Biology A level definitions

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abiotic
an ecological factor that makes up part of the non-biological environment of an organism. such as temperature, pH, rainfall and humidity
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acetylcholine
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 the next
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action potential
change the occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes
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actin
filamentous protein which is involved in contraction within cells, especially muscle cells
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activation energy
energy required to bring about a reaction. the activation energy is lowered by the presence of enzyme
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active immunity
resistance to disease resulting from the activities of an individuals own immune system whereby an antigen induces plasma cells to produce antibodies
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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
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active transport
movement of a substrate from a region where it is low concentration to a region where it is high concentration. the process requires the expenditure of metabolic energy
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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 adenosine diphosphate (ADP) and inorganic phosphate, with the release of energy
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adrenaline
a hormone produced by the adrenal glands in times of stress that prepares the body for an emergency
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aerobic
connected with the presence of free oxygen. aerobic respiration requires free oxygen to release energy from glucose
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allele
one of a number of alternative forms of a gene. for example the gene for the shape of a pea seeds has two alleles one for round and one for wrinkled
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allele frequency
the number of times an allele occurs within the gene pool
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allergen
a normally harmless substance that causes the immune system to produce an immune response
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allergy
the response of the immune system to an allergen such as hay fever and asthma
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anaerobic
connected with the absence of oxygen. anaerobic respiration releases energy from glucose or other foods without the presence of oxygen
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antibiotic
a substance produced by living organisms that can destroy or inhibit the growth of microorganisms
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antibiotic resistance
the development in microorganisms of mechanisms that prevent antibiotics from killing them
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antibody
a protein produced by the lymphocytes in response to the presence of the appropriate antigen
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anticodon
a sequence of three adjacent nucleotides on a molecule of transfer RNA that is complementary to a particular codon on messenger RNA molecule
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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
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antigen
a molecule that triggers an immune response by lymphocytes
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antioxidant
chemical which reduces or prevents oxidation. often used as an additive to prolong the shelf-life of certain foods
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apoplastic pathway
route through the cell walls and intercellular spaces of plants by which water and dissolved substances are transported
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artificial selection
breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or to eliminate others
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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
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atheroma
fatty deposits in the walls of arteries often associated with high cholesterol levels of the blood
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adenosine triphosphate
nucleotide found in all living organisms, which is produced during respiration and is important in the transfer of energy
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automatic nervous system
part of the nervous system controlling the muscles and glands that is not under voluntary control
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autosome
a chromosome which is not a sex chromosome
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axon
a process extending from a neurone that conducts action potentials away from the cell body
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B cell (b lymphocytes)
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
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bendicts test
a simple biochemical reaction to detect the presence of reducing sugars
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biodiversity
the range and variety of genes, species and habitats within a particular region
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biomass
the total mass of living material, normally measured in a specific area over a given period of time
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biotic
an ecological factor that makes up part of the living environment of an organism such as food availability, competition and predation
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biosensor
a device that uses biological molecules to measure the level of certain chemicals
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biuret test
a simple biochemical reaction to detect the presence of protein
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body mass index (BMI)
a persons body mass in kilograms divided by the square of their height in meters
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calvin cycle
a biochemical pathway that forms part of the light-dependant reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrates
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cancer
a disease resulting from cells that break away from an original tumour to form a secondary tumour elsewhere in the body
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carcinogen
a chemical, a form of radiation, or other agent that causes cancer
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cardiac cycle
a continuous series of events which make up a single heart beat
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cardiac muscle
type of muscle found only in the heart. it has fewer striations that skeletal muscles and can contract continuously throughout life without stimulation by nerve impulses
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cardiac output
the total volume of blood that the heart can pump each minute. it is calculated as the volume of blood pumped each beat multiplied by the number of heart beats per minute
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carrier molecule (carrier protein)
a protein on the surface of a cell that helps transport molecules and ions across plasma membranes
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casparian *****
a distinctive band of Suberin around the endodermal cells of a plant root that prevents water passing into xylem via the cells walls. the water is forced through the living part (protoplast) of the endodermal cells
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centrifugation
process of separating out particles of different sizes and densities by spinning them at a high speed in a centrifuge
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cholinesterase
enzyme that breaks down and therefore inactivates the neurotransmitter, acetylcholine in the synapse
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cholesterol
