Edexcel International A Level Biology glossary

130 key terms from the Edexcel International A Level Biology notes, A to Z. Each definition links to the note that teaches it.

A

Absorption spectrum A Level only: A graph showing how much light of different wavelengths is absorbed by a pigment (or by a leaf extract).Revise it: Pigments, spectra and chromatography
Acrosome reaction: Release of enzymes from the acrosome by exocytosis when the sperm contacts the zona pellucida; they digest a path through it.Revise it: Fertilisation in mammals
Action spectrum A Level only: A graph showing the rate of photosynthesis (for example oxygen released or carbon dioxide taken up) at different wavelengths of light.Revise it: Pigments, spectra and chromatography
Active transport: Movement of molecules or ions across a membrane against a concentration gradient, using ATP and carrier proteins.Revise it: Passive transport: diffusion, facilitated diffusion, channel and carrier proteins
Allele: One of the different forms of a gene.Revise it: Genetic terms
Allele frequency A Level only: How common an allele is in the gene pool of a population.Revise it: Evolution: mutation and natural selection
Allopatric speciation A Level only: Formation of new species from populations isolated by a geographical barrier.Revise it: Isolation and speciation
Alternative splicing: Joining exons of pre-mRNA in different combinations, so one gene gives several different mRNAs and proteins.Revise it: One gene, more than one protein
Amniocentesis: Taking amniotic fluid with fetal cells using a needle (about 15 weeks) to test the fetus's DNA or chromosomes.Revise it: Genetic screening: carriers, PGD and prenatal testing
Antibody A Level only: A protein (immunoglobulin) made by a plasma cell that binds specifically to one antigen.Revise it: Antigens, antibodies and antigen-presenting cells
Antigen A Level only: A molecule (usually a protein or glycoprotein) on the surface of a pathogen, a foreign cell or a toxin, that is recognised as foreign by the immune system and stimulates an immune response.Revise it: Antigens, antibodies and antigen-presenting cells
Antigen-presenting cell A Level only: A cell that displays foreign antigen on its surface to T cells, for example a macrophage.Revise it: Antigens, antibodies and antigen-presenting cells
Artefact: A structure seen in the image that is caused by the preparation of the specimen and is not present in the living cell.Revise it: Magnification and resolution
Artificial active immunity A Level only: Immunity from vaccination: the body makes its own antibodies and memory cells.Revise it: Natural, artificial, active and passive immunity

B

Biodiversity: The variety of living organisms in a given area or on Earth, measured as the number and abundance of species (species diversity), the genetic variation within species (genetic diversity) and the range of habitats (habitat diversity).Revise it: Biodiversity, endemism and human threats
Bioinformatics A Level only: Using computer science and statistics to store, analyse and interpret biological data.Revise it: Microarrays and bioinformatics
Blastocyst: A hollow ball of cells with a fluid-filled cavity, an outer layer of cells that forms the placenta, and an inner cell mass of pluripotent cells.Revise it: Stem cells and potency
Bohr effect: Higher CO₂ (lower pH) shifts the curve to the right: lower affinity, so more oxygen is unloaded.Revise it: The oxygen dissociation curve, the Bohr effect and fetal haemoglobin

