Key Words Flashcards
Abiotic
An ecological factor that makes up part of the non-biological environment of an organism (e.g. temperature, pH)
Acetylcholine
One of a group of chemicals, called neurotransmitters, released by neurones. It diffuses across the gap between adjacent neurones and so passes an impulse from one neurone to the next
Action Potential
Change that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes
Actin
Filamentous protein which is involved in contraction within cells, especially muscle cells
Activation Energy
Energy required to bring about a reaction. The activation energy is lowered by the presence of enzymes
Active Site
A group of amino acids that makes up the region of an enzyme into which the substrate fits in order to catalyse a reaction
Active Immunity
Resistance to disease resulting from the activities of an individual’s own immune system whereby an antigen induces plasma cells to produce antibodies
Active Transport
Movement of a substance from a region where it is in a low concentration to a region where it is in a high concentration. The process requires the expenditure of metabolic energy
Adenosine Triphosphate (ATP)
An activated nucleotide found in all living cells that acts as an energy carrier. The hydrolysis of ATP leads to the formation of ADP and inorganic phosphate, with the release of energy
Adrenaline
A hormone produced by the adrenal glands in times of stress that prepares the body for an emergency
Aerobic
Connected with the presence of free oxygen. Aerobic respiration requires free oxygen to release energy from glucose
Allele
One of a number of alternative forms of a gene. For example, the gene for the shape of pea seeds has two alleles: one for ‘round’ and one for ‘wrinkled’
Allele Frequency
The number of times an allele occurs within the gene pool
Allergen
A normally harmless substance that causes the immune system to produce an immune response
Allergy
The response of the immune system to an allergen
Anaerobic
Connected with the absence of oxygen. Anaerobic respiration releases energy from glucose or other foods without the presence of oxygen
Antibiotic
A substance produced by living organisms that can destroy or inhibit the growth of microorganisms
Antibiotic Resistance
The development in microorganisms of mechanisms that prevent antibodies from killing them
Antibody
A protein produced by lymphocytes in response to the presence of the appropriate antigen
Anticodon
A sequence of three adjacent nucleotides on a molecule of transfer RNA that is complementary to a particular codon on a messenger RNA molecule
Antidiuretic Hormone (ADH)
A hormone produced by the hypothalamus that passes to the posterior pituitary gland from where it is secreted. ADH reduces the volume of water in urine by increasing water reabsorption in the kidneys
Antigen
A molecule that triggers an immune response by lymphocytes
Antioxidant
Chemical which reduces or prevents oxidation. Often used as an additive to prolong the shelf-life of certain foods
Apoplastic Pathway
Route through the cell walls and intercellular spaces of plants by which water and dissolved substances are transported
Artificial Selection
Breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or to eliminate others
Asthma
A chronic illness in which there is resistance to air flow to the alveoli of the lungs as a result of the airways becoming inflamed due to an allergic response to an allergen
Atheroma
Fatty deposits in the walls of arteries often associated with high cholesterol levels in the blood.
Autonomic Nervous System
Part of the nervous system, controlling the muscles and glands, that is not under voluntary control
Autosome
A chromosome which is not a sex chromosome
Axon
A process extending from a neurone that conducts action potential away from the cell body
B Cell (B Lymphocyte)
Type of white blood cell that is produced and matures within the bone marrow. B lymphocytes produce antibodies as part of their role in immunity
Benedict’s Test
A simple biochemical reaction to detect the presence of reducing sugars
Biodiversity
The range and variety of genes, species and habitats within a particular region
Biomass
The total mass of living material, normally measured in a specific area over a given period of time
Biotic
An ecological factor that makes up part of the living environment of an organism (food availability, competition, predation)
Biosensor
A device that uses biological molecules to measure the level of certain chemicals
Biuret Test
A simple biochemical reaction to detect the presence of proteins
Body Mass Index (BMI)
A person’s body mass in kilograms divided by the square of their height in metres
Calvin Cycle
A biochemical pathway that forms part of the light-independent reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrates
Cancer
A disease, resulting from cells that break away from an original tumour to form secondary tumours elsewhere in the body
Carcinogen
A chemical, a form of radiation, or other agent that causes cancer
Cardiac Cycle
A continuous series of events which make up a single heart beat
Cardiac Muscle
Type of muscle found only in the heart. It has fewer striations than skeletal muscle and can contract continuously throughout life without stimulation by nerve impulses
