Tuesday, March 11, 2008

Biology Terms and Definitions (A-Z)


Abiotic factor any of the nonliving factors that make up the environment of living organisms.

Abscisic acid a plant growth substance which acts mainly as a growth inhibitor

Absorption the process by which dissolved substances are taken up by cells

Absorption spectrum a graph showing the relative amounts of light of different wavelengths that are absorbed by a pigment

Accomodation the process by which the eye focuses upon an object

Acethylchocine a chemical that allows the transmission of an impulse from one neurone (nerve cells) to another.

Acetylcholineste an enzyme which causes the breakdown of acetylcholine.

Acetylcoenzyme a chemical compound that is an important intermediate in aerobic respiration.

Acid rain this is produced when certain gases in the atmosphere, such as Sulphur dioxide, dissolve in rain water to form acids.

Acrosome a thin cap-like vesicle found at the tip of a sperm.

Actin a protein that forms the thin filaments found in the microscopic fibres of skeletal muscle.

Action potential the rapid change in electrical charge across the membrane of a nerve cells, causing the transmission of an impulse.

Action spectrum a graph showing the relative amount of light of different wavelengths that are used in a particular process, such as photosynthesis.

Activation energy the energy required to activate or begin a chemical reaction.

Active site the part of an enzyme to which a substrate becomes attached during a biochemical reaction.

Active Transport the movement of substances from where they are less concentrated to where they are more concentrated (against a concentration gradient)

Adaptation any feature of the structure or physiology of an organism that makes it well suited to its environment.

Adaptive radiation the process by which a single ancestral type of organism gives rise to a number of different forms.

Adenine a purine nitrogenous base that is found in DNA and RNA.

Adrenal Gland one of a pair of glands found adjacent to the kidney which is responsible for secreting several important hormones.

Adrenaline a hormone secreted by the adrenal glands at times of stress.

Adrenaganic synapse any synapse in which the neurotransmitter is noradrenaline.

Aerobic respiration the breakdown of glucose in the presence of oxygen to yield energy.

Agonist a chemical that binds to a receptor on a cell, triggering a physiological response.

Agrobacterium a type of bacteria commonly used in gene technology.

Aldosterone a hormone involved in regulating the concentration of sodium ions in the blood.

Allele one of the different forms of a gene.

Allopatric speciation the development of one or more species, which occurs when the populations of the parent species become geographically isolated from each other.

All-or nothing a term used to describe the fact that action potentials in nerve cells are always identical in size.
Allosteric enzyme an enzyme that exists in two interchangeable forms, one of which is active and the other inactive.

Alternation of generation a situation where the life cycle of an organism involves alternating spore-producing (diploid) and gametes-producing (haploid)

Alveolus one of the air sacs in the lungs where gas exchange takes place.

Amino acid the basic sub-unit or monomer from which proteins are formed.

Amylase an enzyme which breaks down starch into maltose.

Anabolism the synthesis of complex molecules from simple molecules.

Anaerobic respiration the breakdown of glucose in the absence of oxygen to yield energy.

Angiospermophyta the plant phylum which contains the flowering plants.

Animalia the kingdom containing animals.

Annelida the animal phylum containing earthworms and leeches.

Antagonist a chemical that binds to a receptor in a cell, blocking the normal physiological response.

Anther part of flower in which pollen develops and is later released when the anther ruptures.

Antibiotic a substance produced by a living organism that kills or inhibits the growth of microorganisms.

Antibody a protein molecule produced by lymphocytes in response to stimulation by an antigen.

Anticodon a triplet of bases in transfer RNA (tRNA) that can form base pairs with a specific codon during the synthesis of proteins.

Anti-diuretic hormone(ADH) a hormone which makes the distal convoluted tubules and collecting ducts of a kidney nephron more permeable to water.

Antigen a molecule that triggers the production of antibodies from lymphocytes.

Antiseptic a substance that kills or inhibits the growth of microorganisms and can be used safely on the skin.

Aorta the main artery of the body in mammals.

Apoplastic pathway the route by which water and solutes travel through the cells walls of plants.

Aqueous humour the fluid found in the front part of the eye, between the cornea and the lens.

Arteriole a vessel that receives blood from an artery and carries it to capillaries.

Artery a vessel that carries blood away from the heart.

Arthropoda the animal phylum containing insects, spiders and crustaceans.

Asexual reproduction reproduction involving the formation of new individuals from a single parent without the fusion of gametes.

Atom the smallest part of an element that cannot be broken down further by chemical means.

ATP (Adenosine triphosphate) a molecule that acts as an energy carrier in all living cells.

Atrioventicular node (AVN) a specialized area of muscle between the atria and ventricles of the heart.

Artrioventicular valve one of the valves between the atria and the ventricles of the heart.

Atrium a chamber of the heart which receives blood returning from the organs of the body.

Autonomic nervous system the part of the nervous system that is responsible for controlling the involuntary activities of the body.

Autosome a chromosome that is not a sex chromosome.

Autotrophic nutrition the synthesis of organic molecules from simple inorganic molecules, such as carbon dioxide and water.

Auxin a plant growth substance that acts mainly as a growth stimulator.

Axon the elongated section of a neurone (nerve cell) which carries impulses away from the cell body.


Bacteria a group of microorganisms which play an important role in nutrient recycling, disease transmission and industrial processes.

Bactericidal describes a chemical substance that kills bacteria.

Bacteriostatic describes a chemical substance that prevents the reproduction of bacteria.

Balanced diet a diet that includes all the necessary nutrients in the required amounts to maintain good health.

Basal metabolic rate (BMR) the amount of energy required to sustain an animal at rest.

B-cell a lymphocyte which produces antibodies when stimulated by an antigen.

Benedict’s test a biochemical test that can be used to show the presence of a reducing sugar.

Bicuspid valve the valve between the left atrium and the left ventricle in the heart of a mammal.

Bilateral symmetry a bilaterally symmetrical organism is one that can be divided in only one plane to give two approximately identical halves.

Bile an alkaline solution that is released into the small intestine to aid the digestion of fat.

Bioaccumulation the process by which certain chemicals become concentrated in the bodies of animals found at the top of food chains.

Biochemcical oxygen demand(BOD) the quantity of oxygen that is removed by microorganisms from a sample of water in a given time.

Biochemical tests a group of simple tests that can be used to identify important biological molecules.

Biological control the use of a parasite or a predator to control the number of pest organisms in a particular area.

Biomass the mass of all the organisms present in a given area.

Biotechnology the use of microorganisms or biochemical reactions to make useful products or to carry out industrial processes.

Biotic factor any environmental factor that is associated with living organisms.

Biuret tests a biochemical test that can be used to show the presence of a protein.

Blood a suspension of cells in solution that acts as a transport medium within an animal.

Blood circulation the system that transports blood around the body of many animals.

Blood clotting the production of a blood clot following damage to body tissues.

Blood glucose level the concentration of glucose circulating in the blood plasma.

Blood groups the many types into which an individual’s blood may be classified based on the presence or absence of certain antigens on the surface of red blood cells.

Bohr effect the effect of carbon dioxide concentration on the release of oxygen from haemoglobin in red blood cells.

Bone a supporting tissue found in the skeletons of most vertebrates.

Bwman’s capsule the cup-shaped end of kidney nephron.

Brain the organ is responsible for controlling bodily functions by coordinating the activities of the nervous system.

