AQA Glossary Flashcards
Learn all the key definitions
1
Q
Definition
A
Key word
2
Q
An ecological factor that makes up part of the non-biological environment of an organism
A
Abiotic
3
Q
One of a group of chemicals called neurotransmitters released by neurones. It diffuses across the synapse between adjacent neurones and so passes an impulse from one neurone to another
A
Acetylcholine
4
Q
Change that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes
A
Action potential
5
Q
Filamentous protein which is involved in contraction within cells, especially muscle cells
A
Actin
6
Q
Energy required to bring about a reaction. The _______ ________is lowered by the presence of enzymes
A
Activation energy
7
Q
Resistance to disease resulting from the activities of an individual’s own immune system whereby an antigen induces plasma cells to produce antibodies
A
Active immunity
8
Q
A group of amino acids that makes up the region of an enzyme into which the substrate fits in order to catalyse a reaction
A
Active site
9
Q
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
A
Active transport
10
Q
A hormone produced by the adrenal glands in times of stress that prepared the body for an emergency
A
Adrenaline
11
Q
Connective with the presence of free oxygen. Aerobic respiration requires free oxygen to release energy from glucose
A
Aerobic
12
Q
One form of a gene
A
Allele
13
Q
The number of times an allele occurs within the gene pool
A
Allele frequency
14
Q
A normally harmless substance that causes the immune system to produce an immune response
A
Allergen
15
Q
The response of the immune system to an allergen. E.g. hay fever
A
Allergy
16
Q
Connected with the absence of oxygen. Anaerobic respiration releases energy from glucose or other foods without the presence of oxygen
A
Anaerobic
17
Q
A substance produced by a living organism that can destroy or inhibit the growth of microorganisms
A
Antibiotic
18
Q
The development in microorganisms of mechanisms that prevent antibiotics from killing them
A
Antibiotic resistance
19
Q
A protein produced by lymphocytes in response to the presence of the appropriate antigen
A
Antibody
20
Q
A sequence of three adjacent nucleotides on a molecule of transfer RNA that is complementary to a particular codon on a messenger RNA molecule
A
Anticodon
21
Q
A molecule that triggers an immune response by lymphocytes
A
Antigen
22
Q
A chemical which reduces or prevents oxidation. Often used as an additive to prolong the shelf-life of certain foods
A
Antioxidant
23
Q
Route through the cell walls and intercellular spaces of plants by which water and dissolved substances are transported
A
Apoplastic pathway
24
Q
Breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or eliminate others
A
Artificial selection
25
A chronic illness in which there is resistance to air flow to the alveoli of the lungs as a result of the airways becoming inflames due to an allergic response to an allergen
Asthma
26
Fatty deposits in the walls of arteries, often associated with high cholesterol levels in the blood
Atheroma
27
Nucleotide found in all living organisms, which is produced during respiration and is important in the transfer of energy
ATP (adenosine triphosphate)
28
Part of the nervous system, controlling the muscles and glands that are not under voluntary control
Autonomic nervous system
29
A process extending from a neurone that conductions action potentials away from the cell body
Axon
30
A type of white blood cell that is produced and matures within the bone marrow. They produce antibodies as part of their role in immunity
B cell ((B lymphocyte)
31
A simple biochemical reaction to detect the presence of reducing sugars
Benedict’s test
32
The range and variety of genes, species and habitats within a particular region
Biodiversity
33
The total mass of living material, normally measured in a specific area over a given period of time
Biomass
34
A simple biochemical reaction to detect the presence of protein
Biuret test
35
A person’s body mass in kilograms divided by the square of their height in metres
Body mass index (BMI)
36
A biochemical pathway that forms part of the light-independent reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrate
Calvin cycle
37
A disease, resulting from mutations, that leads to uncontrolled cell division and the eventual formation of a group of abnormal cells called a tumour
Cancer
38
A chemical, form of radiation or other agent that causes cancer
Carcinogen
39
A (continuous series of events which make up a single heartbeat
Cardiac cycle
40
