A-Level Biology: Glossary

A-Level Biology is a whole lot easier when you know the definitions of commonly used words.


A good command of biological terminology can mean the difference between a poor grade and an excellent one! (Since of course you get marks awarded for using the correct technical language in the right context when answering exam questions).


...So, make sure learning and revising these essential biological words and their definitions is an integral  part of your A-Level Biology Revision...





Acetyl: Chemical group derived from acetic acid. Acetyl groups are important in metabolism and are added covalently to some proteins as a post-translational modification.

Acetyl CoA: Acetyl group linked to coenzyme A (CoA). Acetyl CoA is a small water-soluble molecule that carries acetyl groups in cells.

Acetylcholine: A neurotransmitter found in the brain and in the peripheral nervous system.

Acetylcholine receptor: An Ion channel that opens in response to the binding of acetylcholine, resulting in the conversion of  a chemical signal into an electrical one.

Acid hydrolase: Hydrolytic enzymes (e.g. proteases, nucleases, glycosidases) with optimal enzyme activity at approximately pH 5.0 (acidic pH). Acid hydrolase enzymes and are found in lysosomes.

Acquired Immunological Tolerance: An unresponsiveness of the immune system to a given foreign antigen.

Acrosomal vesicle: The region at the head end of a sperm cell. The acrosome contains hydrolytic enzymes which digest the protective coating of the egg.

Actin: Actin is an abundant protein which forms actin filaments in all eukaryotic cells.

Actin filament: A helical protein filament. A major part of the cytoskeleton of all eukaryotic cells and part of the contractile proteins of skeletal muscle.

Actin-binding protein:  Myosin is an example of an actin-binding protein;protein that associates with either actin monomers or actin filaments in cells and modifies their properties.

Action potential (Nerve impulse): The rapid, short-lived, self-propagating electrical excitation in the plasma membrane of neurones and muscle cells. Action potentials, are responsible for long distance signalling in the nervous system.

Activation energy: The 'extra' energy required in order to undergo a particular chemical reaction.

Active site: The active site is the part of an enzyme to which a substrate binds in order to carry out a catalysed reaction.

Active Transport: The energy driven movement of a molecule across a cell membrane.

Adaptive Immune Response: Response of the immune system to a specific antigen that generates immunological memory.

Adenosine Triphosphate: see ATP

Adipocyte: A fat cell.

ADP (Adenosine Diphosphate): A nucleotide that regenerates ATP when phosphorylated by an energy-generating process, i.e. oxidative phosphorylation.

Adrenaline (epinephrine): A hormone released by cells in the adrenal glands (and some neurones) in response to stress. Adrenaline brings about the "fight or flight” response, e.g. increased heart rate and blood sugar levels.

Aerobic: A process that requires oxygen (O2), or occurs in the presence of, gaseous oxygen (O2).

Algae: Unicellular and multicellular eukaryotes which are photosynthetic organisms, e.g. Nitella and Volvox.

Alkane: A compound of carbon and hydrogen that has only single covalent bonds, e.g. ethane.

Alkene: A hydrocarbon with one or more carbon-carbon double bonds,  e.g. ethylene.

Allele: An alternative form of a gene, e.g. fur colour gene; brown or black allele. In a diploid cell each gene will have two alleles, each occupying the same position (locus) on homologous chromosomes.

Alpha helix (α helix): a helical folding pattern in the secondary structure of proteins where a linear sequence of amino acids folds/coils into a right-handed helix stabilised by hydrogen bonding.

Amino acid: Organic molecule containing both an amino group and a carboxyl group. Amino acids are the monomers of proteins.


Anaerobic Threshold: when tissues can not obtain enough oxygen, the cells begin to respire anaerobically. Usually applies to exercise; e.g. “the runner increased her speed so that she crossed the anaerobic threshold, and began to accumulate lactate”


Anticodon: Three bases complementary to a codon. Found on tRNA, allowing it to attach to mRNA. The anticodon for GUG would be CAC.