Bronchiole a small tube in the lungs that carries air between a bronchus and a large number of alveoli.

Bronchus a large tube in the lungs that carries air between the trachea and a number of bronchioles.

Bryophta the plant phylum which contains mosses and liverworts.

Buffer a solution or substance which resists changes in pH by taking in or releasing hydrogen ions.

Bundle of his specialized cardiac muscle fibres that run from the atrioventricular node to the base of the heart.


C3 plant a plant in which the first product of photosynthesis is a three-carbon compound.

C4 plant a plant in which the first product of photosynthesis is a four-carbon compound.

Calcium an essential mineral nutrient for both animals and plants.

Calvin cycle a series of biochemical reactions, forming part of the light independent stage of photosynthesis, which takes place in the stroma of the chloroplasts of C3 plant.

Capillary a very small blood vessel where water, solutes and respiratory gases are exchanged with body tissues.

Capsid the outer protein coat of a virus.

Capsule a thick layer surrounding the cell wall of certain bacteria.

Carbohydrate a compound containing the elements carbon, hydrogen and oxygen, with the general formula Cx(H2O)y.

Carbon cycle one of several nutrient cycles, describing how the element carbon cycles in the environment.

Carbonic anhydrase an enzyme that catalyses the reaction between carbon dioxide and water to form carbonic acid.

Cardiac cycle the sequence of events taking place in a single heartbeat.

Cardiac muscle a specialized type of muscle, found only in the heart of vertebrates.

Cardiovascular centre part of the medulla oblongata in the brain which is responsible for regulating the heart rate.

Carotid body a collection of cells found in the walls of the carotid arteries that are sensitive to the oxygen concentration and pH of blood.

Carrier molecule a molecule that either transports substances across a membrane or carries electrons through an electron transport chain.

Carrying capacity the maximum population that a particular habitat can support .

Cartilage a hard-wearing and flexible connective tissue which plays an important role in the joints of the skeletons.

Casparian strip a band of impermeable suberin found in the walls of endodermal cells in plant roots.

Cell the basic structural and functional unit of most living organisms.

Cell cycle the sequence of events which occur during cell growth and cell division.

Cell fractionation the separation of the different components of a cell.

Cell membrane a membrane which encloses a cell or which surrounds certain organelles within a cell.

Cell sap the solution (of sugars, amino acids and mineral salts) contained in the vacuoles of plant cells.

Cellulose a polysaccharide comprising long, unbranched chains of glucose molecules.

Cell wall a rigid structure found outside the cell surface membrane of plant, fungal and bacterial cells.

Central nervous system(CNS) the CNS consists of the brain and spinal cord, and is responsible for coordinating the activities of the nervous system.

Centrifuge a machine that separates materials of different densities by spinning them in a tube at high speed.

Centrioles structures found in animal cells which play a role in spindle-formation during cell division.

Centromore the part of a chromosome that attaches to the spindle during cell division.

Chemosynthesis the synthesis of complex organic molecules from simple inorganic molecules, using the energy from chemical reactions.

Chiasma(ta) the point(s) at which the chromatids of homologous chromosomes cross over each other during meiosis.

Chitin a nitrogen-containing polysaccharide.

Chlorofluorocarbons(CFCs) manufactured chemicals that act as atmospheric pollutants, degrading the ozone layer.

Chlorophyll a green pigment found in photosynthesis organisms.

Chlroplast an organelle found in the cells of plants and algae which is the site of photosynthesis.

Cholestrol a steroid which performs a number of functions in living organisms.

Cholinergic synapse any synapse in which the neurotransmitter is acetylcholine.

Chordata the animal phylum containing fish, amphibians, reptiles, birds and mammals.

Chromatid one of the two identical strands of genetic material that make up a chromosome.

Chromatography a technique used to separate the individual components of a mixture.

Chromosome a thread-like structure, composed of DNA and protein, found in the nucleus of plant and animal cells.

Cilia short, hair-like structures found on the surface membranes of certain cells.

Circadian rhythm a 24 hour cyclical change in the behaviour or physiology of organisms.

Classification the system by which living organisms are organized into groups.

Clone any of the genetically identical individuals that have arisen from a single parent by asexual reproduction.

Cnidaria the animal phylum containing hydra, jellyfish, sea anemones and corals.

Codominant alleles any two alleles which are both expressed in the phenotype of a heterozygous organisms.

Codon a triplet of bases in messenger RNA (mRNA) that codes for a particular amino acid during the synthesis of proteins.

Coelom a fluid-filled cavity formed in the mesoderm of triploblastic animals.

Coenzyme an organic molecule that associates with an enzyme to catalyse a biochemical reaction.

Cohesion tension theory a theory used to explain the movement of water through the xylem vessels of a plant.

Collagen a fibrous protein found in the connective tissue of skin, tendons, cartilage and bone.

Collecting duct the final section of a kidney nephron.

Collencyma a type of plant tissue in which the cells have additional cellulose thickening in their walls.

Colon part of the large intestine, found between the small intestine and the rectum

Commensalism a relationship between two different species which results in benefit to one without affecting the other.

Community all the living organisms present in a habitat.

Competition the interaction between two or more organisms seeking the same environmental resources that are in short supply such as food or mates.

Competitive inhibitions a reduction in the rate of an enzyme-catalysed reaction by a molecule that is similar in shape to the normal substrate.

Condensation reaction a chemical reaction in which two molecules are joined together and a molecule of water is removed.

Cone cell a light sensitive cell found in the retina of the eye./

Coniferophta the plants phylum which contains pines, firs and other conifers.

Connective tissue an animal tissue consisting of a number of cells embedded in a matrix of protein fibres and salts.

Conservations the process of maintaining an ecosystem in order to retain maximum species diversity.

Consumer any of the heterotrophic organisams in a food chain or food web.

Continuous variation variation of a characteristic within a population, such that a complete range of forms can be seen.

Contractile vacuole an organelle found in some single-celled organisms which removes excess water from the cell.

Corolla a collective name for the petals of a flower.

Coronary heart disease the blockage of one or more of the coronary arteries which supply blood to the muscle of the heart.

Corpus luteum a tissue that forms in the ovary of a mammal following the release of an ovum.

Correlation a relationship between two variables such that a change in one of them is reflected by a change in the other.

Cotyledon the seed leaf of a plant embryo.

Counter-current system a mechanism by which fluids in adjacent systems flow in opposite directions in order to maintain a diffusion gradient between them

Crossing over the exchange of genetic material between homologous chromosomes during meiosis.

Cultural evolution the changes which have taken place in human societies over the course of time.

Cyclic AMP a molecule that acts as a second messenger in many physiological reactions induced by hormones.

Cytochrome an iron-containing protein which forms part of an electron transport chain.

Cytokinin a plant growth substance which stimulates cell division.

Cytoplasm the contents of a cell surrounding the nucleus and enclosed by the cell surface membrane.

Cytosine a pyrimidine nitrogenous base that is found in DNA and RNA.


Deamination the removal of an amino group from an amino acid.

Decomposer an organism that breaks down dead material or waste products.

Deflected succession a type of succession in which a climax community fails to become established due to human influences.

Denitrification the process by which bacteria covert nitrates in the soil to nitrogen gas in the atmosphere.

Density-dependant factor any factor limited the size of a population whose effect is proportional to the density of the population.