lipid that is an important component of cell surface membranes. excess in the blood can lead to atheroma
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chromatid
one of the two strands in a chromosome that are joined together by a centromere prior to cell division
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chromatin
the material that makes up chromosomes. it consists of DNA and the protein histone
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chromosome
a thread-like structure made of protein and DNA by which heredity information is physically passed from one generation to the next
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climax community
the organism that make up the final stage of ecological succession
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clone
a group of genetically identical cells or organisms formed for a single parent as the result of asexual reproduction or by artificial means
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co-dominance
condition in which both alleles for one gene in a heterozygous organism contribute to the phenotype
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codon
a sequence of three adjacent nucleotides in mRNA that codes for one amino acid
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cohesion
attraction between molecules of the same type. it is important in the movement of water up a plant
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collagen
fibrous protein that is the main constituent of connective tissues such as tendons, cartilage and bone
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community
all the living organisms present in an ecosystem at a given time
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complementary DNA
DNA that is made from messenger RNA in a process that is the reverse of normal transcription
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condensation
chemical process in which two molecules combine to form a more complex one with the elimination of the simple substance, usually water. many biological polymers, such as polysaccharides and polypeptides, are formed by condensation
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conservation
method of maintaining ecosystems and the living organisms that occupy them. it requires planning and organisation to make the best use of resources while preserving the natural landscape and wildlife
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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
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continuous variation
variation in which organisms do not fall into distinct categories but show gradations from on extreme to the other
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coronary arteries
arteries that supply blood to the cardiac muscle to the heart
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coronary heart disease
any condition affecting the coronary arteries that supply the heart muscle such as atheroma and thrombosis
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correlation
when a change in one variable is reflected by a change in the second variable
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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
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countercurrent system
a mechanism by which the efficiency of exchange between two substances is increased by having them flowing in opposite directions
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covalent bond
type of chemical bond in which two atoms share a pair of electrons, one from each atom
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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
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cuticle
exposed non-cellular over layer of certain animals and the leaves of plants. it is a waxy and impermeable to water. it therefore helps to reduce water loss
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deciduous
term applied to plants that shed all their leaves together at one season
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denaturation
permanent changes due to the unravelling of the third dimensional structure of a protein as a result of factors such as changes in temperature or pH
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dendrite
a process usually branched extending from the cell body of a neurone which conducts impulses towards the cell body
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denitrifying bacteria
bacteria that converts nitrates to nitrogen gas as part of the nitrogen cycle
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depolarisation
temporary reversal of changes on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted
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diabetes
a metabolic disorder in which the body is unable to regulate the level of blood glucose. there are 2 forms of the disorder - type 1 and type 2
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diastole
the stage in the cardiac cycle when the heart muscle relaxes
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differentiation
the process by which cells become specialised for different functions
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diffusion
the movement of molecules or ions from a region where they are in high concentration to one where there is a low concentration
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diploid
a term applied to cells in which the nucleus contains 2 sets of chromosomes
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dipolar
having a pair of equal and opposite electrical charges
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directional selection
selection that operates towards one extreme in a range of variation
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discontinuous variation
variation shown when the characteristics of organisms fall into distinct categories for examples blood groups in humans
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DNA helicase
enzyme that acts on specific region of the DNA molecule to break the hydrogen bonds between the bases causing the 2 stands to separate and expose the nucleotide bases in that region
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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
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dominant allele
a term applied to an allele that is always expressed in the phenotype of an organism
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ecological niche
describes how an organism fits into its environment. describes what a species is like, where it occurs, how it behaves, its interactions with other species and how it responds to its environment
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ecosystem
all the living and nonliving components of a particular area
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ectothermic
an animal that uses the environment to regulate its body temperature
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effector
an organ that responds to stimulation by a nerve impulse resulting in a change or response
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electron
negatively charged subatomic particle that orbits the positively charged nucleus of all atoms
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electron carrier molecules
a chain of carrier molecules along which electrons pass, releasing energy in the form of ATP as they do so
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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
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endocytosis
the inward transport of large molecules through the cell surface membrane
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endotherm
an animal maintaining its body temperature by physiological mechanisms
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enzyme
a protein or RNA that acts as a catalyst and so alters the speed of a biochemical reaction
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epidemiology
the study of he spread of disease and the factors that affect this spread