Aorta: the main artery from the left ventricle. Has highest pressure of any blood vessel.


Aortic Body: a patch of chemoreceptors (cells sensitive to changes in CO2/ H+ levels in blood). Located on aortic arch just above heart. Sends sensory impulses to medulla. See also; carotid body 


Apoplast Pathway: route of water and ions from soil water to xylem through the plant cell walls and gaps in between cells. This route is non living, and water/ions never cross a membrane so the plan has no control over the rate of flow. Contrast with; Symplast pathway


Arable farming: Growing crops


Arteriole: blood vessels with a particularly muscular wall. Can vasodilate (widen) or vasoconstrict (narrow) to control the blood flow to a particular area.


Artery: blood vessels whose walls have a lot of elastic fibres, so they have good recoil properties to withstand the surges of pressure created when the heart beats. Contrast with arteriole.


Attenuated: Having reduced virulence; live (weakened), non-pathogenic organisms used to activate adaptive immunity. The non-pathogenic organisms administered to the vaccinated individual stimulate lymphocytes in order to produce antibodies and activated T cells, without producing serious disease.

ATP: Adenosine Tri-Phosphate; the cell’s immediate source of energy. In respiration, the energy in glucose (or lipid) is released and used to make ATP from ADP and P. One glucose molecule can yield enough energy to make up to 36 molecules of ATP, which can then be used to provide the energy for muscular contraction, active transport, protein synthesis etc.


Atrial Systole: Contraction of the atria.


Atrium: Upper heart chambers. Receive blood into heart from vena cava and pulmonary vein. Atria are basically loading chambers for the ventricles, so they don’t need thick muscular walls. 


Autosome: Normal chromosome - not a sex chromosome. Humans have 22 pairs.


AV Valve: Valves between the atria and ventricles of the heart, preventing backflow into atria. Old names; bicuspid and tricuspid valves. 


AVN: Atrio-ventricular Node. Vital part of the conducting pathway in the heart. Receives impulses from SAN, creates a delay to allow the ventricles to fill, and then transfer and impulses down the bundles of His.



Bioaccumulation: The accumulation of persistent pesticide in the tissues of organisms in a food chain


Biochemical oxygen demand (BOD): The amount of oxygen used by aerobic micro-organisms as they break down organic materials in water.


Biodiversity: The range of species present in a habitat.


Biological pest control: The use of predators or parasites to control the population of a pest species.


Bohr Effect (Shift): the presence of CO2. (more accurately - the H+ ions resulting from the presence of CO2­) lowers haemoglobin’s affinity for O2 and so the dissociation curve shifts to the right. The significance is that oxygen is released to the respiring tissues that need it. 


Bundle of His: Specialised cardiac muscle fibres which are found in the ventricular septum (muscle wall between ventricles). Receives impulses from the AVN and transfers it to the Purkyne fibres. 


Capillary: Narrow blood vessel with single celled endothelium (wall) to allow exchange of materials. Walls are permeable, which allows the formation of tissue fluid.


Cardiovascular Centre: centre in the medulla (lower part of brain) which can modify the rate of heartbeat. 


Carotid Body: patch of chemoreceptors located in wall of carotid artery. Senses change in blood CO2 and H+ levels. Sends sensory impulses to the medulla. See also; Aortic body. 


Casparian Strip: In plant roots, a waxy waterproof strip, made from suberin, which surrounds endodermal cells. Its function is to block the apoplast pathway, so that all water and ions enter by the symplast route and so the plant has some control over the water/ions entering the xylem. 


CentrioleBundles of protein fibres, found near the nucleus. Centrioles migrate to opposite poles of the cell during mitosis and form the spindle.


CentromereThe point of attachment of two chromatids on a double chromosome.