Detritivore an animal that feeds on dead material or waste products.

Diabetes a disease in which the concentration of glucose in the blood cannot be properly regulated.

Dialysis a method for separating molecules of different sizes by diffusion through a partially permeable membrane.

Dicotyledon a flowering plant that produces seeds with two cotyledons.

Differentiation the process by which cells become specialized for a particular function.

Diffusion the net movement of molecules or ions from a region of high concentration to a region of low concentration.

Digestion the breakdown of food material into simple molecules that can be absorbed by the body.

Dihybrid inheritance the inheritance of two different characteristics.

Diploblastic an animal with a body wall composed of only two layers of cells.

Diploid a cell in which the nucleus contains two complete sets of chromosomes.

Directional selection a type of natural selection which occurs when environmental change favours a form of an organism.

Dissacharide a type of carbohydrate formed from two monosaccharides.

Discontinuos variation a type of variation in which there are clearly defined differences within a population.

Disruptive selection a type of natural selection in which the environment favours forms at the extremes of the range of phenotypic variation.

Diversity a measure of the number of different species present in a particular habitat.

DNA a molecule that forms the genetic material of all living organisms.

DNA probe a single strand of DNA that is used to identify a particular gene.

DNA replication the process by which a DNA polymerase.

Dominant allele any allele that is always expressed in the phenotype of an organism.

Double circulation a type of circulatory system in which blood passes through the heart twice during its journey around the body.

Down’s syndrome a genetic disorder in humans caused by the presence of an additional chromosome.

Duodenum the first section of the small intestine.


Echinodermata the animal phylum containing sea urchins and starfish

Ecology the study of the interactions of organisms with their biotic (living) and abiotic (non-living) environments

Ecosystem a stable but dynamic system, characterized by the interaction of its botic (living) and abiotic (non-living) components

Ectotherm an animal that uses the enviroment to regulate its body temperature

Effector a cell or organ that responds to a stimulus

Electrocardiogram (ECG) a graph showing the electrical activity in the heart during the cardiac cycle

Electron a negatively charged particle normally found orbiting the nucleus of an atom

Electron microscope a microscope that uses a beam of electrons to form highly magnified images of very small objects

Electron transport chain a system carrier molecules which transfer electrons from one to the other, releasing energy for the production of ATP

Electrophoresis a method used to separate a mixture of charged molecules

Element any substance that cannot be broken down further by chemical means

Embryo an animal or plant that develops from a zygote prior to birth, hatching or germination

Emulsion test a biochemical test that can be used to show the presence of lipid

Endocrine glands a gland that secretes a hormone directly into the blood

Endocytosis the uptake of large particles or fluids through the surface membrane of a cell

Endodermis a layer of cells surrounding vascular tissue in the root of a plant

Endometrium the inner lining of the uterus of mammals

Endopeptidase an enzyme that breaks peptide bonds at specific sites within a protein molecule

Endoplasmic reticulum a system of membranes found in the cytoplasm of a cell

Endoskeleton the supportive internal framework of an organism

Endosperm a nutritious tissue found in the seeds of flowering plants

Endotherm an animal that can regulate its body temperature using physiological mechanisms

Enterokinase an enzyme secreted in the small intestine that converts trypsinogen to trypsin

Environment the conditions found in the region in which an organism lives

Enzyme a protein molecule which acts as a catalyst in living organisms
Epidermis the outer layer of cells in a plant or animal

Epistasis a type of gene interaction, in which one gene controls the expression of another gene

Epithelium a layer of closely packed cells which forms a covering around body structures

Ethene a gaseous hydrocarbon (C2 H4) which acts as a plant growth substance

Eukaryotic cells with a distinct nucleus containing a number of chromosomes

Eutrophication a decrease in biodiversity resulting from the pollution of a river or lake by sewage or fertilizers

Evolution the process by which new species arise as the result of gradual change to the genetic make-up of existing species over long periods of time

Excretion the removal of the waste products of cellular metabolism

Exocrine gland a gland that secretes substances through a duct to their place of action

Exocrine gland a gland that secretes substances through a duct to their place of action

Exocytosis the transport of large particles or fluids out of a cell via the the diffusion cell surface membrane

Exopeptidase a protein-digesting enzyme that breaks peptide bonds at the end of polypeptide chain

Exoskeleton the supportive external framework of an organism

Extracellular digestion the breakdown of food molecules that takes place outside the cells


F1 (first filial generation) the offspring resulting from a genetic cross between homozygous parents

Facilitated diffusion the diffusion of a substance across a membrane, facilitated (assisted) by a protein carrier molecule

Fatty acid organic compounds that are usually found as constituents of lipids

Female an organism that produces female gamete (sex cells)

Fermentation the breakdown of organic molecules in the absence of oxygen

Fertilisation the fusion of male and female gametes during the process of sexual reproduction to produce a zygote

Fertiliser a substance that is added to soil in order to increase its productivity

Fetal Haemoglobin the type of haemoglobin found in the blood of a fetus

Fetus (foetus) the stage of development of a mammalian embryo when the main features of the adult form can be seen

Fibrin an insoluble protein that is the foundation of a blood clot

Fick’s Law states that diffusion of a substance across a membrane is directly proportional to the surface area of a membrane and to the difference in concentration on either side, an inversely proportional to the thickness of the membrane

Filicinophyta the plant phylum that contains the ferns

Flagellum a long hair-like structure found on the surface of cells which is involved in locomotion

Florigen a hypothetical plant growth substance that may stimulate flowering in plant

Flower a structure that contains the sexual reproductive organs in certain plants

Fluid mosaic model a theory that describes the structure of a cell membranes

Follicle-stimulating hormone (FSH) a mammalian hormone that stimulates the production of gametes

Food chain a sequence of feeding relationships in an environment

Food web a system of interconnected food chains

Fovea part of the retina of the eye, containing a very large number of cone cells

Fruit the structure that encloses the seeds in a flowering plant

Fungi the kingdom containing mushrooms, toadstools moulds and yeasts


Gall bladder a small sac which stores bile produced by the liver.

Gamete a reproductive cell that fuses with another gamete to form a zygote.

Gametogenesis the formation of gametes

Gametophyte the stage in the life cycle of a plant in which gametes are produced.

Gaseous exchange the movement of gases between an organism and its environment.

Gastric juice a mixture of hydrochloric acid, mucus and digestive enzymes secreted by gastric glands in the stomach wall.

Gene a section of DNA that codes for the production of a particular polypeptide or protein.

Gene mutation any change in the nucleotide sequence of a gene.

Gene pool all the genes and their different alleles that are present in a particular population.

Gene probe a single strand of DNA which has a base sequence complementary to the gene being identified.

Gene technology the process involved in changing the characteristic of an organism by inserting foreign genes into its DNA.

Gene therapy the application of gene technology to alter or replace defective genes.

Generator potential the rapid change in electrical charge across the cell surface membrane of a receptor as a result of a stimulus.

Genetic code the method by which the genetic information in DNA controls the synthesis of specific proteins by the cell.

Genetic conservation the preservation of genetic variation.

Genetic counseling the advice given to people from families with a history of genetic disease about the risks of having children.

Genetic engineering a means of altering the genetic make-up of an organism.

Genetic fingerprint the technique in which an individual’s DNA is analysed to reveal a unique pattern of nucleotide sequences.

