphodiester) bonds to form the sugar–phosphate backbone.Revise it: Roles of helicase and DNA polymerase
DNA replication: The production of exact copies of DNA with identical base sequences.Revise it: Exact copies by semi-conservative replication
Double bond: Two shared pairs of electrons, e.g. C=C in unsaturated fatty acids or C=O in a carboxyl group.Revise it: Carbon atoms and the diversity of carbon compounds

E

Endocytosis HL only: The plasma membrane folds inwards around material outside the cell and pinches off to form a vesicle inside the cell. Example: phagocytosis of bacteria by white blood cells.Revise it: Membrane fluidity, cholesterol and vesicles
Enhancer HL only: A base sequence, which can be some distance from the gene, where activator transcription factors bind to increase the rate of transcription.Revise it: Gene expression and its regulation
Epigenesis HL only: The development of patterns of differentiation in the cells of a multicellular organism. Phenotype changes but not genotype.Revise it: Epigenesis, the transcriptome and epigenetic tags
Epigenetic inheritance HL only: Passing phenotypic changes to daughter cells or offspring without changing the DNA sequence, when tags stay in place through mitosis or meiosis.Revise it: Epigenetic inheritance and the environment
Essential amino acids: Amino acids that cannot be synthesised and must be obtained from food.Revise it: Dietary amino acids and the variety of peptide chains
Evolution: Change in the heritable characteristics of a population.Revise it: Evolution as change in heritable characteristics
Excretion: Removal of the waste products of metabolismRevise it: Processes of life in unicellular organisms
Exocytosis HL only: A vesicle moves to the plasma membrane, fuses with it and releases its contents outside the cell. Examples: secretion of digestive enzymes by pancreatic cells; release of neurotransmitter at a synapse.Revise it: Membrane fluidity, cholesterol and vesicles
Expiratory reserve volume: The extra volume of air that can be breathed out by force after a normal expiration.Revise it: Ventilation and lung volumes

F

Fitness: The survival value and reproductive potential of a genotype: how many offspring it contributes to the next generation compared with other genotypes.Revise it: Overproduction, competition and selection pressures
Fundamental niche: The potential niche of a species, based on its adaptations and tolerance limits, in the absence of competition.Revise it: Fundamental and realized niches, and competitive exclusion

G

Gene: A heritable factor that occupies a specific position (locus) on a chromosome and influences a characteristic.Revise it: Genotype, phenotype, dominance and phenotypic plasticity
Gene expression HL only: The mechanism by which information in genes has effects on the phenotype: transcription, translation and function of the protein product.Revise it: Gene expression and its regulation
Gene pool HL only: All the genes and their different alleles present in a population.Revise it: Gene pools and allele frequencies
Genome: All the genetic information of an organism.Revise it: Unity and diversity of genomes
Genome size: The total amount of DNA in one set of an organism's chromosomes, usually given in base pairs.Revise it: Unity and diversity of genomes
Genotype: The combination of alleles of a gene (or genes) inherited by an organism.Revise it: Genotype, phenotype, dominance and phenotypic plasticity
Glycoprotein: A protein with carbohydrate chains attached, facing out from the plasma membrane; used in cell–cell recognition.Revise it: Glycoproteins and cell–cell recognition

H

Haploid (n): A nucleus with one of each type of chromosome (one full set). Human gametes have 23 chromosomes.Revise it: Meiosis, non-disjunction and variation
Helicase: Unwinds the double helix and separates the two strands by breaking the hydrogen bonds between complementary bases. Each separated strand can then act as a template.Revise it: Roles of helicase and DNA polymerase
Heterotroph: An organism that uses carbon compounds obtained from other organisms to synthesise the carbon compounds that it requires.Revise it: Autotrophs and heterotrophs
Heterotroph (IB wording): An organism that uses carbon compounds obtained from other organisms to synthesise the carbon compounds that it requires.Revise it: Autotrophs and heterotrophs
Heterozygous: Having two different alleles of a gene, for example Aa.Revise it: Genotype, phenotype, dominance and phenotypic plasticity
Highly conserved sequence HL only: A sequence that has remained identical or similar over long periods of evolution, so it is shared by distantly related groups.Revise it: Conserved and highly conserved sequences
Homeostasis: Keeping internal conditions within narrow limitsRevise it: Processes of life in unicellular organisms
Homologous chromosomes: A pair of chromosomes with the same genes in the same sequence, but not necessarily the same alleles.Revise it: Meiosis, non-disjunction and variation
Homozygous: Having two identical alleles of a gene, for example AA or aa.Revise it: Genotype, phenotype, dominance and phenotypic plasticity
Hypertonic: Having a higher solute concentration than another solution.Revise it: Solvation and osmosis
Hypotonic: Having a lower solute concentration than another solution.Revise it: Solvation and osmosis

