A-level biology definitions

A

A band

The dark band, the full length of the thick filaments, including any overlap with thin filaments. · Sliding filaments: shortening a muscle without shortening anything

Abiotic factor

A non-living influence: temperature, light, pH, water, oxygen, wind, soil depth. · Populations and their limits: what stops the numbers rising

Absorption

The movement of the products of digestion from the lumen of the gut, across the epithelium, into the blood or the lymph. · Digestion and absorption: one tube, four chemistries

Accurate

Close to the true value. · Variables and controls: what an experiment can show

Acetyl coenzyme A

The two-carbon acetyl group attached to coenzyme A, which carries it into the Krebs cycle. · Spending ATP to make ATP: glycolysis and the link reaction

Acrosome

A vesicle at the front of a sperm head containing hydrolytic enzymes, which are released to digest a path through the layers around the egg. · Gametes and fertilisation: from two cells to a blastocyst

Activation energy

The minimum energy that colliding particles must have for a reaction to take place. · Enzymes: what a catalyst can and cannot do

Active site

The region of an enzyme, formed by its tertiary structure, to which the substrate binds. · Enzymes: what a catalyst can and cannot do

Active transport

The movement of a substance across a membrane against its concentration gradient, using a carrier protein and energy from the hydrolysis of ATP. · Active transport, co-transport and moving things in bulk

Adaptation

A characteristic that increases an organism's chance of surviving and reproducing in its environment. Adaptations may be anatomical, physiological or behavioural. · Natural selection: variation, selection and allele frequency

Allele frequency

The proportion of all the alleles of a gene in a gene pool that are of one particular kind. · Hardy-Weinberg: what alleles do in a population when nothing acts on them

Allosteric site

A binding site on an enzyme away from the active site, at which a regulatory molecule binds. · Inhibition: how cells, poisons and medicines all turn enzymes down

Amino acid

A molecule containing an amine group, a carboxyl group and a variable side chain attached to the same carbon. · Proteins: from one amino acid to a working shape

Anaerobic respiration

The release of energy from an organic molecule without oxygen, in which glycolysis continues because reduced NAD is oxidised by a reaction other than the electron transport chain. · Anaerobic respiration and the respiratory quotient

Analogous structures

Structures with the same function and different origins, produced by convergent evolution. · Classification and phylogeny: sorting by ancestry, not by looks

Aneurysm

A localised bulge in an artery wall that has been weakened, which may rupture and cause severe internal bleeding. · Cholesterol, atheroma and what a risk factor means

Antagonistic

Describes two effectors or hormones whose effects on a variable are opposite. · Hormones: a message broadcast in the blood that only some cells can read

Anterior pituitary

The lobe that makes its own hormones, under the control of releasing factors from the hypothalamus. · Hormones: a message broadcast in the blood that only some cells can read

Antibiotic

A substance produced by a microorganism, or a synthetic derivative of one, that kills or inhibits the growth of bacteria. · Resistance, drift and the evidence: selection where you can watch it happen

Antibiotic resistance

The ability of a bacterium to survive and reproduce in a concentration of an antibiotic that would kill or inhibit others of its species. · Resistance, drift and the evidence: selection where you can watch it happen

Antibody

A protein with a quaternary structure of four polypeptide chains, produced by plasma cells, with variable regions forming two binding sites complementary to one antigen. · Antibodies, vaccination and the four kinds of immunity

Anticodon

A triplet of bases on tRNA, complementary to a codon on mRNA. · Translation: turning a message into a working protein

Antigen

A molecule, usually a protein or glycoprotein on a cell surface, that the immune system can recognise as self or non-self and that can trigger an immune response. · Pathogens and non-specific defences

Antigen-antibody complex

The structure formed when an antigen binds to the complementary binding site of an antibody. · Antibodies, vaccination and the four kinds of immunity

Antigen-presenting cell

A cell displaying antigens from a pathogen it has destroyed on its own cell-surface membrane. · Pathogens and non-specific defences

Antigenic variation

Change in the antigens on a pathogen's surface, so that memory cells and antibodies produced against the previous form are no longer complementary to it. · Antibodies, vaccination and the four kinds of immunity

Antiparallel

The two strands of a DNA molecule run in opposite directions: the 5′ end of one lies alongside the 3′ end of the other. · DNA and RNA: four letters and a rule about pairing