Genotype the genetic composition of an organism.

Geotropism the directional growth of part of a plant in response to gravity.

Germination the initial growth of a seed, using stored food reserves.

Gibberellin a plant growth substance which acts mainly as a growth stimulator.

Gill the gas exchange organ of many aquatic animals, such as fish, mussels and tadpoles.

Gland a group of cells that is specialized for secretion.

Globular protein a protein whose molecules are curled into a relatively spherical shape and that is often water soluble and metabolically active.

Glomerular filtrate the fluid found in the Bowman’s capsule of a kidney nephron.

Glomerulus a group of capillaries enclosed by the cup-shaped end of a kidney nephron.

Glucagon a hormone involved in the control of blood glucose.

Gluconeogenesis the synthesis of glucose from non-carbohydrate sources, such as lipid and protein.

Glucose a hexose (six-carbon) monosaccharide.

Glycogenesis the conversion of gluscose to glycogen.

Glycogenolysis the conversion of glycogen to glucose.

Glycolysis the first stage of respiration, in which glucose is broken down to form pyruvate.

Glycosidic bond a C-O-C link between two monosaccharide molecules.

Gram staining a staining method used to classify bacteria.

Greenhouse effect the way in which heat is trapped in the earth’s atmosphere.

Growth a permanent increase in size of an organism.

Guanine a purine nitrogenous base that is found in DNA and RNA.


Haber-bosch process a process by which dinitrogen is catalytically reduced to ammonia using hydrogen gas under conditions of relatively high temperature and pressure.

Habitat the place where an organism, a population or a community lives.

Habituation is a form of learning in which repeated application of a stimulus results in decreased responsiveness.

Haemocoel collection of sinuses which are spaces within the body where blood is pumped to at relatively low pressure from the heart.

Haemodialysis the first line of defence when a kidney failure worsens. It is carried out in an artificial kidney often referred to as dialysis machine.

Haemoglobin the red pigment found in red blood cells whose molecules contain four iron ions within a globular protein made up of four polypeptides and that combines reversibly with oxygen.

Halophytes plants found in areas of high salinity, such as estuaries and saltmarshes.

Haploid a term referring to cells which contain only a single copy of each chromosome.

Hardy-weinberg principle states that the proportion of dominant and recessive alleles of a particular gene remains the same if certain conditions are met.

Haustorium a special organ for the digestion and absorption of host cell cytoplasm.

Haversion canal a canal where blood vessels run through the matrix.

Haversion system arrangement of compact bone consisting of osteocytes and matrix in cylinders of bone surrounding central haversion canals.

Helicotrema a tiny hole where compartments communicate at the tip of the cochlea.

Heparin a substance that prevents clotting under normal conditions.

Hepatitis a condition of inflammation of the liver.

Herbivore animals that feed directly on plants.

Heterocysts vegetative cells which are photosynthetic and a few thick-walled cells.

Heterogamy gametes differ in size and structure and in their roles in reproduction.

Heterotrophic nutrition form of feeding in which the organism consumes complex organic material.

Heterozygote a condition in which the alleles of a particular gene are different.

Histology the study of the structure of tissues and organs.

Homeostasis the maintenance of more or less constant internal conditions.

Homoiothermic animals warm blooded animals.

Homologous refers to structures which may have different functions but which have a common evolutionary origin.

Homologous chromosomes a pair of chromosomes having the same gene loci and which are capable of pairing during meiosis.

Homozygote a condition in which the allele of a particular gene are identical.

Humus a black gum-like substance, derived from decayed plant and animal remains.

Huntington’s disease a disease due to an autosomal dominant allele on chromosome 4.

Hyaline cartilage the common form of cartilage in mammals.

Hybridisation the crossing of carefully selected varieties in order to bring together desirable qualities from both parents in the offspring.

Hybridoma an immortal cell formed by the fusion of a cancer cell with a B cell.

Hydrogen bonds a relatively weak bond formed by the attraction between a group with a small positive charge on a hydrogen atom and another group carrying a small negative charge

Hydrolysis the breaking down of large molecules into smaller ones with the addition of a water molecule.

Hydrophonics a means of growing plants without soil.

Hydrophytes plants at the pioneer stage that live in or on water.

Hydrosere a succession that begins with water.

Hypertension abnormally high blood pressure.

Hypertonic when external water potential is more negative than that of the cell (that is, the cell is surrounded by a solution of more negative water potential)

Hypocotyl the base of the radicle, just below the attachment of the cotyledons.

Hypogeal germination a type of germination where the cotyledons remain below ground. The plumule is carried aloft by the growth of the epicotyl. The hooked shape taken on by the plumule as it is drawn up through the soil protects the tip from damage by soil abrasion.

Hypoglycaemia the glucose level in the blood falls.

Hypothalamus the main control center for the autonomic nervous system.

Hypothermia a condition which results from the core body temperature falling below normal.

Hypothyroidism overactivity of the thyroid gland.

Hypotonic when the external water potential is less negative than that of the cell (that is the cell is surrounded by a solution with a less negative water potential)

Hypoxia a deficiency of oxygen reaching the tissues.


Ileum part of the small intestine in which the process of digestion is completed and absorption takes place

Immobilisation a technique used in biotechnology in which enzymes are fixed to an unreactive material

Immune a term used to describe an animal that is not susceptible to infection by certain disease-causing organisms ot that remains unaffected by their toxins

Immune adherence the first cellular binding reaction involving the complement system to be recognized.

Immune complex a more or less large cross-linked network of non-covalently attached antibody (Ab) and antigen (Ag) molecules.

Immune response the specific, collective and coordinated response of a living organism to a particular antigenic challenge

Immunology the biomedical branch that studies specific immunity,

Implantation the process by which an embryo becomes embedded in the lining of the uterus of a mammal

Impulse a signal that travels along a neurone (nerve cell)

Induced fit hypothesis a model that describes the action of an enzyme

Inorganic any compound that is not based on carbon

Inositol a small water-soluble cyclic sugar alcohol

Insulin a hormone involved in the control of blood glucose

Interferon a protein molecule that prevents the replication of viruses

Interspecific competition competition for resources that occurs between members of different species

Intraspecific competition competition for resources that occurs between members of the same species

In vitro any biological processes that occurs outside body, in any artificial situation

In vivo any biological processes that occurs within a living organism

Iodine a nutrient that is required in very small quantities by living organisms

Ion an electrically charged atom or molecule

Ionizing radiation radiation that is able to remove electrons from an atoms or molecules

Iron a nutrient that is required in small quantities by living organism

Islets of Langherhans small group of cell found in the pancreas

Isolation the process by which individuals of the same species are prevented from reproducing

Isomer a molecule that contains the same atoms as another molecule but has a different structure

Isotonic a solution that has the same water potential as another solution

Isotope an atom of the same element as another atom but having a different mass


Kidneys a pair of organs found in mammals which play an important role in excretion and osmoregulation

Kingdom the largest group used in classification

Krebs Cycle an important pathway in aerobic respiration involving decarboxylation and oxidation.


Lactate (or lactic acid) the end product of anaerobic respiration, often produced by muscles during exercise
Lactation the production of milk by mammary glands

Lactose a dissacharide found in milk

Lag phase stage in the growth of a population where the organisms may increase in size but they do not increase in number.