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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
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exocytosis
the outward bulk transport of materials through the cell-surface membrane
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facilitated diffusion
diffusion involving the presence of protein carrier molecules to allow the passive movement of substances across the plasma membranes
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gamete
reproductive cell that fuses with another gamete during fertilisation
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gel electrophoresis
a technique used to separate DNA fragments of different lengths by placing them on agar gel and passing a voltage across them
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gene
section of DNA on a chromosome coding for one or more polypeptides
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gene pool
the total number of alleles in a particular population at a specific time
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gene maker
a section of DNA that is used to indicate the location of a gene or other section of DNA
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gene mutation
a change to one or more nucleotide bases in DNA resulting in a change in genotype which may be inherited
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gene therapy
a mechanism by which genetic diseases such as cystic fibrosis may be cured by masking the effect of the detective gene by inserting a functional gene
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generator potential
depolarisation of the membrane of a receptor as a result of a stimulus
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genetic engineering
recombinant DNA technology
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genetically modified organism
organism that has had its DNA altered as a result of recombinant DNA technology
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genotype
the genetic composition of an organism
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glomerulus
a cluster of blood capillaries enclosed by the renal (bowmans capsule) in the kidney
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glucagon
a hormone produced by a cells of the islets of langerhans in the pancreas that increases blood glucose levels by inhibiting the breakdown of glycogen to glucose
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gluconeogenesis
the conversion of non carbohydrate molecules to glucose
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glycogenesis
the conversion of glucose to glycogen
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glycogenolysis
the conversion of glycogen to glucose
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glycolysis
first part of cellular respiration in which glucose is broken down anaerobically in the cytoplasm to 2 molecules of pyruvate
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glycoprotein
substance made up of a carbohydrate molecule and a protein molecule. parts of cell surface membrane and certain hormones are glycoproteins
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granum
a stack of thylakoids in a chloroplast that resembles a pile of coins. this is the site of the light-dependant reaction of photosynthesis
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guard cell
one of a pair of cells that surround a stoma in plant leaves and control its opening and closing
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habitat
the place where an organism normally lives and which is characterised by physical conditions and the types of other organisms present
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haemoglobin
globular protein in blood that readily combines with oxygen to transport around the body. it compromises four polypeptide chains around an iron-containing haem group
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haploid
term referring to cells that contain only a single copy of each chromosome such as the sex cells (gametes)
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heterozygous
condition in which the alleles of a particular gene are different
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histones
proteins which together with DNA make up the chromosomes of eukaryotic cells
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homeostasis
the maintenance of a more or less constant internal environment
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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 identical however as individual alleles of the same gene may vary. are capable of pairing during meiosis
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homozygous
condition in which the alleles of a particular gene are identical
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human genome
the totality of the DNA sequences on the chromosome of a single human cell
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human genome project
International scientific project to map the entire sequence of all the base pairs of the genes in a single human cell
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hydrogen bond
chemical bond formed between the positive charge on a hydrogen atom and the negative of another atom on the adjacent molecule
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hydrolysis
the breaking down of large molecules into smaller ones by the addition of water molecules
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hyperthermia
a condition that results from the core body temperature rising above normal
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hypothalamus
region of the brain adjoining the pituitary gland that acts as the control centre for the automatic nervous system and regulates body temperature and fluid balance
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hypothermia
a condition that results from the core body temperature falling below normal
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immunity
the means by which the body protects itself from infection
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insulin
a hormone produced by the a 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
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intercropping
the practice of growing two or more crops inclose proximity usually to produce a greater yield on a piece of land
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interspecific competition
competition between organisms of different species
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interspecific variation
differences between organisms of a different species
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intraspecific competition
competition between organisms of the same species
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intraspecific variation
differences between organisms of the same species
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intrinsic proteins
proteins of the cell-surface membrane that completely span the phospholipid bilayer from one side to another
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introns
portions of DNA within a gene that do not code for a polypeptide. the introns are removed from pre-messenger RNA after transcription
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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
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ion channel
a passage across a cell-surface membrane made up of a protein that spans the membrane and open and closes to allow ions to pass in and out of the cell
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islets of langerhans
groups of cells in the pancreas compromising large a cells which produce the hormone glucagon and small B cells which produce the hormone insulin
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isotope
variations of a chemical element that have the same number of protons and electrons but different numbers of neutrons. while their chemical properties are similar they differ in mass.
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in vitro