A type of muscle found only in the heart, can contract continuously throughout life without stimulation by nerve impulses
Cardiac muscle
41
The total volume of blood that the heart can pump in each minute (stroke volume X heart rate)
Cardiac output
42
A protein on the surface of a cell that helps to transport molecules and ions across plasma membrane
Carrier molecule (carrier protein)
43
A distinctive band of suberin around the endodermal cells of a plant root that prevents water passing into xylem via the cell walls
Casparian strip
44
Process of separating out particles of different sizes and densities by spinning them at a high speed in a centrifuge
Centrifugation
45
Lipid that is an important component of cell-surface membranes
Cholesterol
46
An enzyme that breaks down and therefore inactivates the neurotransmitter acetylcholine in the synapse
Cholinesterase
47
One of the two copies of a chromosome that are joined together by a single centromere prior to cell division
Chromatid
48
A thread-like structure made of protein and DNA by which hereditary information is physically passed from one generation to the next
Chromosome
49
The organisms that make up the final stage of ecological succession
Climax community
50
A group of genetically identical organisms formed from a single parent as a result of asexual reproduction or by artificial means
Clone
51
Condition in which both alleles for one gene in a heterozygous organism contribute to the phenotype
Co-dominance
52
A sequence of three adjacent nucleotides in mRNA that codes for one amino acid
Codon
53
Attraction between molecules of the same type. It is important in the movement of water up a plant
Cohesion
54
Fibrous protein that is the main constituent of connective tissues
Collagen
55
All the living organisms present in an ecosystem at a given time
Community
56
DNA that is made from messenger RNA in a process that is the reverse of normal transcription
Complementary DNA
57
A chemical process in which two molecules combine to form a more complex one with the elimination of a simple substance, usually water
Condensation
58
The transfer of DNA from one cell to another by means of a thin tube between the two
Conjugation
59
Method of maintaining ecosystems and the living organisms that occupy them
Conservation
60
Variation in which organisms do not fall into distinct categories but show gradations from one extreme to the other
Continuous variation
61
Arteries that supply blood to the cardiac muscle of the heart
Coronary arteries
62
Any condition affecting the coronary arteries that supply heart muscle
Coronary heart disease (CHD)
63
When a change in one variable is reflected by a change in the second variable
Correlation
64
A mechanism by which the efficiency of exchange between two substances in increased by having them flowing in opposite directions
Countercurrent system
65
A type of chemical bond in which two atoms share a pair of electrons, one from each atom
Covalent bond
66
The process whereby a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that their alleles are exchanged
Crossing over
67
Exposed non-cellular outer layer of certain animals and the leaves of plants
Cuticle
68
Inherited disease in which the body produces abnormally thick mucus that obstructs breathing passages and prevents secretion of pancreatic enzymes
Cystic fibrosis
69
Term applied to plants that shed all their leaves together at one season
Deciduous
70
Permanent changes due to the unravelling of the three-dimensional structure of a protein as a result of factors such as changes in temperature
Denaturation
71
A process, usually branched, extending from the cell body of a neurone which conducts impulses towards the cell body
Dendrite
72
Bacteria that converts nitrates to nitrogen gas as part of the nitrogen cycle
Denitrifying bacteria
73
Temporary reversal of charges on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted
Depolarisation
74
A metabolic disorder in which the body is unable to regulate the level of blood glucose
Diabetes
75
The stage in the cardiac cycle when the heart muscle relaxes
Diastole
76
Any member of the class of flowering plants called Dicotyledonae
Dicotyledonous plants
77
The process by which cells become specialised for different functions
Differentiation
78
The movement of molecules or ions form a region where there are in high concentration to one where their concentration is lower
Diffusion
79
Cells in which the nucleus contains two sets of chromosomes
Diploid
80
Variation shown when the characteristics of organisms fall into distinct categories
Discontinuous variation
81