Chemoreceptor: cells sensitive to chemical change. e.g. in the aortic and carotid bodies there are chemoreceptors which are sensitive to CO2/pH levels. 


Chiasmata: The point at which crossing over occurs. i.e. sites where homologous chromosomes break / rejoin.


Chromatid: One half of a double chromosome (present at the start of cell division)


Chromatin‘Spread out’ DNA in the nucleus of a non-dividing cell i.e. not coiled up into chromosomes


Chromosome: Condensed mass of DNA which appears just before cell division. Each chromosome is one super-coiled DNA molecule containing 1000s of genes.


Clone: A identical genetic copy. You can clone DNA strands, cells or whole organisms


CodonA sequence of three bases coding for a particular amino acid e.g GUG = valine.


Cohesion Tension: The force that draws water up the xylem of a plant.  Water evaporating from the upper surfaces of a leaf creates a negative pressure in the xylem which draws water up through the plant from roots to leaves. Water is cohesive (sticky) so it can form long, continuous unbroken columns all the way up a plant.


Companion cells: a cell in the phloem that is connected to a sieve-tube member by numerous plasmodesmata (strands of cytoplasm). They control the activity of the sieve tubes.


Continuous variation: Variation which can be any value within a range. e.g. height, shoe size. Tends to be controlled by many alleles ( polygenic) which combine to give phenotype.


Coronary Arteries: vital arteries supplying blood to the heart muscle itself. A narrowing of these arteries leads to coronary heart disease. 


Cortex: root tissue between the vascular bundle (phloem and xylem) and epidermis. In plants, it’s a general word for ‘packing tissue’


Crop rotation: Planting different crops in one area in successive years.


Crossover [crossing over]: In meiosis, the process which swaps block of genes between homologous chromosomes, thereby increasing variation.


Cystic fibrosis: Genetic disease due to faulty allele. Sticky mucus blocks lungs and pancreas.

Diastole: Relaxation of heart muscles.



Diploid: A cell / organism which contains two sets of chromosomes. Shows as 2n, e.g. 2n = 46 in humans. Other species have different diploid numbers.


Discontinuous variation: Variation which falls neatly into categories; one thing or the other, e.g. ABO blood groups. Tends to be controlled by single alleles, which are present or not.

Disease: Disfunction of the normal state of an organism, i.e. an irregularity from the normal structure / function of any part of an organism, as recognised by a characteristic set of signs and symptoms.


Dissociation: The splitting of one molecule into two or more molecules e.g. Oxyhameoglobin into oxygen and haemoglobin. Comes from ‘dis-association’ i.e coming apart.


DNA ligase: An enzyme which joins DNA strands, such as those cut by restriction enzymes


DNA polymerase: Enzyme which controls DNA replication by causing the addition of complementary bases on the exposed strands. 


Dominant: An allele which, if present, is shown in the phenotype


Down’s syndrome: A genetic condition cause by three chromosome 21s. It usually arises due to a fault in meiosis so that the eggs have both chromosome 21s instead of one.




Endodermis: in plants, the outer layer of the vascular bundle. Vitally, it contains the Casparian strip which regulates transport of ions into the vascular bundles of roots. Endodermis means ‘inner skin’


Epidermis: In plants, the outer layer of root. Root hair cells are epidermal cells adapted to increase the surface area for absorption. Epidermis means ‘outer skin’


Erythrocytes: posh name for red blood cells.


Eukaryotic cell: Cells with a true nucleus and DNA organised into chromosomes. Complex cell with organelles such as mitochondria, ER etc. Animals, plants, fungi etc.


Eutrophication: Excessive entry of nutrients (nitrate and phosphate) into lakes and rivers.

Fertilisers (artificial / organic): Materials added to the soil which increase nutrient availability for plants.



Frame shiftThe drastic effect of a mutation in which a base is added or deleted. Either way, all the other bases are shifted along, altering all the codons & the amino acids.