refers to experiments carried out outside the living body such as test tubes
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in vivo
refers to experiments that a re carried out within living bodies
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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 acetyl coenzyme A produces from the breakdown of glucose
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latent heat of vaporisation
heat taken in by a liquid in order to transform it into vapour
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ligament
a tough, fibrous connective tissue rich in collagen that joins bone to bone
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light-dependant reaction
stage of photosynthesis in which light energy is required to produce ATP and reduced NADP
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light-independent reaction
stage of photosynthesis which does not require light energy directly but does need the products of the light-dependant reaction to reduce carbon dioxide and so form carbohydrate
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limiting factor
a variable that limits the rate of a chemical reaction
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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
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locus
the position of a gene on a chromosome/DNA molecule
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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
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lumen
the hollow cavity inside a tubular structure such as the gut or a xylem tissue
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lymph
a slightly milky fluid found in lymph vessels and made up of tissue fluid, fats and lymphocytes
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lymphocytes
types of white blood cell responsible for the immune response. they become activated in the presence of antigens. there are 2 types; B lymphocytes and T lymphocytes
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meiosis
the type of nuclear division in which the number of chromosomes is halved
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mesophyll
tissue found between the two layers of epidermis in a plant leaf compromising an upper layer of palisade cells and a lower layer of spongey cells
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metabolism
all the chemical processes that take place in living organisms
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microvilli
tiny finger-like projections from the cell-surface membrane of some animal cells
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middle lamella
layer made up of pectins and other substances found between the walls of adjacent plant cells
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mitosis
the type of nuclear division in which the daughter cells have the same number of chromosomes as the parent cells
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monoclonal antibody
an antibody produced by a single clone of cells
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monomer
one of the many small molecules that combine to form a larger one known as a polymer
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monounsaturated fatty acid
fatty acid that possesses a carbon chain with a single double bond
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motor neurone
neurone that transmits action potentials from the central nervous system to an effector
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multiple alleles
terms used to to describe a gene that has more than 2 possible alleles
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mutagen
any agent that induces a mutation
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mutation
a sudden change in the amount or the arrangement of the genetic material of a cell
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mutualism
a nutritional relationship between two species in which both gain some advantage
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myelin
a fatty substance that surrounds axons and dendrites in certain neurones
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myocardial infraction
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
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myosin
the thick filamentous protein found in skeletal muscle
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NAD (nicotinamide adenine dinucleotide)
a molecule that carries electrons and hydrogen ions during heroic respiration
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NADP (nicotinamide adenine dinucleotide phosphate)
a molecule that carries electrons produced in the light-dependant reaction of photosynthesis
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negative feedback
a series of changes, important in homeostasis, that results in a substance being restored to its normal level
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nephron
basic functional unit of the mammalian kidney responsible for the formation of urine
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neurone
a nerve cell compromising a cell body, axon and dendrites, which is adapted to conduct action potentials
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neuromuscular junction
a synapse that occurs between a neurone and a muscle
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neurotransmitter
one of a number off chemicals that are involved in communication between adjacent neurones or between nerve cells and muscles
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niche
describes how an organism fits into an environment
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nitrifying bacteria
microorganisms that convert ammonium compounds to nitrates and nitrites
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nitrogen fixation
the incorporation of atmospheric nitrogen gas into organic nitrogen-containing compounds
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normal distribution
a bell-shaped curve produced when a certain distribution is plotted on a graph
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node of ranvier
a gap in the myelin sheath that surrounds the axon of a neurone
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nucleotides
complex chemicals made up of an organic base, a sugar and a phosphate. they are basic units of which the nucleic acids DNA and RNA are made
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oestrus
the period in the oestrous cycle immediately after ovulation when the female is most fertile
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oncogenes
mutated versions of photo-oncogenes that result in increased cell division leading to the growth of a tumour
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oral rehydration solution
means of treating dehydration involving giving by mouth a balanced solution of salts and glucose that simulates the gut to reabsorb water
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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
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oxidation
chemical reaction involving the loss electrons
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oxidation-reduction
a chemical reaction in which electrons are transferred from one substance to another. the substance losing electrons is oxidised and the substance gaining electrons is reduced
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oxidative phosphorylation
the formation of ATP in the electron transport system of aerobic respiration
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palisade cells
long, narrow cells packed with chloroplasts that are found in the upper region of a leaf and which carry out photosynthesis
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parasite
an organism that lives on or in a host organism. the parasite gains nutritional advantage and the host is harmed in some way
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passive immunity
resistance to disease that is acquired from the introduction of antibodies from another individual, rather that the individuals own immune system. usually short lived