I

Inspiratory reserve volume: The extra volume of air that can be breathed in by force after a normal inspiration.Revise it: Ventilation and lung volumes
IPBES: The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, which reports evidence of a biodiversity crisis.Revise it: Evidence for and causes of the biodiversity crisis
Isotonic: Having the same solute concentration as another solution.Revise it: Solvation and osmosis

K

Karyogram: An image of the chromosomes of a cell arranged in homologous pairs, in order of decreasing length.Revise it: Chromosome numbers and karyograms
Karyotype: The number and type of chromosomes present in the nucleus of a cell of an organism.Revise it: Chromosome numbers and karyograms

L

Lamarckism: The discarded idea that characteristics acquired during an organism's life, through use or disuse, are passed on to its offspring.Revise it: Natural selection as the mechanism of evolution
Lipids: Substances in living organisms that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents. They include fats, oils, waxes and steroids.Revise it: Lipids: hydrophobic properties, triglycerides and phospholipids
LUCA HL only: The last universal common ancestor: the most recent organism from which all living organisms are descended.Revise it: The last universal common ancestor

M

Mean: The sum of all values divided by the number of values.Revise it: Continuous variation and box-and-whisker plots
Mechanism driving evolutionary change: Natural selection, operating continuously over billions of years and producing the biodiversity of life on Earth.Revise it: Natural selection as the mechanism of evolution
Median: The middle value when the data are in order.Revise it: Continuous variation and box-and-whisker plots
Metabolism: All the enzyme-catalysed reactions in the cellRevise it: Processes of life in unicellular organisms
Mode: The most frequent value (or class).Revise it: Continuous variation and box-and-whisker plots
Molecular clock HL only: A method of estimating when clades diverged from a common ancestor, using the number of differences in base or amino acid sequences.Revise it: Clades and the molecular clock
Mutualism: An interspecific relationship that benefits both species.Revise it: Mutualism

N

Node HL only: A branching point, representing a hypothetical common ancestor that split into two lineages.Revise it: Constructing and analysing cladograms
Non-conjugated protein HL only: A protein made only of polypeptides.Revise it: Quaternary structure: non-conjugated and conjugated proteins
Non-essential amino acids: Amino acids that can be made from other amino acids.Revise it: Dietary amino acids and the variety of peptide chains

O

Organelle: A discrete subunit of a cell that is adapted to perform a specific function.Revise it: Organelles, the nucleus and compartments
Osmoregulation HL only: Regulation of the osmotic concentration of body fluids (the amount of water relative to solutes), in osmoles per litre (osmol L⁻¹).Revise it: The kidney: excretion, ultrafiltration and reabsorption
Osmosis: The net movement of water across a partially permeable membrane from a lower to a higher solute concentration.Revise it: Osmosis, channel proteins and pump proteins