C

Capsid A Level only: The protein coat made of subunits that surrounds the nucleic acid of a virus.Revise it: Structure of bacteria and viruses
Captive breeding programme: Breeding endangered animals in zoos, managing pairings to maintain genetic diversity.Revise it: Evaluating zoos and seed banks in conservation
Chemiosmosis A Level only: Flow of protons down their gradient across a membrane through ATP synthase, providing energy to make ATP.Revise it: Oxidative phosphorylation
Chloroplast envelope A Level only: Two membranes around the chloroplast that control what enters and leaves.Revise it: Chloroplast structure and function
Chorionic villus sampling: Taking placental cells (chorionic villi) from about 11 weeks to test the fetus's DNA or chromosomes.Revise it: Genetic screening: carriers, PGD and prenatal testing
Classification: Organising the variety of life into groups based on relationships, using differences and similarities in phenotypes and genotypes.Revise it: Classification and the three-domain system
Climax community A Level only: The stable end point of succession, in balance with the abiotic environment, with no further net change.Revise it: Succession
Codominance: Both alleles are expressed in the phenotype of the heterozygote, because neither is dominant over the other.Revise it: Genetic terms
Colony-forming unit (cfu) A Level only: A single living cell or small clump of cells that grows into one visible colony on agar.Revise it: Measuring the growth of microorganisms
Community A Level only: All the populations of different species living and interacting in a habitat.Revise it: Population, community, habitat and ecosystem; biotic and abiotic factors
Condensation reaction: A reaction joining two molecules by forming a bond and releasing a molecule of water.Revise it: Glycosidic bonds: condensation and hydrolysis
Cortical reaction: Cortical granules release enzymes that harden the zona pellucida, preventing polyspermy.Revise it: Fertilisation in mammals

D

Deamination A Level only: Removal of the amino group from an excess amino acid in the liver, forming ammonia (then urea) and a keto acid.Revise it: Urea production and ultrafiltration
Decarboxylation A Level only: Removal of carbon dioxide from a substrate molecule.Revise it: Aerobic respiration: the overall reaction
Dehydrogenation A Level only: Removal of hydrogen atoms from a substrate molecule, usually by an enzyme with NAD or FAD as the hydrogen acceptor.Revise it: Aerobic respiration: the overall reaction
Dendrochronology A Level only: Using the pattern and width of annual tree rings to date past events and infer past climate.Revise it: Evidence for climate change
Differential gene expression: Different genes are transcribed in different cell types, giving different active mRNA and proteins, so cells specialise.Revise it: Differential gene expression and specialisation
Dipole: A molecule with a slightly positive end and a slightly negative end because its electrons are unevenly shared.Revise it: Water as a solvent in transport
Dominant: An allele that is expressed in the phenotype when present in only one copy (in the heterozygote).Revise it: Genetic terms
Double circulation: Blood passes through the heart twice per circuit: right side to the lungs, left side to the body.Revise it: Why many animals have a heart and circulation
Double fertilisation: One male nucleus fuses with the egg nucleus (diploid zygote) and the other with the polar nuclei (triploid endosperm).Revise it: Fertilisation in flowering plants
Double-blind trial: A trial in which neither the patients nor the doctors and researchers assessing them know who is receiving the drug and who is receiving the placebo, to avoid bias.Revise it: The development of drug testing

E

Ecosystem A Level only: A community of organisms together with the non-living (abiotic) components of their environment, and the interactions between them.Revise it: Population, community, habitat and ecosystem; biotic and abiotic factors
Endemism: A species being found in only one geographical region, and nowhere else in the world. Such a species is called endemic. Examples: lemurs in Madagascar; many species on isolated islands such as the Galápagos or Hawaii.Revise it: Biodiversity, endemism and human threats
Enzyme: A globular protein that acts as a biological catalyst by lowering the activation energy of a reaction.Revise it: Enzymes: mode of action and specificity
Evolution A Level only: A change in allele frequency in a population over generations.Revise it: Evolution: mutation and natural selection
Extrapolation A Level only: Extending a trend in data beyond the range measured to make a prediction.Revise it: Predicting climate change: extrapolation and models

F

Facilitated diffusion: Passive movement of ions or polar molecules down a concentration gradient through channel or carrier proteins.Revise it: Passive transport: diffusion, facilitated diffusion, channel and carrier proteins
Foam cell: A macrophage that has taken up large amounts of LDL cholesterol in the artery wall; part of the atheroma.Revise it: Atherosclerosis
Forensic entomology A Level only: Using the species and development stage of insects, such as blowfly larvae, on a body to estimate time of death.Revise it: Estimating time of death
Free radical: A highly reactive molecule that damages cells and oxidises LDL cholesterol.Revise it: Dietary antioxidants and the risk of CVD