Cardiac Output
The total volume of blood that the heart can pump each minute. It is calculated as the volume of blood pumped at each beat (stroke volume) multiplied by the number of heart beats per minute (heart rate)
Carrier Molecule (carrier protein)
A protein on the surface of a cell that helps to transport molecules and ions across plasma membranes
Casparian Strip
A distinctive band of suberin around the endodermal cells of a plant root that prevents water passing into xylem via the cell walls. The water is forced through the living part of the endodermal cells
Centrifugation
Process of separating out particles of different sizes and densities by spinning them at high speeds in a centrifuge
Cholinesterase
Enzyme that breaks down and therefore inactivates the neurotransmitter, acetylcholine, in the synapse
Cholesterol
Lipid that is an important component of cell-surface membranes. Excess in the blood can lead to atheroma
Chromatid
One of the two strands of a chromosome that are joined together by a single centromere prior to cell division
Chromosome
A thread-like structure made of protein and DNA by which hereditary information is physically passed from one generation to the next
Climax Community
The organisms that make up the final stage of ecological succession
Clone
A group of genetically identical cells or organisms formed from a single parent as the result of asexual reproduction or by artificial means
Co-dominance
Condition in which both alleles for one gene in a heterozygous organism contribute to the phenotype
Codon
A sequence of three adjacent nucleotides in mRNA that codes for one amino acid
Cohesion
Attraction between molecules of the same type. It is important in the movement of water up a plant
Collagen
Fibrous protein that is the main constituent of connective tissues such as tendons, cartilage and bone
Community
All the living organisms present in an ecosystem at a given time
Complementary DNA
DNA that is made from mRNA in a process that is the reverse of normal transcription
Condensation
Chemical process in which two molecules combine to form a more complex one with the elimination of a simple substance, usually water. Many biological polymers, such as polysaccharides and polypeptides, are formed by condensation
Conservation
Method of maintaining ecosystems and the living organisms that occupy them. It requires planning and organisation to make best use of resources while preserving the natural landscape and wildlife
Consumer
Any organism that obtains energy by ‘eating’ another. Organisms feeding on plants are known as primary consumers and organisms feeding on primary consumers are known as secondary consumers
Continuous Variation
Variation in which organisms do not fall into distinct categories but show gradations from one extreme to the other
Coronary Arteries
Arteries that supply blood to the cardiac muscle of the heart
Coronary Heart Disease (CHD)
Any condition, for example atheroma and thrombosis, affecting the coronary arteries that supply heart muscle
Correlation
When a change in one variable is reflected by a change in the second variable
Co-transport
The transport of one substance coupled with the transport of another substance across a plasma membrane in the same direction through the same protein carrier
Countercurrent System
A mechanism by which the efficiency of exchange between two substances is increased by having them flowing in opposite directions
Covalent Bond
Type of chemical bond in which two atoms share a pair of electrons, one from each atom
Crossing Over
The process whereby a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that their alleles are exchanged
Cuticle
Exposed non-cellular outer layer of certain animals and the leaves of plants. It is waxy and impermeable to water. It therefore helps to reduce water loss
Deciduous
Term applied to plants that shed all their leaves together at one season
Denaturation
Permanent changes due to the unravelling of the three-dimensional structure of a protein as a result of factors such as changes in temperature or pH
Dendrite
A process, usually branched, extending from the cell body of a neurone, which conducts impulses towards the cell body
Denitrifying Bacteria
Bacteria that converts nitrates to nitrogen gas as part of the nitrogen cycle
Depolarisation
Temporary reversal of charges on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted
Diabetes
A metabolic disorder in which the body is unable to regulate the level of blood glucose. There are two forms of the disorder - Type I and Type II diabetes
Diastole
The stage in the cardiac cycle when the heart muscle relaxes
Differentiation
The process by which cells become specialised for different functions
Diffusion
The movement of molecules or ions from a region where they are in high concentration to one where their concentration is lower
Diploid
A term applied to cells in which the nucleus contains two sets of chromosomes
Dipolar
Having a pair of equal and opposite electrical charges
Directional Selection
Selection that operates towards one extreme in a range of variation
Discontinuous Variation