P

Paradigm shift: A fundamental change in the accepted theory and way of thinking in a field of science, when an old framework is replaced by a new one that explains the evidence better.Revise it: Natural selection as the mechanism of evolution
Peptide bond: The –CO–NH– bond formed by condensation between the carboxyl group of one amino acid and the amine group of the next.Revise it: Amino acids and peptide bonds
Phenotype: The observable traits of an organism, resulting from its genotype and environmental factors.Revise it: Genotype, phenotype, dominance and phenotypic plasticity
Population: A group of organisms of the same species living in the same area at the same time.Revise it: The biological species concept and speciation
Population (IB wording): An interacting group of organisms of the same species living in an area. Members normally breed with each other.Revise it: Populations and random sampling
Pressure potential (ψp) HL only: The effect of pressure on water potential. It is generally positive inside cells, because the wall pushes back on the protoplast; it is negative in xylem vessels where sap is transported under tension.Revise it: Water potential
Primary production: Accumulation of carbon compounds in biomass by autotrophs.Revise it: Primary and secondary production
Primary structure HL only: The sequence of amino acids in a polypeptide.Revise it: R-groups and primary structure
Promoter HL only: A base sequence just before (upstream of) a gene where RNA polymerase binds to start transcription.Revise it: Gene expression and its regulation
Prophage HL only: Viral DNA integrated into the host chromosome in the lysogenic cycle, copied each time the host divides.Revise it: Lytic and lysogenic cycles

Q

Quorum sensing HL only: Cell signalling by bacteria in which they detect their own population density and respond only when it is high.Revise it: Receptors, ligands and quorum sensing

R

Realized niche: The actual extent of a species' niche when it is in competition with other species. It is usually smaller than the fundamental niche.Revise it: Fundamental and realized niches, and competitive exclusion
Receptor HL only: A protein with a binding site for a specific signalling chemical.Revise it: Receptors, ligands and quorum sensing
Root HL only: The base of the cladogram, representing the common ancestor of all the organisms in it.Revise it: Constructing and analysing cladograms

S

Sampling error: The difference between the estimate of population size from a sample and the true size of the whole population.Revise it: Populations and random sampling
Secondary production: Accumulation of carbon compounds in biomass by heterotrophs.Revise it: Primary and secondary production
Sessile: Fixed in one place, not moving about: for example plants, barnacles and mussels.Revise it: Populations and random sampling
Single bond: One shared pair of electrons, e.g. C–C or C–H.Revise it: Carbon atoms and the diversity of carbon compounds
Single-nucleotide polymorphism (SNP): A position in the base sequence of a gene or genome where individuals of a species differ in a single base.Revise it: Unity and diversity of genomes
Solute potential (ψs) HL only: The effect of dissolved solutes on water potential. It ranges from zero (pure water) downwards: the more concentrated the solution, the more negative ψs.Revise it: Water potential
Speciation: The splitting of one species into two or more.Revise it: The biological species concept and speciation
Species evenness: How similar the population sizes of the different species are. A community dominated by one species has low evenness.Revise it: Evidence for and causes of the biodiversity crisis
Species richness: The number of different species present in an area.Revise it: Evidence for and causes of the biodiversity crisis
Start codon: AUG, which codes for methionine and marks where translation begins.Revise it: The genetic code
Stop codons: UAA, UAG and UGA, which do not code for an amino acid and signal the end of translation.Revise it: The genetic code
Surface tension: The effect at a water surface where cohesion between the surface molecules makes the surface resist being broken.Revise it: Cohesion and adhesion
System integration: Coordination of the component parts of a system so that they collectively perform an overall function. It is a necessary process in living systems.Revise it: System integration and the hierarchy of subsystems

T

Terminal branch HL only: A branch that ends at a species (or group) being compared, at the tips of the cladogram.Revise it: Constructing and analysing cladograms
Tidal volume: The volume of air breathed in or out in one normal breath.Revise it: Ventilation and lung volumes
Transcription factor HL only: A protein that binds to a specific base sequence in DNA, such as a promoter or enhancer, and increases or decreases the rate of transcription of a gene.Revise it: Gene expression and its regulation

U

Universality: The same codons code for the same amino acids in almost all living organisms, with only minor variations. This is evidence for common ancestry and allows genes to be transferred between species.Revise it: The genetic code

V

Vital capacity: The largest volume of air that can be breathed out after the deepest possible breath in: tidal volume + inspiratory reserve + expiratory reserve.Revise it: Ventilation and lung volumes

Test yourself on these terms with the IB Diploma Biology HL flashcards, or tick them off on the specification checklist.