Fungicide: Chemical which kills fungi (many crop diseases are caused by fungi)


Gene therapy: Treating genetic disease using healthy copies of the faulty allele


GeneA length of DNA which codes for making one protein or polypeptide


Generic nameName of the Genus; first part of a Latin name. Always has capital, e.g. Homo


Genetic Marker: An extra gene inserted into a plasmid that allows transformed bacteria to be recognised e.g. antibiotic resistance gene


Genome: The entire DNA sequences in an organisms chromosomes.


Genotype: The alleles an organism possesses. e.g. Aa or AA, AaBb or Aabb etc


Guard cells: Specialised epidermal cells surrounding the stoma. The cell walls contain radial microfibrils of cellulose, allowing them to change shape and bend when water is absorbed, thus opening the stoma.




Haemoglobin: A vital protein found in red blood cells, consisting of 4 polypeptide chains with a haem group on each unit. One oxygen molecule will bind to each Haem group for oxygen carriage in the blood. Key function – to pick up oxygen where abundant (lungs), and release it where needed (tissues).


Haemophilia: Sex linked genetic disease, faulty allele prevents blood clotting. 


Haploid: A cell/organism which contains a single set of chromosomes e.g. eggs & sperm. Shown as n e.g. n = 23 in humans


Herbicide: Chemical which inhibits growth of, or kills, plants (weeds).


Heterozygous: Possessing two different alleles of a particular gene. Written as Aa or Bb etc


Hierarchy: In taxonomy, a layered system of groups within groups, with no overlap.


Homologous chromosomes: A pair of chromosomes that have the same genes in the same places (loci) although not always the same alleles. Human have 23 homologous pairs


Homozygous: Possessing two alleles the same e.g. AA or aa. Said to be “true breeding”


Huntington’s disease: Genetic disease caused by dominant allele that makes a brain-degenerating protein. Does not begin until age 35-40, often after gene has been passed on


Hydrostatic Pressure: the pressure of fluid within a vessel, e.g.: blood in an artery has high hydrostatic pressure. Created by the ventricles.



Inbreeding: The breeding of genetically similar individuals e.g. bothers and sisters. This is bad. It promotes homozygosity and increases the chance of genetic disease.

Incidence (incidence rate): Number of new cases of a particular disease in a population.

Incomplete dominance: When two alleles both exert an effect. e.g. in the Human ABO blood groups, A and B are codominant over O.  

Incubation period: The time taken for signs and symptoms to appear following entry of a pathogen  into a host organism.


Independent assortment: A key feature of meiosis, which separates the homologous chromosomes so that any one of a pair of alleles can pass into a gamete with one from any other pair.

Index case: The first recorded case of a disease in an outbreak (endemic, epidemic or pandemic).


Induced Mutations: Mutation(s) caused by direct exposure to a known mutagen.

Infection: The invasion of a host by a microorganism with subsequent establishment and multiplication of the agent. An infection may or may not lead to overt disease.


Insecticide: Chemical that kills insects.


Intensive farming: Production of high yields from a small land area.



Lactate (Lactic acid in solution): The first stage of respiration converts glucose into pyruvate. If there is no oxygen available the pyruvate is turned into lactate which accumulates in muscles, causing fatigue. Lowered pH interferes with enzymes in muscles, which can’t work as hard. Lactate can be broken down in muscles when oxygen arrives, but some diffuses into the blood and is removed by the liver, which either respires it or converts it into glycogen. 


Lignin: a polymer that strengthens plant cell walls, particularly of xylem vessels – known as secondary cell wall. Major component of wood. 


Locus: The position of a gene on a chromosome (plural = loci)


Lumen: the hole through a tube e.g. blood vessel.  


Lymph: Fluid which flows in the lymphatic vessels, produced by drainage of tissue fluid. Lymph is basically tissue fluid plus some protein and lipid molecules that are secreted by cells but too large to pass directly into the blood.