Leguminous plant a plant that plays an important role in the nitrogen cycle

Lens a transparent structure in the eye which is responsible for focusing light onto the retina

Lenticels small pores found in the stems of woody plants

LH lutenising hormone

Ligament a tough but flexible connective tissue that hold bones together as a movable joint

Ligase any of a group of enzymes that are important in the synthesis and repair of many biological molecule

Light-dependent reaction stage in photosynthesis in which light energy is required to produce ATP and reduced NADP.

Light-independent reaction Stage of photosynthesis which does not require light energy and in which carbon dioxide is reduced to form carbohydrate.

Lignin a complex polymer found in the cell walls of plant tissues

Limiting factor a variable which limits the rate of a particular reaction.

Linkage term used to describe the situation in which two or more genes are found on the same chromosomes.

Link reaction the reaction that links glycolysis and the Krebs cycle during aerobic respiration

Lipase an enzyme that breaks down lipids into fatty acids and glycerol

Lipid any of a large group of organic substances which are insoluble in water but soluble in organic solvents such as ethanol

Lipoprotein a compound consisting of a lipid combined with a protein

Liver a large organ found in vertebrates that performs many important metabolic reactions

Lock and key hypothesis simple model to explain the way in which the active site of the enzymes binds to the substrate to catalyze a reaction.

Locus the position on a chromosome occupied by a particular gene

Loop of Henle the hairpin-shaped section of a kidney nephron

Log phase stage in the growth of a population in which there is a rapid increase in numbers.

Lumen the space inside a vessel or duct

Lung one of a pair of gas exchange organs found in air-breathing vertebrates

Luteinising hormone (LH) a hormone that stimulates ovulation in females and the production of testosterone in males

Lymph an almost colorless fluid, very similar in composition to blood plasma but with fewer plasma proteins, that is present in lymph vessels.

Lymphocte a type of white blood cell that is involved in the immune response; unlike phagocytes they become active only in the presence of a particular antigen that ‘matches’ their specific receptors or antibodies

Lysosome a small membrane-bound organelle found in the cytoplasm of many cells


Male an organism that produces male gamete (sex cells)

Maltose a disaccharide found in germinating seeds

Mass transport the transport of molecules in bulk from one part of an organism to another

Magnification the number of times greater that an image is than the actual object. Magnification= image size / object size.

Mean the arithmetic average of a set of numbers

Mechanoreceptor a sensory receptor that responds to a mechanical stimulus such as touch sound and pressure

Medulla the inner part of a kidney

Medulla oblongata part of the brain which is responsible for regulating the breathing rate and blood flow around the body

Meiosis the type of nuclear division in which the number of chromosomes is halved.

Menstrual cycle the cycle of events associated with ovulation and the development and breakdown of the endometrium

Memory cells lymphocytes which develops during an immune response and retain the ability to respond quickly when an antigen enters the body on a second, or any subsequent, occasion.

Mental disease a disease that affects the mind.

Meristem a group of plant cells which are capable of mitotic division to bring about growth

Messenger RNA(mRNA) a type of RNA which carries genetic information from the nucleus to the cytoplasm during protein synthesis

Metabolic rate a measure of the rate of energy release by the body.

Metabolism all the chemical processes that take place in living organisms

Micronutrient nutrients required in very small quantities in order to maintain health

Microorganism any organism that can be observed only with the aid of a microscope

Mitochondria a rod-shape organelle found in the cytoplasm of eukaryotic cells

Mitosis the type of nuclear division in which the daughter cells have the same number of chromosomes as the parent cell.

Molecule a substance formed by the combination of atoms

Monocotyledon a flowering plant that produces seeds with only one cotyledon (seed leaf)

Monoculture term used to describe a large area of land in which only one type of crop is grown.

Monocyte a type of white blood cells

Monohybrid inheritance the inheritance of a single characteristic

Monomer a molecule that can combine with similar molecules to form a polymer

Monosaccharide the simplest form of carbohydrate

Mucus a glycoprotein which has lubricating and protective functions

Multiple alleles term used to describe a gene which has more than two possible alleles.

Muscle a tissue consisting of contractile fibres that can produce movement

Mutation a change in the amount or the arrangement of the genetic material in a cell.

Mutualism a nutritional relationship between two species in which both gains some advantage

Mycelium a network of hyphae that forms the body of many fungi

Myelin a mixture of phospholipids and cholesterol which forms a sheath around many nerve cells

Myofibrils the microscopic fibres that make up the larger fibres of skeletal muscle

Myoglobin an iron-containing protein found in muscle

Myosin a protein that forms the thick filaments in the microscopic fibres of skeletal muscle


NAD a molecule that acts as a hydrogen acceptor during respiration

NADP a molecule that acts as a hydrogen acceptor during photosynthesis

Natural immunity immunity gained by infected (active) or by receiving antibodies from the mother across the placenta or in the breast milk (passive).

Natural selection the mechanism responsible for the evolution of new species

Negative feedback a series of changes which result in a substance being restored to its normal level-an important part of homeostasis.

Nephron the functional unit of the kidney

Net productivity a measure of the rate at which an organism accumulates new body substances.

Neromuscular junction the point at which a motor neurone connects with a muscle

Neurone a cell specialized for the conduction of nerve impulses

Neurotransmitter a chemical that permits the transmission of nerve impulses across a synapse

Niche the precise point at which an organism fits into its environment, both in terms of where it lives and what it does.

Nitrification the conversion of ammonium compounds to nitrites and nitrates.

Nitrogen cycle one of several nutrient cycles, describing how the element nitrogen cycles in the environment

Nitrogen fixation the incorporation of atmospheric nitrogen gas into organic nitrogen-containing compounds.

Non-competitive inhibition the inhibition of enzyme activity due to the binding of an inhibitor molecule at a site other than the active sites

Non-disjunction the failure of chromosomes to separate properly during meiosis.

Non-infectious disease a disease that is not caused by an organism.

Noradrenaline a chemical that allows the transmission of an impulse from one neurone (nerve cell) to another

Nucleic acid a biological molecule consisting of a long chain of nucleotides

Nucleolus part of the nucleus of a cell where ribosomal RNA is produced

Nucleotide the basic unit from which nucleic acids are formed

Nucleus a large organelle found in eukaryotic cells

Null hypothesis a statement based on the assumption that there is no significant difference between two or more sets of experimental results.

Nutrient a substance that is required in the diet, e.g. proteins, carbohydrates, fats, vitamins and minerals; water and fibers are not generally regarded as nutrients.

Nutrient cycling the way in which elements move through ecosystems


Obesity a form of malnutrition in which energy consumption is much higher than energy expenditure, leading to storage of excessive amounts of fat; a person who is 20% or more above the recommend weight for their height is considered to be obese.

Oesophagus the section of the digestive system that lies between the mouth and the stomach

Oestrogen a sex hormone produced by the ovaries in female

Oogenesis the process by which female gametes are formed in animal.

Organ a structure within a multicellular organism that is made up different types of tissues working together to perorm a particular function, e.g. the stomach in a human or a leaf in a plant.

Organelle a functionally and structurally distinct part of cell, for example a ribosome or mitochondrion

Organic any molecules based on carbon

Ornithine cycle biochemical pathway in the liver that converts ammonia to urea

Osmoregulation the ability to control the concentration of body fluids.

Osmosis the net movement of water molecules from a region of high water potential to a region of loe water potential through a partially permeable membrane, as a result of their random motion.