Variation shown when the characteristics of organisms fall into distinct categories (e.g. blood groups in humans)
DNA Helicase
Enzyme that acts on a specific region of the DNA molecule to break the hydrogen bonds between the bases causing the two strands to separate and expose the nucleotide bases in that region
DNA Replication
The process in which the double helix of a DNA molecule unwinds and each strand acts as a template on which a new strand is constructed
Dominant Allele
A term applied to an allele that is always expressed in the phenotype of an organism
Ecological Niche
Describes how an organism fits into its environment. It describes what a species is like, where it occurs, how it behaves, its interactions with other species and how it responds to its environment
Ecosystem
All the living and non-living components of a particular area
Ectothermic
An animal that uses the environment to regulate its body temperature
Effector
An organ that responds to stimulation by a nerve impulse resulting in a change or response
Electron
Negatively charge subatomic particle that orbits the positively charge nucleus of all atoms
Electron Carrier Molecules
A chain of carrier molecules along which electrons pass, releasing energy in the form of ATP as they do so
Emphysema
A disease in which the walls of the alveoli break down, reducing the surface area for gaseous exchange, thereby causing breathlessness in the patient
Endocytosis
The inward transport of large molecules through the cell-surface membrane
Endotherm
An animal maintaining its body temperature by physiological mechanisms
Enzyme
A protein or RNA that acts as a catalyst and so alters the speed of a biochemical reaction
Epidemiology
The study of the spread of disease and the factors that affect this spread
Eukaryotic Cell
A cell that has a membrane-bound nucleus and chromosomes. The cell also possesses a variety of other membranous organelles, such as mitochondria and endoplasmic reticulum
Exocytosis
The outward bulk transport of materials through the cell-surface membrane
Facilitated Diffusion
Diffusion involving the presence of protein carrier molecules to allow the passive movement of substances across plasma membranes
Gamete
Reproductive (sex) cell that fuses with another gamete during fertilisation
Gel Electrophoresis
A technique used to separate DNA fragments of different lengths by placing them on agar gel and passing a voltage across them
Gene
Section of DNA on a chromosome coding for one or more polypeptides
Gene Pool
The total number of alleles in a particular population at a specific time
Gene Marker
A section of DNA that is used to indicate the location of a gene or other section of DNA
Gene Mutation
A change to one or more nucleotide bases in DNA resulting in a change in genotype which may be inherited
Gene Therapy
A mechanism by which genetic diseases (e.g. cystic fibrosis) may be cured by masking the effect of the defective gene by inserting a functional gene
Generator Potential
Depolarisation of the membrane of a receptor cells as a result of the stimulus
Genetic Engineering
The process by which genes are manipulated, altered or transferred from organism to organism. Also know as recombinant DNA technology
Genetically Modified Organism (GMO)
An organism that has had its DNA altered as a result of recombinant DNA technology/ genetic engineering
Genotype
The genetic composition of an organism
Glomerulus
A cluster of blood capillaries enclosed by the renal (Bowman’s capsule) in the kidney
Glucagon
A hormone produced by alpha-cells of the islets of Langerhans in the pancreas that increases blood glucose levels by initiating the breakdown of glycogen to glucose
Gluconeogenesis
The conversion of non-carbohydrate molecules to glucose
Glycogenesis
The conversion of glucose to glycogen
Glycogenolysis
The conversion of glycogen to glucose
Glycolysis
First part of cellular respiration in which glucose is broken down anaerobically in the cytoplasm to two molecules of pyruvate
Glycoprotein
Substance made up of a carbohydrate molecule and a protein molecule. Parts of the cell-surface membrane and certain hormones are glycoproteins
Granum (plural grana)
A stack of thylakoids in a chloroplast that resembles a pile of coins. This is the site of the light-dependent reaction of photosynthesis
Guard Cell
One of a pair of cells that surround a stoma in plant leaves and control its opening and closing
Habitat
The place where an organism normally lives and which is characterised by physical conditions and the types of other organisms present
Haemoglobin
Globular protein in blood that readily combines with oxygen to transport it around the body. It comprises four polypeptide chains around an iron-containing haem group
Haploid
Term referring to cells that contain only a single copy of each chromosome
Heterozygous
Condition in which the alleles of a particular gene are different
Histones
Proteins which together with DNA, make up the chromosomes of eukaryotic cells
Homeostasis
The maintenance of more or less constant internal environment
Homologous Chromosomes
A pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features. They are not necessarily identical, however, as individual alleles of the same gene many vary. They are capable of pairing during meiosis
