Edexcel International AS Level Biology glossary
70 key terms from the Edexcel International AS Level Biology notes, A to Z. Each definition links to the note that teaches it.
A
- 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
- 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
- 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
- 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
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
- 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
- Captive breeding programme
- Breeding endangered animals in zoos, managing pairings to maintain genetic diversity.Revise it: Evaluating zoos and seed banks in conservation
- 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
- Codominance
- Both alleles are expressed in the phenotype of the heterozygote, because neither is dominant over the other.Revise it: Genetic terms
- 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
- 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
- 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
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
- 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
- Genotype
- The alleles that an organism has for a particular gene (or genes).Revise it: Genetic terms
H
- Heterozygote
- An organism with two different alleles of a gene.Revise it: Genetic terms
- Homozygote
- An organism with two identical alleles of a gene.Revise it: Genetic terms
- 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
- 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
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
- 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
- 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
- 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
- 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
Q
- 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
- 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
- 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
- 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
- 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
- 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
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
- 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
- Translation
- Making a polypeptide at a ribosome using the codons of mRNA, with tRNA bringing amino acids.Revise it: Transcription
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 AS Level Biology flashcards, or tick them off on the specification checklist.