Ovary an organ that produces female gametes

Ovum a gamete produced by female vertebrates

Oxidation a chemical reaction involving the loss of electrons.

Oxidative phosphorylation the formation of ATP through the oxidation of hydrogen atoms in the electron transport system.

Oxidoreductase a class of enzymes that catalyse biochemical reactions involving oxidation and reduction

Oxygen debt the amount of oxygen needed to oxidize the lactic acid that accumulates during anaerobic respiration.

Oxygen dissociation curve a graph illustrating the relationship between the partial pressure of oxygen and the percentage saturation of haemoglobin with oxygen.


Palisade wall a type of cell found just beneath the upper epidermis pf a leaf

Palmitoylation a process by which a protein can be attached to a membrane, which can be the plasma membrane or an intracellular membrane

Pancreas a secretory organ found just below the stomach in vertebrates

Papilla (e) cellular outgrowths which look like little bumps or fingers on the surface of cells.

Parasite an organism that lives on or within a host (another organism); it obtains nutrients from the host without benefiting or killing (although it may damage) the host;

Parasitism a type of symbiotic relationship in which one organism benefits and the other does not.

Parasympathetic nervous system the part of the automatic nervous system which regulates physiological functions when the body is at rest

Partially permeable membrane a membrane that is permeable to small molecules but does not allow the passage of larger molecules

Parenchyma A generalized cell or tissue in a plant which may manufacture or store food, and can often divide or differentiate into other kinds of cells.

Passive transport a process allowing small hydrophobic molecules to cross the membranes phospholipid bilayer, without any intervention of proteins or any consumption of energy.

Pectin a polysaccharide found inplant cell walls

Pentose a monosaccharide containing five carbon atoms.

Pepsin a protein digesting enzyme produced by the stomach

Peptidase an enzyme of the hydrolase group which hydrolyzes peptide bonds within peptide chains.

Peptide a small natural or synthetic chain of amino acids connected by peptide bonds.

Peptide bond an amide linkage between the carboxyl group of an amino acid and the amino group of another one which is a covalent bond, and it is formed by loss of a water molecule.

Peristalsis the movement of material along a tube by waves of muscular contraction

Pesticide any chemical compound that are used to kill pests

Petal one of the outer appendages of a flower, located between the outer sepals and the stamens. Petals often display bright colors that serve to attract pollinators.

pH an unit measuring the acidity of a solution.

Phagocyte a specialized cell able to ingest and digest extracellular materials (pathogens, cell debris, particles ...) by phagocytosis.

Phagocytosis the mechanism by which cells ingest extracellular large substances, including macromolecules, parts or totality of other cells and microorganisms

Phagosome a cytosolic vacuole containing materials captured by phagocytosis, and which are destined to be destroyed.

Pharynx cavity in the digestive tract just past the mouth itself.

Phatogenic Organism which causes a disease within another organism.

Phenotype the physical or functional characteristics of an organism, produced by the interaction of genotype and environment during growth and development.

Phenotypic variance variance of the phenotype due to genotypic and environmental factors combined.

Phloem Nutrient-conducting tissue of vascular plants.

Phosphate an ion consisting of a phosphorus atom and four oxygen atoms.

Phospholipids a lipid whose one of the alcohol group is esterified by phosphoric acid.

Photoautograph an organism that synthesis complex organic molecules from simple inorganic molecules, using light energy

Photophosphorylation the synthesis of ATP using light energy during photosynthesis

Photoreceptor a cell which is sensitive to light

Photsystem a system of photosynthesis pigments found in chloroplasts

Photosythesis biochemical process in which light energy is absorbed by chlorophyll, and is used to fuel the building of sugar molecules.

Phototropism a change in growth in response to light

Phragmoplast the cell plate formed during cell division.

Phylum a group used for the classification of organisms

Phylogenetics Field of biology that deals with the relationships between organisms which includes the discovery of these relationships and the study of the causes behind this pattern.

Physiology the study of the processes which sustain life in organisms, such as nutrition, respiration and reproduction

Phytochrome a pigment found in plants which are important in regulating growth

Phytolysis the splitting of water molecules that takes place during photosynthesis

Pigment a coloured substance

Pinocytosis one of the types of endocytosis. It is the mechanism by which cells ingest extracellular fluids

Pinosome a small fluid-filled vesicle found in the cytoplasm during pinocytosis

Pith any central region of parenchyma tissue within a plant stem.

Pituitary gland an endocrine gland situated at the base of the brain

Pituitary hormone a hormone released from the pituitary gland.

Placenta a tissue formed within the uterus through which nutrients are passed from the mother to the embryo (and later the fetus) and its wastes are removed

Plasmodesma the unique class of intercellular junction which, in plant cells, connects the cytoplasm of adjacent cells.

Plantae the kingdom containing plants

Plant growth substance an organic compund that plays an important role in the coordination of plant growth and development

Plasma membrane the structure which envelops a cell, surrounds its cytoplasm and separates it from the extracellular environment

Plasma protein any of the protein constituents of blood plasma

Plasmid circular loop of DNA in prokaryotes.

Plasmodesmata cytoplasmic connections between neighboring cells in plant tissues.

Plasmogamy a process of fusion of the cytoplasm of two cells; the first step in syngamy.

Plasmolysis the shrinkage of cytoplasm away from the cell wall in plant cells

Plastid any of several pigmented cytoplasmic organelles found in plant cells and other organisms, having various
physiological functions, such as the synthesis and storage of food.

Platelets disk-shaped structures, found in the blood of all mammals.

Platyhelminthes the animal phylum containing flatworms, tapeworms and flukes

Plumule the part of a plant embryo that develops into a shoot

Pneumocyte a cell of the walls of the pulmonary alveoli.

Pollen the microspore of seed plants.

Pollen tube a tube produced by germinating pollen grain

Polllination process of transferring the pollen from its place of production to the place where the egg cell is produced.

Pollinator animal which carries pollen from one seed plant to another, unwittingly aiding the plant in its reproduction.

Polymer a large molecule constructed from many smaller identical units.

Polymerase Chain Reaction a technique invented by Kary Mullis in 1984 to amplify a specific segment of DNA in vitro,
through primer extensions

Polynucleotide a polymer of mononucleotides, or nucleotides.

Polypeptide a long chain of amino acids connected by peptide bonds.

Polyploid an individual containing more than two sets of genes and chromosomes.

Polysaccharides carbohydrates which release more than ten monosaccharides during hydrolysis.

Population a group of organisms, usually a group of sexual organisms that interbreed and share a gene pool, and are normally relatively isolated from other groups of the same species.

Porins multipass transmembrane proteins in which the transmembrane segments are arranged as a b sheet rather than as a helices.

Potocytosis the process by which ions or molecules are internalized into cells, through transient invaginations of the plasma membrane

Predator organism which hunts and eats other organisms.

Prenyl group a long unsaturated fatty acyl chain which allows proteins to be attached to the plasma membrane or to intracellular membranes

Prenylation a post-translational lipid modification which can add an isoprenoid group, or prenyl group, to proteins, and attach them to both the plasma membrane and intracellular membranes

Prey organism hunted and eaten by a predator.

Primary lymphoid organs the sites of B lymphocyte and T lymphocyte generation and maturation.

Producer any organism which brings energy into an ecosystem from inorganic sources.