Homozygous
Condition in which the alleles of a particular gene are identical
Human Genome
The totality of the DNA sequences on the chromosomes of a single human cell
Human Genome Project
International scientific project to map the entire sequence of all the base pairs of the genes in a single human cell
Hydrogen Bond
Chemical bond formed between the positive charge on a hydrogen atom and the negative charge on another atom of an adjacent molecule
Hydrolysis
The breaking down of large molecules into smaller ones by the addition of water molecules
Hyperthermia
A condition that results from the core body temperature rising above normal
Hypothalamus
Region of the brain adjoining the pituitary gland that acts as the control centre for the autonomic nervous system and regulates the body temperature and fluid balance
Hypothermia
A condition that results from the core body temperature falling below normal
Immunity
The means by which the body protects itself from infection
Insulin
A hormone, produced by the beta-cells of the islets of Langerhans in the pancreas, which decreases blood glucose levels by, amongst other things, increasing the rate of conversion of glucose to glycogen
Intercropping
The practice of growing two or more crops in close proximity usually to produce a greater yield on a piece of land
Interspecific Competition
Competition between organisms of different species
Interspecific Variation
Differences between organisms of different species
Intraspecific Competition
Competition between organisms of the same species
Intraspecific Variation
Differences between organisms of the same species
Intrinsic Proteins
Proteins of the cell-surface membrane that completely span the phospholipid bilayer from one side to the other
Introns
Portions of DNA within a gene that do not code for a polypeptide. The introns are removed from pre-messenger RNA after transcription
Ion
An atom or group of atoms that has lost or gained one or more electrons. Ions therefore have either a positive or negative charge
Ion Channel
A passage across the cell-surface membrane made up of a protein that spans the membrane and opens and closes to allow ions to pass in and out of the cell
Islets of Langerhans
Groups of cells in the pancreas comprising large alpha-cells, which produce the hormone glucagon, and small beta-cells, which produce the hormone insulin
Isotope
Variation of a chemical element that have the same number of protons and electrons but a different number of neutrons. While their chemical properties are similar they differ in mass
In Vitro
Refers to experiments carried out outside the living body (e.g. in test tubes)
In Vivo
Refers to experiments that are carried out within living bodies
Krebs Cycle
Series of aerobic biochemical reactions in the matrix of the mitochondria of most eukaryotic cells by which energy is obtained through the oxidation of acetylcoenzyme A produced from the breakdown of glucose
Latent Heat of Vaporisation
Heat taken in by a liquid in order to transform it into a vapour
Ligament
A tough, fibrous connective tissue, rich in collagen, that joins bone to bone
Light-dependent Reaction
Stage of 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 directly but does need the products of the light-dependent reaction to reduce carbon dioxide and so form carbohydrate
Limiting Factor
A variable that limits the rate of a chemical reaction
Link Reaction
The process linking glycolysis with the Krebs cycle in which hydrogen and carbon dioxide are removed from pyruvate to form acetylcoenzyme A in the matrix of the mitochondria
Locus
The position of a gene on a chromosome/DNA molecule
Loop of Henle
The portion of the nephron that forms a hairpin loop that extends into the medulla of the kidney. It has a role in the reabsorption of water
Lumen
The hollow cavity inside a tubular structure such as the gut of a xylem vessel
Lymph
A slightly milky fluid found in lymph vessels and made up of tissue fluid, fats and lymphocytes
Lymphocytes
Types of white blood cells responsible for the immune response. They become activated in the presence of antigens. There are two types: B and T
Meiosis
The type of nuclear division in which the number of chromosomes is halved
Mesophyll
Tissue found between the two layers of epidermis in a plant leaf comprising an upper layer of palisade cells and a lower layer of spongy cells
Metabolism
All the chemical processes that take place in living organisms
Microvilli
Tiny finger-like projections from the cell-surface membrane of some animal cells
Middle Lamella
Layer made up of pectins and other substances found between the walls of adjacent plant cells
Mitosis
The type of nuclear division in which the daughter cells have the same number of chromosomes as the parent cell
Monoclonal Antibody
An antibody produced by a single clone of cells
Monomer
One of many small molecules that combing to form a larger one know as a polymer
Mono-unsaturated fatty acid
Fatty acid that possesses a carbon chain with a single double bond
Motor Neurone
Neurone that transmits action potential from the central nervous system to an effector (e.g. muscle or gland)