Apoplast pathway

Movement of water through the cellulose cell walls and the spaces between them, without entering the cytoplasm of any cell. · Xylem and the transpiration stream: the pull comes from the top

Artefact

A structure seen in a prepared specimen that was not present in the living cell, produced by the preparation or imaging process. · Microscopy: magnification, resolution and what you can trust

Atheroma

A plaque in the wall of an artery, consisting of lipid, foam cells and cell debris beneath a fibrous cap, which narrows the lumen and stiffens the wall. · Cholesterol, atheroma and what a risk factor means

ATP

Adenosine triphosphate: a phosphorylated nucleotide made of adenine, ribose and three phosphate groups, used as the immediate source of energy in every cell. · Spending ATP to make ATP: glycolysis and the link reaction

Atrioventricular node

The only electrical connection between atria and ventricles; it delays the impulse so the atria empty first. · The heart and the cardiac cycle: pressure decides everything

Atrioventricular valves

The valves between each atrium and its ventricle; tricuspid on the right, bicuspid or mitral on the left. · The heart and the cardiac cycle: pressure decides everything

Attachment protein

A protein on a virus's surface with a shape complementary to a receptor on its host cell, which determines which cells it can infect. · Prokaryotic cells and viruses: less machinery, same problems

B

Bilayer

Two layers of phospholipid with the hydrophobic tails facing inwards and the hydrophilic heads facing the watery solutions on each side. · The fluid mosaic model: what a membrane is made of

Binomial

The two-part international name of a species: genus then specific name, italicised, genus capitalised. · Classification and phylogeny: sorting by ancestry, not by looks

Biotic factor

A living influence: competitors, predators, prey, parasites, pathogens, mates. · Populations and their limits: what stops the numbers rising

Bivalent

The structure formed when a homologous pair comes together in prophase I: two chromosomes, four chromatids. · Meiosis: two divisions, and why no two gametes come out the same

Bohr shift

The rightward shift of the oxygen dissociation curve caused by an increased partial pressure of carbon dioxide and the fall in pH that goes with it, so that oxygen dissociates more readily at any given partial pressure. · Haemoglobin: an S-shaped curve and everything it explains

C

Capsid

The protein coat surrounding a virus's genetic material. · Prokaryotic cells and viruses: less machinery, same problems

Capsule

A layer of slime outside the cell wall which retains water, aids attachment and gives some protection from phagocytosis. · Prokaryotic cells and viruses: less machinery, same problems

Carbonic anhydrase

The enzyme in red blood cells catalysing the reversible reaction between carbon dioxide and water to form carbonic acid. · Haemoglobin: an S-shaped curve and everything it explains

Carrier

An individual heterozygous for a recessive allele, so not showing the characteristic but able to pass the allele on. · Monohybrid inheritance: genetic diagrams that score

Carrier protein

An intrinsic protein with a binding site specific to one substance, which changes shape to move it across the membrane. · Active transport, co-transport and moving things in bulk

Carrying capacity

The population size that a particular environment can support over a sustained period, given the resources available in it. · Populations and their limits: what stops the numbers rising

Cartilage

A firm, flexible supporting tissue. In the trachea it forms incomplete rings, and in the bronchi irregular blocks, holding the airway open against the pressure drop of inspiration. · Lungs: seventy square metres, folded into a chest

Case-control study

People who already have the disease are compared with matched people who do not, and both groups are asked about past exposure. Fast and efficient for rare diseases; yields an odds ratio, not a risk. · Diet, energy and how to read a health claim

Casparian strip

A band of suberin in the walls of the endodermal cells that is impermeable to water, so the apoplast route is blocked and water must cross a plasma membrane. · Xylem and the transpiration stream: the pull comes from the top

Catalyst

A substance that increases the rate of a reaction by lowering its activation energy, and which is not used up in the process. · Enzymes: what a catalyst can and cannot do

Causation

A relationship in which changing one variable changes the other. It cannot be read off a scatter graph. · Variables and controls: what an experiment can show

Chemiosmosis

The synthesis of ATP driven by the movement of protons down an electrochemical gradient through ATP synthase. · The Krebs cycle and oxidative phosphorylation

Chiasma

The visible point at which two non-sister chromatids cross during a bivalent's crossover. · Meiosis: two divisions, and why no two gametes come out the same