An 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 helicase
82
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
DNA replication
83
An allele that is always expressed in the phenotype of an organism
Dominant allele
84
All conditions and resources required for an organism to survive, reproduce and maintain a viable population
Ecological niche
85
More of less self-contained functional unit in ecology made up of all the interacting biotic and abiotic factors in a specific area
Ecosystem
86
An animal that uses the environment to regulate its body temperature
Ectothermic
87
An organ that responds to stimulation by a nerve impulse resulting in a change or response
Effector
88
Negatively charged sub-atomic particle that orbits the positively charged nucleus of all atoms
Electron
89
A chain of carrier molecules along which electrons pass, released energy in the form of ATP as they do so
Electron carrier molecule
90
A disease in which the walls of the alveoli break down, reducing the surface area for gaseous exchange, thereby causing breathlessness
Emphysema
91
An animal maintaining its body temperature by physiological mechanisms
Endotherm
92
A protein or RNA that acts as a catalyst and so alters the speed of a biochemical reaction
Enzyme
93
The study of the spread of disease and the factors that affect this spread
Epidemiology
94
A cell that has a membrane-bound nucleus and chromosomes
Eukaryotic cell
95
Consequence of an increase in nutrients, especially in nitrates and phosphates, in fresh water lakes and rivers, that often leads to a decrease in biodiversity
Eutrophication
96
Diffusion involving the presence of protein carrier molecules to allow the passive movement of substances across plasma membranes
Facilitated diffusion
97
Reproductive cell that fuses with another gamete during fertilisation
Gamete
98
A technique used to separate DNA fragments of different lengths by placing them on an agar gel and passing a voltage across them
Gel electrophoresis
99
A section of DNA on a chromosome coding for one or more polypeptide
Gene
100
The total number of alleles in a particular population at a specific time
Gene pool
101
A section of DNA that is used to indicate the location of a gene or other section of DNA
Gene marker
102
A mechanism by which genetic diseases may be cured by masking the effect of the defective gene by inserting a functional gene
Gene therapy
103
Depolarisation of the membrane of a receptor cell as a result of a stimulus
Generator potential
104
Organism that has had its DNA altered as a result of recombinant DNA technology
Genetically modified organism (GMO)
105
The genetic composition of an organism
Genotype
106
A hormone produced by α cells at the islets of Langerhans in the pancreas that increases blood glucose levels by initiating the breakdown of glycogen to glucose
Glucagon
107
The conversion of non-carbohydrate molecules into glucose
Gluconeogenesis
108
The conversion of glucose to glycogen
Glycogenesis
109
First part of cellular respiration in which glucose is broken down anaerobically in the cytoplasm to two molecules of pyruvate
Glyconeolysis
110
Substance made up of a carbohydrate molecule and a protein molecule
Glycoprotein
111
A stack of thylakoids in a chloroplast that resembles a pile of coins, this is the site of the light-dependent reaction of photosynthesis
Granum
112
Gases such as methane and carbon dioxide which in the atmosphere cause more heat energy to be trapped, so raising the temperature at the Earth’s surface
Greenhouse gas
113
One of a pair of cells that surround a stoma in plant leaves and controls its opening and closing
Guard cell
114
The place where an organism normally lives which is characterised by physical conditions and the types of other organisms present
Habitat
115
Globular protein in blood that readily combines with oxygen to transport it around the body
Haemoglobin
116
Cells that only contain a single copy of each chromosome
Haploid
117
Condition in which the alleles of a particular gene are different
Heterozygous
118
An inactive, dormant state, accompanied by a very low body temperature that certain animals go into during periods of prolonged cold
Hibernation
119
A compound of protein and lipid molecules found in blood plasma, it transports cholesterol from other cells to the liver
High-density lipoprotein (HDL)
120
Substance released on tissue injury that causes dilation of blood vessels
Histamine
121
The maintenance of a more or less constant internal environment
Homeostasis
122
A pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features.
