GCE A/L Biology key terms

The number of times an image is enlarged compared with the real object, calculated by dividing the image size by the actual size, usually expressed with a multiplication sign.

The smallest distance between two points at which they can still be seen as separate objects; a property of the lens system and wavelength of illumination, not of enlargement alone.

An instrument that uses a beam of electrons focused by electromagnets to form images, giving far higher resolution than light microscopes and revealing organelles such as ribosomes and cristae.

A double sheet of phospholipid molecules with hydrophobic tails facing inwards and hydrophilic heads facing outwards, forming the basic framework of all cell membranes.

The description of membranes as a fluid phospholipid bilayer with proteins, glycoproteins and cholesterol embedded in or attached to it, able to move laterally within the layer.

Passive movement of substances across a membrane down their concentration gradient through specific carrier or channel proteins, requiring no metabolic energy.

Movement of ions or molecules across a membrane against their concentration gradient using carrier proteins and energy from ATP hydrolysis, for example the sodium-potassium pump.

The net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

The tendency of water to move from one place to another, measured in pressure units; pure water at atmospheric pressure has a value of zero and solutes lower it.

A biological catalyst, usually a globular protein, that lowers the activation energy of a reaction by forming a temporary enzyme-substrate complex at its active site.

The specific region of an enzyme, formed by a few amino acids and often a cofactor, that binds the substrate through complementary shape and chemistry.

Loss of the three-dimensional shape of a protein or enzyme caused by heat, extremes of pH or heavy metals, breaking hydrogen and ionic bonds so the active site no longer fits the substrate.

Nuclear division producing two genetically identical daughter nuclei with the same chromosome number as the parent cell, used in growth, repair and asexual reproduction.

Two successive nuclear divisions that halve the chromosome number and produce four genetically varied haploid cells, forming gametes or spores.

Exchange of corresponding segments between non-sister chromatids of a homologous pair during prophase I of meiosis, producing new combinations of alleles.

The random orientation and separation of each homologous pair or of unlinked gene pairs at meiosis, so alleles are combined in gametes independently of one another.

One of the alternative forms of a gene occupying the same locus on homologous chromosomes and differing slightly in base sequence.

The particular combination of alleles present in an organism for one or more genes, written as pairs of letters such as homozygous or heterozygous combinations.

The observable characteristics of an organism resulting from the interaction between its genotype and the environment.

An allele that is expressed in the phenotype even when only one copy is present, masking the effect of a recessive allele in a heterozygote.

The synthesis of a messenger RNA molecule from a DNA template strand by RNA polymerase, occurring in the nucleus of eukaryotic cells.

The assembly of a polypeptide at a ribosome, in which transfer RNA molecules bring amino acids in the order specified by the codons of messenger RNA.

A sequence of three bases on messenger RNA that specifies one amino acid or a stop signal during protein synthesis.

A change in the base sequence of DNA, such as substitution, deletion or insertion, which may alter the amino acid sequence of a protein or the regulation of a gene.

The loss of water vapour from the aerial parts of a plant, mainly through stomata, driven by evaporation and pulling water up the xylem in the transpiration stream.

Vascular tissue containing dead, lignified hollow cells that transports water and dissolved mineral ions upward from the roots to the leaves.

Vascular tissue of living sieve tube elements and companion cells that transports sucrose and amino acids from sources to sinks in the plant.

The movement of dissolved organic substances such as sucrose through the phloem from a region where they are produced or stored to a region where they are used or stored.

A pore in the leaf epidermis bounded by two guard cells, whose turgor changes open and close it to regulate gas exchange and water loss.

The maintenance of a stable internal environment within narrow limits, achieved by negative feedback mechanisms controlling factors such as temperature, blood glucose and water potential.

A control mechanism in which a deviation from a set point triggers a response that reduces the deviation, restoring the factor towards its normal level.

The control of the water and solute content of body fluids, carried out in mammals mainly by the kidneys under the influence of antidiuretic hormone.

A substance produced by a microorganism, or made synthetically, that kills or inhibits the growth of bacteria, usually by interfering with cell wall synthesis, protein synthesis or DNA replication.

The ability of a bacterium to survive and multiply in the presence of an antibiotic, arising by mutation or gene transfer and spreading through a population under selection pressure.

A microorganism or virus that causes disease in a host, entering the body and damaging tissues or releasing toxins.

A protein of the immunoglobulin group produced by plasma cells in response to a specific antigen, binding to it to neutralise or help destroy the pathogen.

A molecule, usually a protein or polysaccharide on the surface of a cell or particle, that is recognised as foreign and triggers an immune response.

The process by which individuals with inherited characteristics better suited to their environment survive and reproduce more successfully, so allele frequencies change over generations.

A group of organisms with similar morphology and physiology that can interbreed to produce fertile offspring, and which is reproductively isolated from other such groups.

The maximum population size of a species that an ecosystem can support indefinitely, determined by the available resources and other environmental limiting factors.

A self-contained unit comprising a community of interacting organisms and the physical environment in which they live, through which energy and nutrients flow.

The position an organism occupies in a food chain, such as producer, primary consumer or secondary consumer, defined by the number of energy-transfer steps from the start.

All the individuals of one species living in the same area at the same time, which can interbreed and are affected by the same environmental factors.

The deliberate mating of organisms chosen for desirable inherited characteristics over many generations to produce improved varieties or breeds.

The direct manipulation of an organism's DNA, for example by inserting a gene from another organism into a plasmid vector and transferring it into a host cell.

A technique that makes many copies of a specific DNA sequence in vitro using primers, DNA polymerase and repeated cycles of heating and cooling.

A factor in an experiment that is kept constant across all trials so that its effect cannot influence the measured outcome.

The factor deliberately changed by the investigator in an experiment, whose effect on the measured outcome is being tested.

The factor that is measured in an experiment and that changes in response to the independent variable.