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pathogen
any microorganism that causes disease
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pentose sugar
a sugar that possesses 5 carbon atoms such as ribose and deoxyribose
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peptide bond
the chemical bond formed between 2 amino acids during condensation
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phagocytosis
mechanism by which cells engulf particles to form a vesicle or a a vacuole
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phenotype
the characteristics of an organism, often visible resulting in both its genotype and the effects of the environment
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phloem
plant tissue that transports the products of photosynthesis from leaves to the rest of the plant
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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
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photolysis
splitting of a water molecule by light such as occurs during the light-dependant reaction of photosynthesis
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photomicrograph
photograph of an image produced by a microscope
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pioneer species
a species that can colonise bare rock or ground
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plasmid
a small circular piece of DNA found in bacterial cells
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plasmodesmata
fine strands of cytoplasm that extend through pores in adjacent plant cell walls and connect the cytoplasm of one cell with another
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plasmolysis
the shrinkage of cytoplasm away from the cell wall that occurs as a plant cell loses water by osmosis
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polygenes
group of genes that are responsible for controlling a characteristic
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polymer
large molecules made up of repeating smaller molecules (monomers)
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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
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polymerases
group of enzymes that catalyse the formation of long-chain molecules (polymers) from similar basic units (monomers)
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polyunsaturated fatty acid
fatty acid that possess carbon chains with many double bonds
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population
a group of individuals of the same species that occupy the same habitat at the same time
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positive feedback
process which results in a substance that departs from its normal level becoming further from its norm
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primary structure of a protein
the sequence of amino acids that makes up the polypeptides of a protein
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primary succession
the progressive colonisation of bare rock or other barren terrain b living organisms
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producer
an organism that synthesises organic molecules from simple inorganic ones such as carbon dioxide and water. most producers are photosynthetic and form the first trophic level in a food chain
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prokaryotic cell
a cell of an organism belonging to the kingdom prokaryotae that is characterised by lacking a nucleus and membrane-bound organelles such as bacteria
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proton
positively charged sub-atomic particle found in the nucleus of an atom
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protoplast
the living portion of a plant cell, that is the nucleus and cytoplasm along with the organelles it contains
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quaternary structure of a protein
a number of polypeptide chains linked together and sometimes associated with non-protein groups to form a protein
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receptor
a cell adapted to detect changes in the environment
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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
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recognition site
a nucleotide sequence, usually of 4,6 or 8 nucleotides that is recognised by a restriction endonuclease and to which it attaches
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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
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reduction
chemical process involving the gain of electrons
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reflex arc
the nerve pathway in the body taken by an action potential that leads to rapid, involuntary response to a stimulus
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refractory period
period during which the membrane of the axon of a neurone cannot be depolarised and no new action potential can be initiated
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renal capsule
the cup shaped portion of the start of the nephron that encloses the glomerulus
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repolarisation
return to the resting potential in the axon of a neurone after an action potential
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resting potential
the difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated
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restriction endonucleases
a group of enzymes that cut DNA molecules at a specific sequence of bases called a recognition sequence
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RNA polymerase
enzyme that joins together nucleotides to form a messenger RNA during transcription
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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
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saprobiotic microorganism
also known as a saprophyte this is an organism that obtains its food from the dead or decaying remains of other organisms
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sarcomere
a section of myofibril between two Z-lines that forms the basic structural unit of skeletal muscle
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saturated fatty acid
a fatty acid in which there are no double bonds between the carbon atoms
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Schwann cell
cell around a neurone whose cell-surface membrane wraps around the dendron or axon to form the myelin sheath
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secondary structure of a protein
the way in which the chain of amino acids of the polypeptides of a protein is folded
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secondary succession
the recolonisation of an area after an earlier community has been removed or destroyed
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selection
process that results in the best-adapted individuals in a population serving to breed and so pass their favourable alleles to the next generation
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selection pressure
the environmental force altering the frequency of alleles in population
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selective breeding
breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or eliminate others
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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
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sensory neurone
a neurone that transmits an action potential from a sensory receptor to the central nervous system
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serum
clear liquid that is left after blood has clotted and the clot has been removed. it is therefore blood plasma without the clotting factors
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sickle-cell anaemia
inherited blood disorder in which abnormal haemoglobin leads to red cells becoming sickle-shaped and less able to carry oxygen