Homologous chromosomes
123
Condition in which the alleles of a particular gene are identical
Homozygous
124
The totality of the DNA sequences on the chromosomes of a single human cell
Human genome
125
International scientific project to map the entire sequence of all the base pairs of the genes in a single human cell
Human genome project
126
Chemical bond formed between the positive charge on a hydrogen atom and the negative charge on another atom of an adjacent molecule
Hydrogen bond
127
The breaking down of large molecules into smaller ones by the addition of water molecules
Hydrolysis
128
A condition that results from the core body temperature rising above normal
Hyperthermia
129
Region of the brain adjoining the pituitary gland that acts as the control centre for the autonomic nervous system and regulates body temperature and fluid balance
Hypothalamus
130
A condition that results from the core body temperature falling below normal
Hypothermia
131
The means by which the body protects itself from infection
Immunity
132
A hormone produced by the β cells of the islets of Langerhans in the pancreas which decreases blood glucose levels by increasing the rate of conversion of glucose to glycogen
Insulin
133
Differences between organisms of different species
Interspecific variation
134
Differences between organisms of the same species
Intraspecific variation
135
Proteins of the cell-surface membrane that completely span the phospholipid bilayer from one side to the other
Intrinsic proteins
136
Portions of DNA within a gene that do not code for a polypeptide
Introns
137
An atom or group of atoms that has lost or gained one or more electrons
Ion
138
A passage across a 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
Ion channel
139
Groups of cells in the pancreas comprising large α cells, which produce the hormone glucagon, and small β cells, which produce the hormone insulin
Islets of Langerhans
140
Solutions that possess the same concentration of solutes and therefore have the same water potential
Isotonic
141
Variations of a chemical element that have the same number of protons and electrons but different numbers of neutrons
Isotope
142
Refers to experiments carried out outside the living body, e.g. test tubes
In vitro
143
Refers to experiments that are carried out within living bodies
In vivo
144
Energy that an object possesses due to its motion
Kinetic energy
145
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
Krebs cycle
146
A tough, fibrous connective tissue rich in collagen that joins bone to bone
Ligament
147
Stage of photosynthesis in which light energy is required to produce ATP and reduced NADP
Light-dependent reaction
148
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
Light-independent reaction
149
A variable that limits the rate of a chemical reaction
Limiting factor
150
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
Link reaction
151
The position of a gene on a chromosome/DNA molecule
Locus
152
A compound containing both protein and lipid molecules that occurs in blood plasma and lymph, it carries cholesterol from the liver to other cells in the body
Low-density lipoprotein (LDL)
153
The hollow cavity inside a tubular structure such as the gut
Lumen
154
A slightly milky fluid found in lymph vessels and made up of tissue fluid, fats and lymphocytes
Lymph
155
Types of white blood cell responsible for the immune response, they become activated in the presence of antigens
Lymphocytes
156
The type of nuclear division in which the number of chromosomes is halved
Meiosis
157
Tissue found between the two layers of epidermis in a plant leave comprising an upper layer of palisade cells and a lower layer of spongey cells
Mesophyll
158
All the chemical processes that take place in living organisms
Metabolism
159
Tiny finger-like projections from the cell-surface membrane of some animals
Microvilli
160
Layer made up of pectins and other substances found between the walls of adjacent plant cells
Middle lamella
161
The temporary movement of a population of organisms from one locality to another
Migration
162
The type of nuclear division in which the daughter cells have the same number of chromosomes as the parent cell
Mitosis
163
Term used to describe a large area of land in which only one type of crop is grown
Monoculture
164
One of many small molecules that combine to form a larger one known as a polymer
Monomer
165
Fatty acid that possesses a carbon chain with a single double bond
Mono-unsaturated fatty acid
166
Neurone that transmits action potentials from the central nervous system to an effector
Motor neurone
167
Term used to describe a gene that has more than two possible alleles