Medulla (oblongata): The part of the lower brain which controls involuntary activities, such as control of breathing and heart rate.


Meiosis: Cell division that shuffles the genes on the chromosomes so that no two gametes are the same. Has two divisions, so a diploid cell gives rise to four haploid cells.


Messenger RNA [mRNA]: A mobile copy of a gene, a single strand of nucleotides which directs protein synthesis. Made in transcription, used in translation.


Mitosis: Normal cell division. One diploid cell gives rise to two identical diploid cells 


Monoculture: Growing the same crop on a large scale over several years.


Mutagen: An agent which causes mutation. e.g. ionising radiation, mustard gas


Myocardium: posh word for heart muscle.


Myogenic: the ability of heart muscle to beat on its own, without the stimulation of nerves. The word means ‘muscle originating’


Myoglobin: red pigment found in muscles. Essentially one quarter of a haemoglobin molecule. It has a higher affinity for oxygen than haemoglobin, so the curve is to the left. Its function is to store oxygen in the muscles, to allow aerobic respiration to continue for longer. More myoglobin = more stamina.



Natural selection: Basic mechanism of evolution; the best adapted (‘fittest’) individuals will survive to pass more of their genes on to the next generation. Proposed by Darwin in 1858.



Organic effluents: Waste materials derived from organic matter e.g. animal slurry, silage runoff.


Outbreeding: The breeding of genetically different individuals. This increases diversity (or heterozygosity) & is good; faulty alleles are usually masked by healthy ones


Oxygen Dissociation Curve: Graph displaying how saturation of oxygen in Haemoglobin varies with the partial pressure of oxygen.  Good way of showing the properties of haemoglobin

Palisade cells: tall cylindrical cells, packed with chloroplasts, found towards the upper surface of a leaf. The main site of photosynthesis in most plants. 




Partial pressure: A measure of the amount of a particular gas in a mixture. You can’t just say ‘concentration of oxygen, or carbon dioxide’, because the rate at which it passes into our blood depends on the partial pressure. 20% of the air is oxygen, and at sea level the atmospheric pressure is 100KPa. So the partial pressure of oxygen is 20 KPa (20% of 100). If you were to climb a mountain, there would be less air pressing down on you, the atmospheric pressure would fall and so would the partial pressure of oxygen. There would be less of a partial pressure gradient between the air and your blood, so oxygen would enter more slowly. That’s why you would find it harder to breathe.

Pathogen: Any “agent” that can causes disease, e.g. virus, bacterium, toxin, protozoan etc.


Pathogenic (pathogenicity): The ability to cause disease.


PCR: The polymerase chain reaction. DNA cloning in a test tube; a quick method of making millions of copies of a tiny DNA sample. e.g. in forensics.


Pedigree: Of known ancestry (a “family tree”). Applies to humans as well as animals


Persistent pesticide: (non-biodegradable) Pesticide which remains in the same chemical form over a long period of time.


Pest (agricultural): Organism that causes damage to crop plants.


Pesticide: Chemical toxic to living organisms that is used to control pests.


Phenotype: The observable features of an organism (genotype + environment = phenotype)


Phloem: vital vascular (transport) tissue in a plant. Transports the products of photosynthesis (e.g. sucrose) from sources to sinks in the process of translocation. . Consists of living cells, in contrast to xylem. 


PhylogeneticIn taxonomy, from origin of phyla i.e. what evolved from what? Bit like a family tree. Based on shared features, DNA and protein studies.


Pits: In plant cells; holes in adjacent cellulose cell walls, through which plasmodesmata pass


Plasma: Liquid part of the blood.


Plasmid: Tiny circle of DNA found in the cytoplasm of bacteria. Contains useful rather than essential genes e.g for antibiotic resistance. Used as a vector, to transfer genes into bacteria in genetic engineering.


Plasmodesmata: Thin strands of cytoplasm that penetrate the cells walls (at pits), connecting adjacent cells. Important in the symplast pathway, and in phloem where companion cells communicate with sieve tubes via plasmodesmata.