G

Gene: A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain, found at a particular locus on a chromosome.Revise it: Genetic terms
Gene flow A Level only: The movement of alleles between populations by interbreeding.Revise it: Isolation and speciation
Genotype: The alleles that an organism has for a particular gene (or genes).Revise it: Genetic terms
Grana A Level only: Stacks of thylakoids that give a large surface area for photosynthetic pigments.Revise it: Chloroplast structure and function
Gross primary productivity (GPP) A Level only: The total chemical energy fixed by photosynthesis in a given area in a given time (for example kJ m⁻² year⁻¹).Revise it: Gross and net primary productivity

H

Habitat A Level only: The place where an organism, population or community lives, with its own set of environmental conditions.Revise it: Population, community, habitat and ecosystem; biotic and abiotic factors
Habituation A Level only: A decrease in response to a repeated stimulus that is harmless.Revise it: Habituation
Heterozygote: An organism with two different alleles of a gene.Revise it: Genetic terms
Homeostasis A Level only: Maintenance of a stable internal environment within narrow limits, a state of dynamic equilibrium.Revise it: Homeostasis and thermoregulation
Homozygote: An organism with two identical alleles of a gene.Revise it: Genetic terms
Hospital-acquired infection A Level only: An infection acquired in hospital, for example MRSA or C. difficile.Revise it: Hospital-acquired infections and codes of practice
Hydrolysis: A reaction in which a molecule of water is used to break a bond, for example a glycosidic bond.Revise it: Glycosidic bonds: condensation and hydrolysis

I

Independent assortment: The random orientation of homologous pairs at the equator in metaphase I, giving random combinations of maternal and paternal chromosomes in gametes.Revise it: Meiosis and genetic variation

L

Latency A Level only: A state in which viral nucleic acid persists in the host cell without producing new virus particles, and no symptoms need occur; the virus can later become active.Revise it: The terms lytic and latency
Linkage: Genes on the same chromosome are said to be linked, because they tend to be inherited together.Revise it: Gene loci and linkage
Linked genes: Genes at different loci on the same chromosome, which tend to be inherited together.Revise it: Gene loci and linkage
Locus: The location of a gene on a chromosome. Plural: loci.Revise it: Gene loci and linkage
Lysosome: A membrane-bound sac made by the Golgi apparatus, containing hydrolytic enzymes that digest worn organelles and engulfed material.Revise it: Ultrastructure of eukaryotic cells
Lytic cycle A Level only: A viral life cycle in which the virus uses the host cell to make many copies of itself at once; the host cell bursts (lyses) and releases the new particles.Revise it: The terms lytic and latency

M

Magnification: How many times larger the image is than the real object.Revise it: Magnification and resolution
Mass transport: Movement of fluid (blood) in bulk through vessels over long distances by pressure from a pump.Revise it: Why many animals have a heart and circulation
Microarray A Level only: A slide with thousands of single-stranded DNA probes used to find which genes are active.Revise it: Microarrays and bioinformatics
Middle lamella: A layer rich in pectins (calcium pectate) between the walls of neighbouring plant cells that cements the cells together.Revise it: Plant cell ultrastructure and its functions
Morula: A solid ball of about 16 cells formed by early cell divisions (cleavage) of the zygote.Revise it: Stem cells and potency
Motor neurone A Level only: A neurone that carries impulses from the central nervous system to an effector, a muscle or gland.Revise it: Sensory, relay and motor neurones
Multiple alleles: More than two alleles of a gene in a population (for example Iᴬ, Iᴮ and Iᴼ for ABO blood groups).Revise it: Multiple alleles, polygenic inheritance and variation

N

Negative feedback A Level only: A change in a factor is detected and the response reverses it, returning the factor to its set point.Revise it: Negative and positive feedback
Net primary productivity (NPP) A Level only: The chemical energy remaining in plant biomass after the energy lost in respiration has been taken away. It is the energy available to the next trophic level.Revise it: Gross and net primary productivity
Niche: The role of a species within its habitat: how it obtains food and other resources, the abiotic conditions it tolerates and the biotic interactions it has with other species (such as feeding relationships, competition and predation).Revise it: Niche and adaptations to the environment