Multiple alleles
168
Any agent that induces a mutation
Mutagen
169
A sudden change in the amount or the arrangement of the genetic material in the cell
Mutation
170
A fatty substance that surrounds axons and dendrites in certain neurones
Myelin
171
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
Myocardial infarction
172
The think filamentous protein found in skeletal muscle
Myosin
173
A molecule that carriers electrons and hydrogen ions during aerobic respiration
NAD (nicotinamide adenine dinucleotide)
174
A molecule that carries electrons produced in the light-dependent reaction of photosynthesis
NADP (nicotinamide adenine dinucleotide phosphate
175
A series of changes, important in homeostasis, that results in a substance being restored to its normal level
Negative feedback
176
A nerve cell, comprising a cell body, axon, and dendrites which is adapted to conduct action potentials
Neurone
177
A synapse that occurs between a neurone and a muscle
Neuromuscular junction
178
One of a number of chemicals that are involved in communication between adjacent neurones or between nerve cells and muscles
Neurotransmitter
179
Microorganisms that convert ammonium compounds to nitrites and nitrates
Nitrifying bacteria
180
Incorporation of atmospheric nitrogen gas into organic nitrogen-containing compounds
Nitrogen fixation
181
A gap in the myelin sheath that surrounds the axon of a neurone
Node of Ranvier
182
A bell-shaped curve produced when a certain distribution is plotted on a graph
Normal distribution
183
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
Nucleotides
184
The period in the oestrous cycle immediately after ovulation when the female is most fertile
Oestrus
185
Mutated versions of proto-oncogenes that result in increased cell division leading to the growth of a tumour
Oncogene
186
Means of treating dehydration involving giving, by mouth, a balanced solution of salts and glucose that stimulates the gut to reabsorb water
Oral rehydration solution (ORS)
187
The passage of water from a region of high water potential to a region where its water potential is lower, through a partially permeable membrane
Osmosis
188
Degeneration of the cartilage of the joints, causing pain and stiffness in these joints
Osteoarthritis
189
Chemical reaction involving the loss of electrons
Oxidation
190
A chemical reaction I which electrons are transferred from one substance to another substance. Losing electrons is oxidised, gaining electrons is reduced
Oxidation-reduction
191
The formation of ATP in the electron transport system of aerobic respiration
Oxidative phosphorylation
192
Long, narrow cells packed with chloroplasts that are found in the upper region of a leaf and which carry out photosynthesis
Palisade cells
193
An organism that lives on or in a host organism. It gains a nutritional advantage and the host is harmed in some way
Parasite
194
Resistance to disease that is acquired from the introduction of antibodies from another individual, rather than an individual’s own immune system
Passive immunity
195
Any microorganism that causes disease
Pathogen
196
A sugar that possesses five carbon atoms
Pentose
197
The chemical bond formed between two amino acids during condensation
Peptide bond
198
Mechanism by which cells engulf particles to form a vesicle or a vacuole
Phagocytosis
199
The characteristics of an organism, often visible, resulting from both its genotype and the effects of the environment
Phenotype
200
Plant tissue that transports the products of photosynthesis from leaves to the rest of the plant
Phloem
201
Triglycerides in which one of the three fatty acid molecules is replaced by a phosphate molecule
Phospholipid
202
Slitting of a water molecule by light such as occurs during the light-dependent reaction of photosynthesis
Photolysis
203
Photograph of an image produced by a microscope
Photomicrograph
204
A species that can colonise bare rock or ground
Pioneer species
205
A small circular piece of DNA found in bacterial cells
Plasmid
206
Fine strands of cytoplasm that extend through pores in adjacent plant cell walls and connect with the cytoplasm of one cell with another
Plasmodesmata
207
The shrinkage of cytoplasm away from the cell wall that occurs as a plant cell loses water by osmosis
Plasmolysis
208
Group of genes that are responsible for controlling a characteristic
Polygenes
209
Large molecule made up of repeating smaller molecules
Polymer
210
Group of enzymes that catalyse the formation of long-chain molecules from similar basic units
Polymerase
211
Process of making many copies of a specific sequence of DNA or part of a gene
Polymerase chain reaction (PCR)