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sinoatrial node
and area of heart muscle in the right atrium that controls and coordinates the contraction of the heart also known as the pacemaker
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skelteal muscle
the muscle that makes up the bulk of the body and which works under conscious control also known as voluntary muscle/ smooth muscle
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smooth muscle
is found in the alimentary canal and the walls of blood vessels. its contraction is not under conscious control
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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
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speciation
the evolution of 2 or more species from existing species
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species
a group of similar organisms that can breed together to produce fertile offspring
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species diversity
the number of different species and the number of individuals of each species within any one community
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stabilising selection
selection that tends to eliminate the extremes of the phenotype range within a population. it arises when environmental conditions are constant
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stem cells
undifferentiated dividing cells that occur in embryos and in adult tissues that require constant replacement such as bone marrow
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stimulus
a detectable alteration in the internal or external environment of an organism that produces some change in that organism
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stoma
a pore, mostly found in the lower epidermis of a leaf through which gases diffuse in and out of the leaf
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stroke volume
the volume of blood pumped at each ventricular contraction of the heart
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stroma
matrix of a chloroplast where the light-independent reaction of photosynthesis takes place
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substrate
a substance that is acted on or used by another substance or process, in microbiology the nutrient medium is used to grow microorganisms
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substrate-level phosphorylation
the formation of ATP by the direct transfer of a phosphate group from a reactive intermediate to ADP
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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
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symplastic pathway
route through the cytoplasm and plasmodesmata of plant cells by which water and dissolved substances are transported
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synapse
a junction between neurones in which they do not touch but have a narrow gap, the sympathetic cleft, across which a neurotransmitter can pass
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systole
the stage in the cardiac cycle in which the heart muscle contacts. it occurs in 2 stages atrial systole when the atria contract and ventricular systole when the ventricles contract
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tendon
tough, flexible but inelastic connective tissue that joins muscle to bone
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tertiary structure of a protein
the folding of a whole polypeptide chain in a precise way as determined by the amino acids of which it is composed
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threshold level/value
the minimum intensity that a stimulus must reach in order to trigger an action potential in a neurone
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thrombosis
formation of a blood clot within a blood vessel that may lead to blockage
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thylakoid
series of flattened membranous scar in a chloroplast that contain chlorophyll and the associated molecules needed for the light-dependant reaction of photosynthesis
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tidal volume
the volume of air breathed in and out during a single breath when at rest
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tissue
a group of similar cells organised into a structural unit that serves a particular function
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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
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T cell (t lymphocytes)
type of white blood cell that is produced in the bone marrow but matures in the thymus gland. t lymphocytes coordinate the immune response and kill infected cells.
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transcription
formation of messenger RNA molecules from the DNA that makes up a particular gene. it is the first stage of protein synthesis
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transducer cells
cells that convert a non-electrical signal such as light or sound into an electrical (nervous) signal and vice versa
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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 virus
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transpiration
evaporation of water from a plant
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triglyceride
an individual lipid molecule made up of a glycerol molecule and 3 fatty acids
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trophic level
the position of an organism in a food chain
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tumour
a swelling in an organism that is made up of cells that continue to divide in an abnormal way
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tumour suppressant gene
a gene that maintains normal rates of cell division and so prevents the development of tumours
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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
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ultrafilteration
filtration assisted by blood pressure such as in the formation of tissue fluid
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unsaturated fatty acid
a fatty acid in which there are one or more double bonds between the carbon atoms
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vaccination
the introduction of a vaccine containing appropriate disease antigens into the body by injection or mouth in order to induce artificial immunity
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vasoconstriction
narrowing of the internal diameter of blood vessels
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vasodilation
widening of the internal diameter of blood vessels
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vector
a carrier. such as plasmid , which carries DNA into a cell, or to an organism that carries a parasite to its host
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voltage-gated channel
protein channel across a cell-surface membrane that opens and closes according to changes in the electrical potential across the membrane
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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 0
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xerophyte
plant adapted to living in dry conditions
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xylem vessels
dead, hollow, elongated tubes, with lignified side walls and no end walls, that transported water in most plants
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Other cards in this set

Card 2

Front

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 the next

Back

acetylcholine

Card 3

Front

change the occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes

Back

Preview of the back of card 3

Card 4

Front

filamentous protein which is involved in contraction within cells, especially muscle cells

Back

Preview of the back of card 4

Card 5

Front

energy required to bring about a reaction. the activation energy is lowered by the presence of enzyme

Back

Preview of the back of card 5
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