IB Biology – Definitions required by the syllabus

IB Biology syllabus – definitions* Definition given in IB Biology syllabusTOPIC WORD DEFINITION

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.*

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IB Biology – Definitions required by the syllabus

TOPIC WORD DEFINITION

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.*

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IB Biology – Definitions required by the syllabus

TOPIC WORD DEFINITION

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 be sustained in a habitat over the long term. Usually refers to a particular species, but can be applied to more than one.

Random sample A method to ensure that every individual in a population has an equal chance of being observed.

Evolution The process of cumulative change in the heritable characteristics of a population.*

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IB Biology – Definitions required by the syllabus

TOPIC WORD DEFINITION

Topic 5: Human Health and Physiology

Pathogen An organism or virus that causes a disease.*

Excretion This is a general term for the discharge of substances from the body. Release of an absorbed or endogenous substance or of a waste product, in urine, faeces, or other products normally leaving the body. Excretion of most chemical compounds from the body occurs mainly through the kidney and the gut, although volatile compounds may be largely eliminated by exhalation. Excretion by perspiration and through hair and nails may also occur. Excretion by the gastrointestinal tract may take place by various routes such as the bile, the shedding of intestinal cells and transport through the intestinal mucosa.

Topic 8: Genetics

Recombination The reassortment of genes or characters into different combinations from those of the parents. Recombination occurs for linked genes by crossing over and, for unlinked genes, by chromosome assortment* due to random alignment during metaphase I and II of meiosis.

Linkage group Genes that are located on the same chromosome are part of a linkage group; the closer together the genes are, the lower the probability that they will be separated by crossing-over during meiosis in, and hence the greater the probability that they will be inherited together.

Polygenic inheritance

A trait or characteristic that is influenced by the expression of more than one gene, e.g. hair colour in humans. Continuous variation occurs as a result.

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IB Biology – Definitions required by the syllabus

TOPIC WORD DEFINITION

Topic 10: Defence Against Infectious Disease

Active immunity Immunity due to the production of antibodies by the organism itself after the body’s defence mechanisms have been stimulated by invasion of foreign micro-organisms.*

Passive immunity

Immunity due to the acquisition of antibodies from another organism in which active immunity has been stimulated, including via the placenta or in colostrum.*

Natural immunity

Immunity due to infection.*

Artificial immunity

Immunity due to inoculation with vaccine.*

Topic 11: Nerves, Muscles and Movement

Resting potential

The difference in electrical charge across the plasma membrane of a neuron when not conducting nerve impulses (i.e. resting). In humans, the value is around -70mV, meaning the inside is negative relative to the outside of the cell.

Action potential Voltage change generated across the membrane of a nerve or muscle cell when the cell is activated by electrical, chemical or mechanical stimuli. In humans, the value is around +40mV. The action potential is propagated along the nerve by voltage-gated ion channels.

Topic 12: Excretion

Osmoregulation The control of the water balance of the blood, tissue or cytoplasm of a living organism.*

Topic 13: Plant Science

Transpiration The loss of water vapour from the leaves and stems of plants.*

Pollination The transfer of pollen from the male organ, where it is formed, to the receptive region of a female organ, e.g. from anther to stigma.

Option G: Ecology and Conservation

Gross production

The amount of energy fixed by autotrophs, in kJ m-2 yr-1.

Net production Gross production less the energy used by the autotrophs for their own respiration, in kJ m-2 yr-1.

Biomass A quantitative estimate of the entire amount of living organisms in a particular habitat; it can be measured in terms of volume, mass, or caloric energy.

Option H: Further Human Physiology

Partial pressure The pressure exerted by an individual gas in a mixture of gases; measured in kPa (or mm Hg).

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