212
Fatty acid that possesses carbon chains with many double bonds
Polyunsaturated fatty acid
213
A group of individuals of the same species that occupy the same habitat at the same time
Population
214
Process which results in a substance that departs from its normal level becoming further from its norm
Positive feedback
215
The sequence of amino acids that makes up the polypeptides of a protein
Primary structure of a protein
216
The progressive colonisation of bare rock or other barren terrain by living organisms
Primary succession
217
An organism that synthesises organic molecules from simple inorganic ones such as carbon dioxide and water
Producer
218
A cell that does not have a nucleus or membrane bound organelles
Prokaryotic cell
219
Positively charged sub-atomic particle found in the nucleus of an atom
Proton
220
The living portion of a plant cell (i.e. the nucleus and cytoplasm along with the organelles it contains)
Protoplast
221
A number of polypeptide chains linked together, and sometimes associated with non-protein groups to form a protein
Quaternary structure of a protein
222
A cell adapted to detect changes in the environment
Receptor
223
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
Recessive allele
224
A nucleotide sequences that is recognised by a restriction endonuclease and to which it attaches
Recognition site
225
General term that covers the processes by which genes are manipulated, altered or transferred from organism to organism
Recombinant DNA technology
226
Chemical processes involving the gain of electrons
Reduction
227
The nerve pathway in the body taken by an action potential that leads to a rapid, involuntary response to a stimulus
Reflex arc
228
Period during which the membrane of the axon of a neurone cannot be depolarised and no new action potential can be initiated
Refractory period
229
Return to the resting potential in the axon of a neurone after an action potential
Repolarisation
230
The difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated
Resting potential
231
A group of enzymes that cut DNA molecules at a specific sequence of bases called a recognition sequence
Restriction endonuclease
232
Enzyme that joins together nucleotides to form messenger RNA during transcription
RNA polymerase
233
Propagation of a nerve impulse along a myelinated Dendron or axon in which the action potential jumps from one node of Ranvier to another
Saltatory conduction
234
A fatty acid in which there are no double bonds between the carbon atoms
Saturated fatty acid
235
Also known as a saprophyte, this is an organism that obtains its food from the dead or decaying remains of other organisms
Saprobiotic microorganism
236
A section of myofibril between two Z-lines that forms the basic structural unit of the skeletal muscle
Sarcomere
237
Cell around a neurone whose cell-surface membrane wraps around the Dendron or axon to form the myelin sheath
Schwann cell
238
The way in which the chain of amino acids of the polypeptides of a protein is folded
Secondary structure of a protein
239
The recolonization of an area after an early community has been removed or destroyed
Secondary succession
240
Process that results in the best-adapted individuals in a population surviving to breed and so pass their favourable alleles to the next generation
Selection
241
The environmental force altering the frequency of alleles in a population
Selection pressure
242
Breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or eliminate others
Selective breeding
243
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
Semi-conservative replication
244
A neurone that transmits an action potential from a sensory receptor to the central nervous system
Sensory neurone
245
Clear liquid that is left after blood has clotted and the clot has been removed (blood plasma without clotting factors)
Serum
246
Inherited blood disorder in which abnormal haemoglobin leads to red cells becoming dickle-shaped and less able to carry oxygen
Sickle-cell anaemia
247
An area of heart muscle in the right atrium that controls and coordinates the contraction of the heart (pacemaker)
Sinoatrial node (SAN)
248
The muscle that makes up the bulk of the body and which works under conscious control
Skeletal muscle
249
Found in the alimentary canal and the walls of blood vessels, its contraction is not under conscious control (unstriated muscle)
Smooth muscle
250
Protein channels across cell-surface membranes that use ATP to move sodium ons out of the cell in exchange for potassium ions that move in
Sodium-potassium pump
251
The evolution of two or more species from existing species
Speciation
252