Multiple Alleles
Term used to describe a gene that has more than two possible alleles
Mutagen
Any agent that induces a mutation
Mutation
A sudden change in the amount or the arrangement of the genetic material in the cell
Myelin
A fatty substance that surrounds axons and dendrites in certain neurones
Myocardial Infarction
Otherwise known as a heart attack, results from the interruption of the blood supply to the heart muscle, causing damage to an area of the heart with consequent disruption to its function
Myosin
The thick filamentous protein found in skeletal muscle
NAD (Nicotinamide Adenine Dinucleotide)
A molecule that carries electrons and hydrogen ions during aerobic respiration
NADP (Nicotinamide Adenine Dinucleotide Phosphate)
A molecule that carries electrons produced in the light-dependent reaction of photosynthesis
Negative Feedback
A series of changes, important in homeostasis, that result in a substance being restored to its normal level
Nephron
Basic functional unit of the mammalian kidney responsible for the formation of urine
Neurone
A nerve cell, comprising a cell body, axon and dendrites, which is adapted to conduct action potential
Neuromuscular Junction
A synapse that occurs between a neurone and a muscle
Neurotransmitter
One of a number of chemicals that are involved in communication between adjacent neurones or between nerve cells and muscles. Two important examples are acetylcholine and noradrenaline
Nitrifying Bacteria
Microorganisms that convert ammonium compounds to nitrites and nitrates
Nitrogen Fixation
The incorporation of atmospheric nitrogen gas into organic nitrogen-containing compounds
Node of Ranvier
A gap in the myelin sheath that surrounds the axon of a neurone
Normal Distribution
A bell-shaped curve produced when a certain distribution is plotted on a graph
Nucleotides
Complex chemicals made up of an organic base, a sugar and a phosphate. They are the basic units of which the nucleic acids DNA and RNA are made
Oestrus
The period in the oestrous cycle immediately after ovulation when the female is the most fertile
Oncogenes
Mutated versions of proto-oncogenes that result in increased cell division leading to the growth of a tumour
Oral Rehydration Solution (ORS)
Means of treating dehydration involving giving, by mouth, a balanced solution of salts and glucose that stimulates the gut to reabsorb water
Osmosis
The passage of water from a region of high water potential to a region where its water potential is lower, through a selectively permeable membrane
Oxidation
Chemical reaction involving the loss of electrons
Oxidation-Reduction (Redox)
A chemical reaction in which electrons are transferred from one substance to another substance. The substance losing electrons is oxidised and the substance gaining the electrons is reduced
Oxidative Phosphorylation
The formation of ATP in the electron transport system of aerobic respiration
Palisade Cells
Long, narrow cells, packed with chloroplasts, that are found in the upper region of a leaf and which carry out photosynthesis
Parasite
An organism that lives on or in a host organism. It gains a nutritional advantage and the host is harmed in some way
Passive Immunity
Resistance to disease that is acquired from the introduction of antibodies from another individual, rather than an individual’s own immune system. It is usually short-lived
Pathogen
Any microorganism that causes disease
Pentose Sugar
A sugar that possesses five carbon atoms. Two examples are ribose and deoxyribose
Peptide Bond
The chemical bond formed between two amino acids during condensation
Phagocytosis
Mechanism by which cells engulf particles to form a vesicle or a vacuole
Phenotype
The characteristics of an organism, often visible, resulting from both its genotype and the effects of the environment
Phloem
Plant tissue that transports the products of photosynthesis from leaves to the rest of the plant
Phospholipid
Triglyceride in which one of the three fatty acid molecules is replaced by a phosphate molecule. They are important in the structure and functioning of plasma membranes
Photolysis
Splitting of a water molecule by light such as occurs during the light-dependent reaction of photosynthesis
Photomicrograph
Photograph of an image produced by a microscope
Pioneer Species
A species that can colonise bare rock or ground
Plasmid
A small circular piece of DNA found in bacterial cells
Plasmodesmata
Fine strands of cytoplasm that extend through pores in adjacent plant cell walls and connect the cytoplasm on one cell with another
Plasmolysis
The shrinkage of cytoplasm away from the cell wall that occurs as a plant cell loses water by osmosis
Polygenes
Group of genes that are responsible for controlling a characteristic
Polymer
Large molecule made up of repeating smaller molecules (monomers)
Polymerase Chain Reaction (PCR)
Process of making many copies of a specific sequence of DNA or part of a gene. It is used extensively in gene technology and genetic fingerprinting
Polymerases
Group of enzymes that catalyse the formation of long-chain molecules (polymers) from similar basic units (monomers)
Polyunsaturated Fatty Acid (PUFA)
