IB Biology syllabus – definitions
* Definition given in IB Biology syllabus
Topic 1: Cells Organelle A discrete structure within a cell, with a specific function.*
Tissue A group of cells similar to each other, along with their associated intercellular substances, which perform the same function within a multicellular organism.
Organ A group of tissues which work together as a single unit to perform a particular function within a multicellular organism.
Organ system A group of organs, vessels, glands, other tissues, and/or pathways which work together to perform a body function within a multicellular organism.
Diffusion The passive movement of molecules from a region of high concentration to a region of low concentration.
Osmosis The passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.*
Topic 2: The Chemistry of Life Organic Compounds containing carbon that are found in living organisms (except hydrogencarbonates, carbonates and oxides of carbon).*
Enzyme Proteins that act as biological catalysts, speeding the rate at which biochemical reactions proceed but not altering the direction or nature of the reactions.
Active site A specific region of an enzyme where a substrate binds and catalysis takes place.

Denaturation A structural change in a protein that results in a loss (usually permanent) of its biological properties. Refer only to heat and pH as agents.*
Degenerate Having more than one base triplet to code for one amino acid.*
Universal Found in all living organisms.*
Cell respiration Controlled release of energy in the form of ATP from organic compounds in cells.*

Topic 3: Genetics Gene A heritable factor that controls a specific characteristic. (The differences between structural genes, regulator genes and genes coding for tRNA and rRNA are not expected at SL).*
Allele One specific form of a gene, differing from other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene.*
Genome The whole of the genetic information of an organism.*
Gene mutation A change in the base sequence of a gene on DNA. This could be by substitution, deletion, or insertion of a base or bases. The terms point mutation or frameshift mutation will not be used.
Homologous chromosomes A pair of chromosomes containing the same linear gene sequences, each derived from one parent.

Genotype The alleles possessed by an organism.*
Phenotype The characteristics of an organism.*
Dominant allele An allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state.* Dominant alleles mask the effects of recessive alleles.
Recessive allele An allele that only has an effect on the phenotype when present in the homozygous state.*
Codominant alleles Pairs of alleles that both affect the phenotype when present in a heterozygote. (The terms incomplete and partial will no longer be used.)*
Locus The particular position on homologous chromosomes of a gene.*
Homozygous Having two identical alleles of a gene.*
Heterozygous Having two different alleles of a gene.*
Carrier An individual that has a recessive allele of a gene that does not have an effect on their phenotype.*
Test cross Testing a suspected heterozygote by crossing it with a known homozygous recessive. (The term backcross is no longer used.)*
Sex linkage Genes found on sex chromosomes (X and Y chromosomes) are said to be sex linked.
Genetic screening Testing an individual for the presence or absence of a gene.*
Clone A group of genetically identical organisms or a group of cells artificially derived from a single parent cell.*

Topic 4: Ecology and Evolution Ecology The study of relationships between living organisms and between organisms and their environment.*
Ecosystem A community and its abiotic environment.*
Population A group of organisms of the same species who live in the same area at the same time.*
Community A group of populations living and interacting with each other in an area.*
Species A group of organisms which can interbreed and produce fertile offspring.*
Habitat The environment in which a species normally lives or the location of a living organism.*
Autotroph (producer) An organism that fixes chemical energy in the form of organic molecules; it "makes its own food" and does not require previously formed organic materials from the environment; e.g. photoautotroph, chemoautotroph.
Heterotroph (consumer) An organism that requires chemical energy from already formed organic molecules.
Detritivore Animal that consumes decomposing organic particles, deriving nutrition primarily from microbes on the particles.
Saprotroph (decomposer) Organism that feeds by secreting digestive enzymes onto its food source and absorbing the products of digestion. Fungi and bacteria are examples.
Trophic level Stage in a food chain or web leading from primary producers (lowest trophic level) through primary consumers (herbivores) to secondary and tertiary consumers (carnivores).

Carrying capacity Maximum average number or biomass of organisms that can