Point mutation: A mutation of one base (which can change the amino acid and the whole protein)


Polygenic: Controlled by many alleles (see continuous variation) e.g. height in humans


Polyploid: A cell/organism containing many sets of chromosomes e.g. wheat can have 8


Prokaryotic cell: Simple cells with no true nucleus or complex organelles. Bacteria are prokaryotic.

Purkyne (or Purkinje) Fibres: Specialised muscle fibres (not nerves) along the ventricle walls that coordinate ventricular contraction., starting at the apex (bottom), ensuring that blood is forced up into the arteries. 



Recessive: An allele which is only shown in the phenotype in the absence of the dominant


Recombinant DNA: DNA from two different species. e.g. the human insulin gene can be spliced into bacterial DNA, producing recombinant DNA.


Replica plating: Growing bacteria on a series of agar plates to see which ones have been transformed (i.e. accepted new genes, such as those for antibiotic resistance)


Replication: The copying of DNA into two identical strands


Reproductive isolation: Stage 1 of speciation; when part of a population are unable to reproduce with the other part, so natural selection causes them to evolve along different lines


Restriction enzyme: An enzyme which cuts DNA at specific (recognition) site. Used to cut out genes in genetic engineering. Found in bacteria e.g EcoR1 comes from E coli.


Reverse transcriptase: Enzymes which make DNA from RNA (thus reversing transcription) Such enzymes are found in retroviruses, such as HIV


Ribosome: A small organelle, the site of translation in protein synthesis. 


RNA polymerase: An enzyme which moves along a gene, causing the assembly of mRNA on the sense strand i.e. the enzyme which controls transcription.



SAN (Sino-Atrial Node): Cells found in wall of right atrium that generate impulses for atrial contraction. The SAN is the pacemaker, and can speed up or slow the heart according to motor impulses from the cardiovascular centre.


Secondary Cell Wall: innermost layer of a cell wall deposited after cell enlargement has ceased, contains lignin. Important in maintaining cell shape


Semi-conservative: The mechanism of DNA replication. In each new strand, half is original (has been conserved) and half is new. This was shown by Meselsohn and Stahl


Semi-lunar Valve: Found in veins, and between ventricles and aorta/pulmonary artery to prevent backflow. 


Sense strand: The side of the DNA molecule (in a gene) which codes for making the protein


Sex chromosome: Chromosomes that decide the sex of an individual. Humans have one pair of sex chromosomes. XX = female, XY  = male


Sickle cell anaemia: Genetic disease caused by a recessive allele. Haemoglobin polymerises, distorting the red cells into sickle shapes, causing clots and other problems


Sieve element: a conducting cell in the phloem. Has no nucleus but does have some organelles around the outside.


Sieve plate: the end wall of a sieve-tube element that is perforated by sieve plate pores.


Speciation: How new species develop. Usually has three basic stages; 1 Part of a population becomes isolated; it cannot interbreed with the rest. 2 Natural selection acts differently on the two sub-populations 3 In time, genetic differences accumulate, -----> different species


Specific name: Name of the species; second part of Latin name. No capital; e.g. sapiens


Spindle: Cradle of protein fibres which organise/move chromosomes during cell division


Sticky ends: Staggered cuts in DNA resulting from cutting by restriction enzymes


Stoma (Stomata - plural): Pores (holes) in the lower epidermis of a plant leaf, surrounded by guard cells, allowing for gaseous exchange and transpiration. Generally, stomata are open in the day to allow gas exchange (the rapidly photosynthesising palisade cells need CO2) and closed at night to reduce water loss. 


Stretch Receptors: Nerve cells which are sensitive to tension changes. Found in the lungs where they provide feedback about the degree of inflation of the lungs, so we know when to stop breathing in and start breathing out. 