Fatty acid that possesses carbon chains with many double bonds
Population
A group of individuals of the same species that occupy the same habitat at the same time
Positive Feedback
Process which results in a substance that departs from its normal level becoming further from its norm
Primary Structure of a Protein
The sequence of amino acids that makes up the polypeptides of a protein
Primary Succession
The progressive colonisation of bare rock or other barren terrain by living organisms
Producer
An organism that synthesises organic molecules from simple inorganic ones such as carbon dioxide and water. Most are photosynthetic and form the first trophic level in a food chain
Prokaryotic Cell
A cell of an organism belonging to the kingdom Prokaryotae that is characterised by lacking a nucleus and membrane-bound organelles
Proton
Positively charge sub-atomic particle found in the nucleus of an atom
Protoplast
The living portion of a plant cell, that is, the nucleus and cytoplasm along with the organelles it contains
Quaternary Structure of a Protein
A number of polypeptide chains linked together, and sometimes associated with non-protein groups, to form a peptide
Receptor
A cell adapted to detect changes in the environment
Recessive Allele
The condition in which the effect of an allele is apparent in the phenotype of a diploid organism only in the presence of another identical allele
Recognition SIte
A nucleotide sequence, usually of 4,6 and 8 nucleotides, that is recognised by a restriction endonuclease and to which it attaches
Recombinant DNA Technology
General term that covers the processes by which genes are manipulated, altered or transferred from organism to organism. Also known as genetic engineering
Reduction
Chemical process involving the gain of electrons
Reflex Arc
The nerve pathway in the body taken by an action potential that leads to a rapid, involuntary response to a stimulus
Refractory Period
Period during which the membrane of the axon of a neurone cannot be depolarised and no new action potential can be initiated
Renal Capsule
The cup shaped portion of the start of the nephron that encloses the glomerulus
Repolarisation
Return to the resting potential in the axon of a neurone after an action potential
Resting Potential
The difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated
Restriction Endonucleases
A group of enzymes that cut DNA molecules at a specific sequence of bases called a recognition sequence
RNA Polyermase
Enzyme that joins together nucleotides to form mRNA during transcription
Saltatory Conduction
Propagation of a nerve impulse along a myelinated dendron or axon in which the action potential jumps from one node of Ranvier to another
Saprobiotic Microorganism
Also known as a saprophyte, this is an organism that obtains its food from the dead or decaying remains of other organisms
Sacromere
A section of myofibril between two Z-lines that form the basic structural unit of skeletal muscle
Saturated Fatty Acid
A fatty acid in which there are no double bonds between the carbon atoms
Schwann Cell
Cell around a neurone whose cell-surface membrane wraps around the dendron or axon to form the myelin shaft
Secondary Structure of a Protein
The way in which the chain of amino acids of the polypeptides of a protein is folded
Secondary Succession
The recolonisation of an area after an earlier community has been removed or destroyed
Selection
Process that results in the best-adapted individual in a population surviving to breed and so pass their favourable alleles to the next generation
Selection Pressure
The environmental force altering the frequency of alleles in a population
Selective Breeding
Breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or eliminate others
Semi-conservative Replication
The means by which DNA makes exact copies of itself by unwinding the double helix so that each chain acts as a template for the next. The new copies therefore possess one original and one new strand of DNA
Sensory Neurone
A neurone that transmits an action potential from sensory receptor to the central nervous system
Serum
Clear liquid that is left after blood has clotted and the clots has been removed. It is therefore blood plasma without the clotting factors
Sickle-cell Anaemia
Inherited blood disorder in which abnormal haemoglobin leads to read cells becoming sickle-shaped and less able to carry oxygen
Sinoatrial Node (SAN)
An area of heart muscle in the right atrium that controls and coordinates the contraction of the heart. Also known as the pacemaker
Skeletal Muscle
The muscle that makes up the bulk of the body and which works under conscious control. Also known as voluntary muscle
Smooth Muscle
Also known as involuntary or unstriated muscle, is found in the alimentary canal and the walls of blood vessels. Its contraction is not under conscious control
Sodium-potassium Pump
Protein channels across cell-surface membranes that use ATP to move sodium ions out of the cell in exchange for potassium ions that move in
Speciation
The evolution of two or more species from existing species
Species
A group of similar organisms that can breed together to produce fertile offspring
Species Diversity