Progesterone a progestational steroid hormone which inhibits ovulation during pregnancy, induces differentiation and
maturation of ovocyte, stimulates mammary glands and maintains an optimal intrauterine environment for sustaining

Prokaryotes the simplest organisms found on our earth constituting of a single small cell with no membrane-bounded nucleus

Prokaryotic cells that have no internal membranes or cytoskeleton.

Prolactin a hormone secreted by anterior pituitary gland.

Prostele When a plant's vascular tissue develops in a solid central bundle

Prosthetic group the non-polypeptidic constituent of a heteroprotein which is essential for its biological activity.

Protein a polypeptide constructed from amino acids.

Proteinaceous describes any structure which is composed of protein.

Prothrombin a protein which is one of the blood coagulation factors, and is secreted by liver in the presence of vitamin K.

Protist An organism that belongs to the Kingdom Protista, which includes forms with both plant and animal affinities

Protoplasm All the contents of a cell, including the nucleus.

Purine bases aromatic nitrogen-containing compounds, which are not naturally found free and have urate as the end
product of their degradation.


Q10 the increase in the rate of a process when the temperature is increased by 10 degrees celsius.

Quadrat a square frame used to mark out an area in which organisms can be investigated

Quatenary structure the type of structure shown by a protein made up from more than one polepeptide chain

Quinones hydrophobic molecules which are able to transport electrons and are conjugated aromatic diketones, derived from benzene.


Radial symmetry describes an organism that can be divided in any longitudinal plane to give two approximately identical halves.

Radicle the part of plant embryo that develops into a root

Receptor a cell or group of cells speacialised to detect a particular stimulus

Recessive allele any allele that is only exposed in the phenotype of an organism if the dominant allele is not present

Receptors proteins that can bind to other specific molecules eg to antibodies or hormones.

Recombinant an individual or a cell which is the product of a genetic recombination of alleles not present in the parents

Red blood cell a cell speacialised for transporting respiratory gases in blood

Reducing sugar any carbohydrate which has a reducing properties

Reduction the addition of an electron to a molecule

Reductase an enzyme of the oxidoreductase group which catalyzes the reduction of a given substrate.

Reflex a rapid automatic response to a particular stimulus

Refractory period the time interval during which a nerve cell is incapable of responding to a stimulus

Renin an enzyme secreted by the kidney in response to a fall in blood pressure

Reproductive isolation the prevention of interbreeding between members of different species that inhabit the same geographical area.

Resistance the ability of an organism to tolerate substances which would normally be toxic to them

Respiration the oxidation of organic molecules by cells in order to release energy

Respiratory quotient (RQ) the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration

Resting potential the difference in electrical charges across the cell surface membrane of a nerve cell.

Restriction endonuclease an enzyme that cuts DNA molecules at specific sites

Restriction enzymes endonucleases present in most bacteria whose ability is to restrict the possibilities for a foreign DNA to take over the transcription and translation machinery of the bacterium.

Reproduction the manufacture of offspring as part of an organism's life cycle.

Resting ion channels ion channels which are permanently open

Retina a layer of light-sensitive cells found at the back of the eye.

Retinal a photosensitive compound derived from beta-carotene.

Retinoate an oxidized derivative of retinol is formed by carboxylation of the aldehyde group of retinal.

Retrovirus a type of virus that contains RNA as its nucleic acid

Rhizobium a genus of nitrogen-fixing bacteria which form a mutualistic relationship with leguminous plants.

Rhizoid a cellular outgrowth of a plant that usually aids in anchoring to the surface and increasing surface area to acquire water or nutrients

Rhizome a horizontal underground stem, such as found in many ferns, where only the leaves may stick up into the air

Ribonucleases globular proteins which are mainly composed of a helices and b sheets.

Ribonucleoproteins conjugated proteins consisting in a non covalent association between a ribonucleic acid and a basic protein (RNP protein)

Ribonucleoside a compound built of a nitrogenous base in N-glycosidic linkage with a sugar

Ribonucleotide a compound built of a ribonucleoside in linkage with one, two or three phosphate groups

Ribose a five-carbon sugar found in RNA

Ribosomal RNA an RNA molecule which is an integral component of a ribosome.

Ribosome small structures present in all prokaryotic and eukaryotic cells which contains large complexes of RNA and protein molecules, about 20 to 30 nm in diameter.

Ribosome receptor a specific receptor which is able to anchor ribosomes to the cytosolic face of the endoplasmic reticulum (ER) membrane.

Ribulose biphosphate (RuBP) a five-carbon sugar which combines with carbon dioxide during the light-independent reaction of photosynthesis.

RNA(ribonucleic acid) a type of nucleic acid molecule

RNA polymerase an enzyme that catalyses the synthesis of RNA

Rod cell a light sensitive cell found in the retina of the eye

Rough endoplasmic reticulum the region of endoplasmic reticulum (ER) whose outer surface bears ribosomes.

Root the part of the plant that goes downwards, normally into the ground, anchoring the plant and absorbing and conducting water and mineral nutrients.

Root pressure the force exerted by roots which pushes water into the xylem and up the plant

Ruminant any mammal that posseses a rumen (a chamber found at the interior of the gut)


Salinity a measure of the salt concentration of water.

Saliva a watery fluid secreted by the salivary glands in the mouth

Saltatory conduction the mechanism by which an impulse is transmitted along a myelinated nerve cell.

Sampling the selection of a number of items or individuals from a population

Saprophytes any organism that obtains its nutrients from dead or decaying matter by secreting digestive enzymes onto the food and then absorbing the soluble products of digestion

Sarcolemma the membrane that surrounds a muscle fibre

Saturated fatty acid a fatty acid which has the maximum number of hydrogen atoms attached to its carbon

Scavenger an organism that feeds upon dead and dying organisms.

Schwann cell a cell that produces the mylin sheath around a nerve cell

Sclerenchyma a support tissue in plants

Sclerotin an insoluble tanned protein permeating and stiffening the chitin of the cuticle of arthropods

Second messenger a molecule found within a cell which is activated by the binding of a chemical messenger to a receptor on the cell surface membrane

Secondary structure the way in which a polypeptide is coiled or folded

Sediment any solid material that has settled out of a state of suspension in liquid.

Seed a structure produced by seed plants which encapsulates the embryo.

Seed bank a collection of seeds which is used as a store of genetic material

Selection process which favors one feature of organisms in a population over another feature found in the population.

Selective breeding the breeding of plants or animals in order to select certain favourable characteristics within the population

Semi-conservative replication the mechanism by which DNA produces identical copies of itself

Semilunar valve a valve that prevents the backflow of blood

Sepal the outermost structures of a flower that makes up the calyx.

Septum partition which divides up a larger region into smaller ones

Serum blood plasma from which the fibrinogen and other clotting factors have been removed.

Sex chromosomes a pair of chromosomes which determine the sex of an organism

Sex determination the mechanism by which sex is determined in a species

Sex linkage the inheritance of a characteristic which is determined by a gene on one of the sex chromosomes.

Sexual reproduction the fusion of male and female gametes to produce a zygote

Shoot usually, the above ground portion of a plant, bearing the leaves.

Single circulation a type of circulatory system in which the blood passes only once through the heart in each complete circuit of the body

Sink part of an organism where substances are removed from a mass transport system

Sino-atrial node (SAN) a collection of specialized muscle cells in the right atrium of the heart

Sinusoid a blood-filled space in the liver

Skeletal muscle muscle tissue that is found attached to the skeleton

Skeleton support structure in animals, against which the force of muscles acts.