O

Osmosis: The net movement of free water molecules through a partially permeable membrane, down a water potential gradient.Revise it: Osmosis and water potential

P

PGD: Pre-implantation genetic diagnosis: cells taken from IVF embryos are tested, and only unaffected embryos are implanted.Revise it: Genetic screening: carriers, PGD and prenatal testing
Phenotype: The observable characteristics of an organism, resulting from its genotype and the environment.Revise it: Genetic terms
Photolysis A Level only: The light-driven splitting of water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons replace those lost from chlorophyll.Revise it: The light-dependent reactions
Photophosphorylation A Level only: The addition of phosphate to ADP to make ATP, using light energy.Revise it: The overall reaction and the role of ATP
Phytochrome A Level only: A light-sensitive plant pigment that exists as Pr and Pfr, detecting red and far-red light.Revise it: Phytochrome
Pioneer species A Level only: The first species to colonise bare ground, tolerant of harsh conditions, which changes the environment for others.Revise it: Succession
Placebo: An inactive substance that looks identical to the drug, given to the control group so the effect of the drug can be separated from the effect of believing one is being treated.Revise it: The development of drug testing
Plasmid: A small, circular piece of DNA separate from the main DNA, carrying extra genes such as antibiotic resistance.Revise it: Ultrastructure of prokaryotic cells
Pluripotent: Able to differentiate into almost any type of cell in the body, but not extra-embryonic tissues, so cannot form a complete organism.Revise it: Stem cells and potency
Polygenic inheritance: A characteristic controlled by many genes at different loci, each with a small additive effect.Revise it: Multiple alleles, polygenic inheritance and variation
Polyspermy: Fertilisation of an egg by more than one sperm; it gives an abnormal number of chromosomes.Revise it: Fertilisation in mammals
Population A Level only: All the organisms of one species living in a habitat at the same time.Revise it: Population, community, habitat and ecosystem; biotic and abiotic factors
Positive feedback A Level only: The response increases the change away from the starting point, e.g. oxytocin in labour.Revise it: Negative and positive feedback
Provirus A Level only: HIV DNA, made from RNA by reverse transcriptase, integrated into the host chromosome.Revise it: The terms lytic and latency
Pyruvate A Level only: The 3C product of glycolysis. It enters the mitochondrion in aerobic respiration or is converted to lactate in anaerobic respiration in animals.Revise it: Glycolysis

Q

Q₁₀ A Level only: The factor by which the rate of a reaction (or process) increases when the temperature is raised by 10 °C.Revise it: Temperature, enzyme activity and Q₁₀
Quaternary structure: More than one polypeptide chain in a protein (for example the four chains of haemoglobin).Revise it: Amino acids, peptide bonds and polypeptides

R

Recessive: An allele that is expressed in the phenotype only when two copies are present (in the homozygote).Revise it: Genetic terms
Recombinant DNA A Level only: DNA made by joining DNA from two different sources, such as a gene and a plasmid vector.Revise it: Making recombinant DNA
Reflex A Level only: A rapid, automatic response to a stimulus that does not involve conscious thought.Revise it: The spinal reflex arc
Reliable: The results are consistent, and similar results would be obtained if the study were repeated.Revise it: Evaluating the design of studies
Reproductive isolation: Populations can no longer exchange genes, so their gene pools become separate.Revise it: The Hardy-Weinberg equation, mutation and natural selection
Resolution: The smallest distance between two points at which they can still be seen as two separate points.Revise it: Magnification and resolution
Respiratory quotient (RQ) A Level only: The volume of carbon dioxide produced divided by the volume of oxygen consumed in a given time.Revise it: Respiratory quotient (RQ)
Respiratory substrate A Level only: An organic molecule, such as glucose, that is oxidised in respiration to release energy.Revise it: Aerobic respiration: the overall reaction
Risk factor: Something linked to an increased probability of developing a disease, not a guarantee of it.Revise it: Risk factors for cardiovascular disease