The number of different species and the number of individuals of each species within any one community
Stabilising Selection
Selection that tends to eliminate the extremes of the phenotype range within a population. It arises when environmental conditions are constant
Stem Cells
Undifferentiated dividing cells that occur in embryos and in adult animal tissues that require constant replacement (e.g. bone marrow)
Stimulus
A detectable alteration in the internal or external environment of an organism that produces some change in that organism
Stoma (plural stomata)
A pore, mostly found in the lower epidermis of a leaf, through which gases diffuse in and out of the leaf
Stroke Volume
The volume of blood pumped at each ventricular contraction of the heart
Stroma
Matrix of a chloroplast where the light-independent reaction of photosynthesis takes place
Substrate
A substance that is acted on or used by another substance or process. In microbiology, the nutrient medium used to grow microorganisms
Substrate-level Phosphorylation
The formation of ATP by the direct transfer of a phosphate group from a reactive intermediate to ADP
Supernatant Liquid
The liquid portion of a mixture left at the top of the tube when suspended particles have been separated out at the bottom during centrifugation
Symplastic Pathway
Route through the cytoplasm and plasmodesmata of plant cells by which water and dissolved substances are transported
Synapse
A junction between neurones in which they do not touch but have a narrow gap, the synaptic cleft, across which a neurotransmitter can pass
Systole
The stage in the cardiac cycle in which the heart muscle contracts. It occurs in two stages; atrial systole when the atria contract and ventricular systole when ventricles contract
Tendon
Tough, flexible but inelastic connective tissue that joins muscle to bone
Tertiary Structure of a Protein
The folding of a whole polypeptide chain in a precise way, as determined by the amino acid of which it is composed
Threshold Level/Value
The minimum intensity that a stimulus must reach in order to trigger an action potential in a neurone
Thrombosis
Formation of a blood clot within a blood vessel that may lead to a blockage
Thylakoid
Series of flattened membranous sacs in a chloroplast that contain chlorophyll and the associated molecules for the light-dependent reaction of photosynthesis
Tidal Volume
The volume of air breathed in and out during a single breath when at rest
Tissue
A group of similar cells organised into a structural unit that serves a particular function
Tissue Fluid
Fluid that surrounds the cells of the body. Its composition is similar to that of blood plasma except that it lacks proteins. It supplies nutrients to the cells and removes waste products
T Cell (T Lymphocyte)
Type of white blood cell that is produced in the bone marrow but matures in the thymus gland. They coordinate the immune response and kill infected cells
Transcription
Formation of mRNA molecules from the DNA that makes up a particular gene. It is the first stage of protein synthesis
Transducer Cells
Cells that convert a non-electrical signal, such as light or sound, into an electrical (nervous) signal and vice versa
Transduction
The process by which one form of energy is converted into another. In microbiology, the natural process by which genetic material is transferred between one host cell and another by a virus
Transpiration
Evaporation of water from a plant
Triglyceride
An individual lipid molecule made up of a glycerol molecule and three fattya acids
Trophic Level
The position of an organisms in a food chain
Tumour
A swelling in an organism that is made up of cells that continue to divide in an abnormal way
Tumour Suppressor Gene
A gene that maintains normal rates of cell division and so prevents the development of tumours
Turgid
A plant cell that contains the maximum volume of water it can. Additional entry of water is prevented by the cell wall stopping further expansion of the cell
Ultrafiltration
Filtration assisted by blood pressure
Unsaturated Fatty Acid
A fatty acid in which there are one or more double bonds between the carbon atoms
Vaccination
The introduction of a vaccine containing appropriate disease antigens into the body, by injection or mouth, in order to influence artificial immunity
Vasoconstriction
Narrowing of the internal diameter of blood vessels
Vasodilation
Widening of the internal diameter of blood vessels
Vector
A carrier. The term may refer to something such as a plasmid, which carriers DNA into a cell, or to an organisms that carries a parasite to its host
Voltage-gated Channel
Protein channel across a cell-surface membrane that opens and closes according to changes in the electrical potential across the membrane
Water Potential
The pressure created by water molecules. It is the measure of the extent to which a solution gives out water. The greater the number of water molecules present, the higher (less negative) the water potential. Pure water has a water potential of zero
Xerophyte
A plant adapted to living in dry conditions
Xylem Vessels
Dead, hollow, elongated tubes, with lignified side walls and no end walls, that transport water in most plants