Symplast Pathway: Water and ions transport route through the plant cell cytoplasm. This is a route through living tissue with membranes, so the plant has some control over what passes into the xylem. See apoplast pathway. 


Systole: Contraction of the heart muscles. 



TAQ polymerase: DNA polymerase enzyme from the bacterium Thermus aquaticus (hence the TAQ); a thermostable enzyme widely used in PCR, optimum temp about 72 degrees C


Taxonomy: The very exciting science of classification.

Test cross: Cross with a homozygous recessive (aa) to find out if an individual is AA or Aa


Thermostable enzyme: Active across a range of temperatures, not denatured at the usual 50/60oC. From thermophilic (heat loving) bacteria in hot springs e.g. TAQ polymerase. 


Tissue Fluid: Fluid which surrounds all cells and is similar to plasma without the plasma proteins. Cells obtain nutrients from the tissue fluid and deposit cellular waste (urea and CO2).


Tracheids: the main conducting cell of the xylem, characterized by an elongated shape and lignified secondary cell wall. See also vessel element. 


Transcription: First step in protein synthesis, basically ‘gene copying’. Involves transferring the genetic code from DNA to mRNA. Happens in the nucleus.


Transfer RNA [tRNA]: a molecule that brings specific amino acids to ribosomes. Clover leaf shaped, has an anticodon on one end & carries an amino acid at the other.


Transgenic organism: An organisms whose DNA has been altered, such as the bacteria with human genes inserted


Translation: Second step in protein synthesis. Using the code on the mRNA to build a protein. Takes place on ribosomes


Translocation: The movement of the products of photosynthesis (sucrose, amino acids etc) throughout a plant in phloem tissue. The driving force behind translocation is called the mass flow hypothesis, or Munch’s theorem. 


Transpiration stream: the passage of water through the plant from roots to leaves and out into the atmosphere.


Transpiration: Loss of water vapour from the plant’s upper surfaces, mainly leaves.


True breeding: Homozygous individuals which always produce offspring on the same type


Vaccine: A preventative treatment of prepared microorganisms by means of either killed (dead) microorganisms,  living (i.e. weakened / attenuated) microorganisms. or inactivated bacterial toxins. Vaccines are administered to induce the immune response and provide protection against a pathogen or a toxin.

Vein: Blood vessel which returns blood to the heart. Adapted to minimise the resistance to blood flow despite low pressure. Has; thin walls, large lumen & valves. Relies heavily on action of surrounding skeletal muscles.


Ventricle: Lower chamber of heart, contracts to force blood into aorta or pulmonary artery.


Ventricular Systole: Contraction of the ventricle.


Vessel element: one of the different types of cell that makes up xylem tissue.

Virion: Complete virus particle, the simplest consists of just a protein capsid surrounding a single nucleic acid molecule (either DNA or RNA).


Virology: The branch of microbiology that is concerned with the study (structure, function, classification, epidemiology etc.) of viruses and viral diseases.


Virulence: The level of pathogenicity as indicated by fatality rates (cases) and/or the ability to invade host tissues and cause disease. So, in simple terms: Virulence is how pathogenic an organism is.


Virus: A simple acellular infectious agent. An obligate intracellular parasite”. A complete virus particle is called a virion consisting of a protein coat and a nucleic acid genome. Viruses lack independent metabolism and replication only takes place within living host cells.



Water Potential: The tendency of a cell/solution/area to absorb water. It’s a negative scale. Pure water has a water potential of zero. The more solute that is dissolved, the lower the water potential. Dry air has the lowest water potential (up to -30 000 kPa). Throughout a plant water moves down a water potential gradient, from soil water to the atmosphere.  



Xerophyte: a plant which has adapted to dry environments.


Xylem: the water-conducting tissue of plants, comprised mainly of dead cells strengthened by lignin and cellulose.


Zygote: A fertilised egg



*Note this list is not comprehensive and will be updated and added to regularly...