S

Saltatory conduction A Level only: The action potential jumps from node of Ranvier to node along a myelinated axon, by local circuits.Revise it: Myelination and saltatory conduction
Sarcomere A Level only: The repeating contractile unit of a myofibril, from one Z line to the next.Revise it: Structure of a skeletal muscle fibre
Saturated lipid: One whose fatty acids have no C=C double bonds in the hydrocarbon chain.Revise it: Saturated and unsaturated lipids
Secondary thickening: Extra layers of cellulose and lignin laid down inside the primary wall, making cells like xylem vessels and sclerenchyma fibres rigid and waterproof.Revise it: Cell wall structure, secondary thickening and plant fibres
Sensory neurone A Level only: A neurone that carries impulses from a receptor to the central nervous system.Revise it: Sensory, relay and motor neurones
Solvent: A liquid in which other substances (solutes) dissolve to form a solution.Revise it: Water as a solvent in transport
Species: A group of similar organisms that can interbreed to produce fertile offspring.Revise it: Classification and the three-domain system
Species richness: The number of different species in a habitat.Revise it: Measuring biodiversity: species richness and heterozygosity index
Stem cell: An undifferentiated cell that can divide by mitosis to produce more stem cells and can differentiate into specialised cell types.Revise it: Stem cells and potency
Sticky ends A Level only: Short single-stranded overhangs at a cut that can base-pair with complementary sticky ends.Revise it: Making recombinant DNA
Substrate-level phosphorylation A Level only: Formation of ATP by the direct transfer of a phosphate group from a substrate molecule to ADP, without the electron transport chain.Revise it: Glycolysis
Sustainability: Meeting the needs of the present without harming the ability of future generations to meet their own needs.Revise it: Plant fibres, starch and sustainability
Sustainable resource A Level only: One used no faster than it is replaced, so it remains available for future generations.Revise it: Reforestation, sustainable resources and biofuels
Sympatric speciation A Level only: Formation of new species within the same area through reproductive isolation, for example by polyploidy.Revise it: Isolation and speciation

T

Template (antisense) strand: The DNA strand copied by RNA polymerase; mRNA is complementary to it.Revise it: Transcription
Tensile strength: The maximum force per unit cross-sectional area that a material can withstand when it is pulled before it breaks. Units: N mm⁻² or Pa (1 N mm⁻² = 1 MPa).Revise it: Core practical 8: tensile strength of plant fibres
Tidal volume A Level only: The volume of air breathed in (or out) in one breath. It is about 0.5 dm³ at rest and increases during exercise.Revise it: Core practical 17: exercise and spirometer traces
Timing mismatch A Level only: When interacting species respond at different rates, for example food peak and hatching of young no longer coincide.Revise it: Effects of climate change on plants and animals
Totipotent: Able to differentiate into any type of cell, including the cells of the extra-embryonic tissues such as the placenta, so can form a complete organism.Revise it: Stem cells and potency
Transcription: Making mRNA from the template strand of a gene, by RNA polymerase, in the nucleus.Revise it: Transcription
Transcription factor A Level only: A protein that binds to a specific DNA sequence and switches transcription of a gene on or off.Revise it: Hormones and transcription factors
Translation: Making a polypeptide at a ribosome using the codons of mRNA, with tRNA bringing amino acids.Revise it: Transcription
Tubercle A Level only: A ball of macrophages, T cells and dead tissue formed around TB bacteria in the lungs.Revise it: How Mycobacterium tuberculosis and HIV cause disease

V

Valid: The data measure what the study claims to measure, with other variables controlled.Revise it: Evaluating the design of studies

Z

Zona pellucida: A jelly-like layer of glycoproteins around the egg cell, with sperm receptors; it hardens after fertilisation.Revise it: Specialised mammalian gametes

Test yourself on these terms with the Edexcel International A Level Biology flashcards, or tick them off on the specification checklist.