A group of similar organisms that can breed together to produce fertile offspring
Species
253
The number of different species and the number of individuals of each species within any one community
Species diversity
254
Selection that tends to eliminate the extremes of the phenotype range within a population. It arises when environmental conditions are constant
Stabilising selection
255
Undifferentiated dividing cells that occur in embryos and in adult animal tissues that require constant replacement
Stem cell
256
A detectable in the internal or external environment of an organism that produces some change in that organism
Stimulus
257
Pore, mostly in the lower epidermis of a leaf through which gases diffuse in and out of the leaf
Stoma (plural stomata)
258
The volume of blood pumped at each ventricular contraction of the heart
Stroke volume
259
Matrix of a chloroplast where the light-independent reaction of photosynthesis takes place
Stroma
260
A substance that is acted on or used by another substance or process
Substrate
261
The formation of ATP by the direct transfer of a phosphate group from a reactive intermediate to ADP
Substrate-level phosphorylation
262
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
Supernatant liquid
263
Route through the cytoplasm and plasmodesmata of plant cells by which water and dissolved substances are transported
Symplastic pathway
264
A junction between neurones in which they do not touch but have a narrow gap, the synaptic cleft, across which a neurotransmitter can pass
Synapse
265
The stage in the cardiac cycle in which the heart muscle contracts
Systole
266
Tough, flexible but inelastic connective tissue that joins muscles to bone
Tendon
267
The folding of a whole polypeptide chain in a precise way, as determined by the amino acids of which it is composed
Tertiary structure of a protein
268
The minimum intensity that a stimulus must reach in order to trigger an action potential in a neurone
Threshold level/value
269
Formation of a blood clot within a blood vessel that may lead to a blockage
Thrombosis
270
Series of flattened membranous sacs in a chloroplast that contain chlorophyll and the associated molecules needed for the light-dependent reaction of photosynthesis
Thylakoid
271
The volume of air breathed in and out during a single breath when at rest
Tidal volume
272
A group of similar cells organised into a structural unit that serves a particular function
Tissue
273
Fluid that surrounds the cells of the body. Its supplies nutrients to the cells and removes waste products
Tissue fluid
274
A type of white blood cell that is produced in the bone marrow but matures in the thymus gland. Coordinates the immune response and kills infected cells
T cell (T lympphocyte)
275
Formation of messenger RNA molecules from the DNA that makes up a particular gene
Transcription
276
Cells that convert a non-electrical signal such as sight or sound into an electrical (nervous) signal and vice versa
Transducer cells
277
The process by which one form of energy is converted into another
Transduction
278
The transfer of a pathogen from one individual to another
Transmission
279
Evaporation of water from a plant
Transpiration
280
An individual lipid molecule made up of a glycerol molecule and three fatty acids
Triglyceride
281
The position of an organism in a food chain
Trophic level
282
A swelling in an organism that is made up of cells that continue to divide in an abnormal way
Tumour
283
A gene that maintains normal rates of cell division and so prevents the development of tumours
Tumour suppressor gene
284
A plant cell that is full of water, additional entry of water is prevented by the cell wall stopping further expansion of the cell
Turgid
285
Filtration assisted by blood pressure
Ultrafiltration
286
A fatty acid in which there are one or more double bonds between the carbon atoms
Unsaturated fatty acid
287
The introduction of a vaccine containing appropriate disease antigens into the body by injection or mouth in order to induce artificial immunity
Vaccination
288
Narrowing of the internal diameter of blood vessels
Vasoconstriction
289
Widening of the internal diameter of blood vessels
Vasodilation
290
A carrier
Vector
291
Protein channel across a cell-surface membrane that opens and closes according to changes in the electrical potential across the membrane
Voltage-gated channels
292
The pressure created by water molecules. The measure of the extent to which a solution gives out water
Water potential
293
A plant adapted to living in dry conditions
Xerophyte
294
Dead, hollow, elongated tubes with lignified side walls and no end walls, that transport water in most plants
Xylem vessels