Sliding-filament hypothesis a hypothesis to account for the mechanism of contraction in skeletal muscle

Smooth muscle muscle tissue that is not under conscious control

Solute potential the component of water potential that is due to the presence of solute molecules

Source part of organism where substances enter a mass transport system

Speciation the formation of a new species of organisms

Species an important classificatory category, which can be variously defined by the biological species concept, cladistic species concept, ecological species concept, phenetic species concept, and recognition species concept.

Sperm a gamete produced by male animals

Spermatogenesis the production of sperms by the testis

Sphincter muscle a specialized muscle surrounding an opening within the body

Spinal cord part of the central nervous system that is enclosed by the backbone

Spiracles pores in the exoskeleton of an insect which lead to the trachea

Sporangium a chamber inside of which spores are produced through meiosis.

Spore a single cell that is dispersed as a reproductive unit, or that encapsulates a cell during unfavorable environmental conditions

Sporophyte the stage in the life cycle of a plant in which spores are produced.

Stabilising selection a type of natural selection in which the environment acts against forms at the extreme of the range of phenotypic ratio

Stamen part of a flower, the tip of which produces pollen

Standard deviation a measure of the spread of data about the mean

Starch a complex polymer of glucose, used by plants and green algae to store surplus sugar for later use.

Steroid a type of lipid molecule with a number of biological functions

Stigma the sticky tip of a pistil or the dense region of pigments found in many photosynthetic protists which is sensitive to light

Stimulus a change in the internal or external environment which produces a response in an organism

Stomach part of the digestive system, located between the oesophagus and the duodenum

Stomata openings in the epidermis of a stem or leaf of a plant which permit gas exchange with the air.

Style the narrow stalk of the pistil, located above the ovary but below the stigma.

Suberin a waxy substance consisting of a long chains of fatty acids

Substrate the substance acted upon by an enzyme in biochemical reactions

Succession the progressive change which occurs in a community of organisms over a period of time

Sucrose a disaccharide found in plants

Summation a process which occurs in synapses due to the addictive effect of a number of stimuli

Supernatant the liquid layer which is left on top after a suspension has been centrifuged

Surface area to volume ratio the ratio between the surface area of an organism to its volume

Symbiosis a close interaction between individuals at different species

Sympathetic nervous system the part of the autonomic nervous system which regulates physiological functions when the
body is active

Sympatric speciation the development of one or more species which occurs when populations of the parent species living in the same area are prevented from interbreeding

Symplastic pathway the route by which water and solutes travel through the cytoplasm and plasmodesmata of plant cells

Synapse the junction between two nerve cells

Synergistic effect two substances whose combined action produces a greater effect than would be expected from adding the individual effects of each substance

Syngamy the process of union of two gametes; sometimes called fertilization

Synovial joint a moveable joint between two bones


Taxonomy the study of the classification of living organisms

T-cell one of a group of lymphocytes which play an important role in the immune response of an organism

Tendon a band of connective tissue that attaches skeletal muscle to bone

Tertiary structure the irregular folding of a polypeptide

Testa the protective outer covering of a seed

Test cross a genetic cross used to identify the genotype of an individual

Testis an organ that produces male gametes

Testosterone a sex hormone produced by the testis in males

Termal pollution an increase in the temperature of the environment due to the release heat from industrial processes

Thermoregulation the control of body temperature in animal

Thylakoid one of a collection of flattened membrane-bound sacs found in a chloroplast

Thymine a pyramidine nitrogenous base that is found in DNA

Thyroid-stimulating hormone(TSH) a hormone secreted by the pituitary gland which stimulates the secretion by the hypothalamus which stimulates the secretion of thyroxine

Thyrotrophic-releasing hormone(TRH) a hormone secreted by the hypothalamus which stimulates the secretion of thyroid-stimulating hormone

Thyroxine a hormone secreted by the thyroid gland

Tissue a group of cells with a specific function in the body of an organism.

Tissue fluid the fluid which surrounds the cells in an animal

Trachea(mammal) a tube which links the mouth and nose to the lungs in air-breathing vertebrates

Trachea(insects) tubes which form part of the gas exchange

Tracheid a type of cell found in the xylem tissue of plants

Transamination a biochemical reaction in which an amino group is transferred from one amino acid to form a new amino acid

Transcription the copying of the genetic code from DNA to messenger RNA during protein synthesis

Transect a technique used in ecology to sample the organisms across an area

Transfer RNA a type of RNA which carries amino acids to a ribosome during protein synthesis

Translation the transport of substances from one part of a plant to another

Transpiration the loss of water vapour from a plant

Tricuspid valve the valve between the right atrium and the right ventricle in the heart of a mammal

Triglyceride a lipid molecule consisting of three fatty acids linked to a molecule of glycerol

Trioblastic any animal with a body cmposed of three layers of cells

Trophic level the position occupied by an organism in a food chain

Tropism the directional growth of part of a plant in response to an external stimulus

Trypsin a protein-digesting enzyme produced by pancreas

Tuber an underground stem which has been modified for storage of nutrients, such as a potato.

Turgid cell a cell which contains the maximum amount of water

Turgor pressure force exerted outward on a cell wall by the water contained in the cell.


Ultrafiltration filtration which occurs on a molecular scale

Ultrastructure the structure of cells and organelles as seen using an electron microscope

Unsaturated Fatty Acid a fatty acid which has less than the maximum number of hydrogen atoms attached to its carbon atom

Uracil a pyrimidine nitrogenous base that is found in RNA

Urea the main nitrogenous waste product of mammals

Urine an aqueous solution produced by the kidneys in mammals, that contains urea, inorganic salts and water.

Uterus the organ in which the embryo develops in female mammals


Vacuole a membrane-bound space within the cytoplasm of a cell

Variation the phenotypic differences between individuals within a species

Vascular System a system of vessels for the transportation of substances from one part of an organism to another

Vasoconstriction a decrease in the internal diameter of an article

Vasodilation an increase in the internal diameter of an arteriole

Vector any organism which acts as a carrier

Vein(animal) a vessel that carries blood toward the heart

Vein(plant) part of the vascular system in the leaves of plants

Vena Cava the main vein of the body

Ventilation the mechanism by which respiratory gases are delivered to and from a gas exchange surface

Ventricle a chamber of the heart that pumps blood to the organs of the body

Venule a vessel that receives blood from capillaries and carries it to a vein

Vesicle a small-membrane sac found in the cytoplasm of a cell

Vessel a type of cell found in the xylem tissue of plants

Villus a finger-shaped projection of the lining of the small intestine

Virus a microorganism which can only reproduce inside a living thing

Vitamin an organic molecule required by living things organisms in small amounts in order to maintain normal health


Water Potential a measure of the ability of a solution to give out water

White Blood Cell a nucleated cell found in the blood

Wilting the condition that occurs in plants when more water is lost by transpiration than is absorbed from the soil


x-Chromosome one of the sex hormones

Xerophyte a plant which is adapted for growing in dry environments

Xylem the plant vascular tissue concerned with the transport of water and inorganic salts


Y-Chromosome one of the sex chromosomes


Zygote a diploid cell that results from the fusion of two gametes during sexual reproduction

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