Chloride shift

The movement of chloride ions into a red blood cell as hydrogencarbonate ions move out, maintaining electrical neutrality. · Haemoglobin: an S-shaped curve and everything it explains

Cholinergic synapse

A synapse that uses acetylcholine as its neurotransmitter. · The synapse: transmission, and why it runs one way

Chromatid

One of the two identical copies of a chromosome, joined to its sister at the centromere after DNA replication. · The cell cycle: growth, copying, and one division that changes nothing

Ciliated epithelium

An epithelium whose cells carry cilia that beat in a co-ordinated wave, sweeping mucus and trapped particles up towards the throat. · Lungs: seventy square metres, folded into a chest

Climax community

The final, relatively stable community, which persists until something disturbs it. · Succession: how bare rock becomes woodland, and why we stop it

Clonal expansion

The repeated division by mitosis of the selected lymphocyte, producing a large clone of genetically identical cells. · The specific response: selected, cloned, remembered

Clonal selection

The binding of an antigen to the one lymphocyte whose receptor is complementary to it, activating that cell and no others. · The specific response: selected, cloned, remembered

Co-transport

The transport of two substances across a membrane by the same carrier protein at the same time, in which one moves down its concentration gradient and provides the means for the other to move against its own. · Active transport, co-transport and moving things in bulk

Codon

A sequence of three bases in mRNA that specifies one amino acid or a stop signal. · The genetic code and transcription

Coenzyme

A non-protein molecule that an enzyme needs in order to work. NAD accepts hydrogen from a substrate and carries it elsewhere, so it is a hydrogen carrier. · Spending ATP to make ATP: glycolysis and the link reaction

Cohort study

A group of people free of the disease is recruited, their exposure is recorded, and they are followed forward in time to see who develops it. Gives incidence and relative risk directly, and gets the time order right by construction. · Diet, energy and how to read a health claim

Community

All the populations of all the species living in the same place at the same time. · Populations and their limits: what stops the numbers rising

Companion cell

A cell alongside each sieve tube element with dense cytoplasm, a nucleus and many mitochondria, connected to it by numerous plasmodesmata and responsible for loading it and keeping it alive. · Phloem and translocation: pumped at the ends, flowing in the middle

Compensation period

The time taken from first light for a plant to reach its compensation point. The term appears on some specifications rather than all; check whether your board uses it. · Limiting factors and primary productivity

Compensation point

The light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so there is no net exchange of carbon dioxide or oxygen. · Limiting factors and primary productivity

Competitive exclusion principle

Two species cannot occupy the same niche in the same place indefinitely; the better competitor eliminates the other. · Populations and their limits: what stops the numbers rising

Competitive inhibitor

A molecule with a shape similar to the substrate that binds to the active site, preventing the substrate from binding. · Inhibition: how cells, poisons and medicines all turn enzymes down

Complementary base pairing

Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three, so the sequence of one strand fixes the sequence of the other. · DNA and RNA: four letters and a rule about pairing

Complementary DNA (cDNA)

DNA made from an mRNA template by reverse transcriptase, and therefore containing no introns. · Recombinant DNA technology: isolating, inserting and expressing a gene

Concentration gradient

The difference in concentration between two points, divided by the distance between them. · Surface area to volume ratio: why size limits diffusion

Condensation

A reaction that joins two molecules with a covalent bond and releases a molecule of water. · Carbohydrates: from one sugar to a store of thousands

Confounding variable

A variable that changes with the independent variable and could itself explain the change in the dependent variable, so the two effects cannot be separated by the data collected. · Variables and controls: what an experiment can show

Conjugated protein

A protein with a non-protein prosthetic group permanently associated with it. · Haemoglobin: an S-shaped curve and everything it explains

Constant region

The part of an antibody that is the same in all antibodies of a class, and that binds to receptors on phagocytes. · Antibodies, vaccination and the four kinds of immunity

Control experiment

An extra run of the method, differing in one deliberate way, done to rule out an explanation the main runs cannot rule out on their own. · Variables and controls: what an experiment can show

Control sequence

A region of DNA that is not translated but determines whether, when and how strongly a gene is transcribed. · Mutation and cancer: what goes wrong, and where it goes wrong

Control variable

A quantity held the same across every treatment, so that it cannot be the reason the dependent variable changed. · Variables and controls: what an experiment can show

Coronary arteries

The vessels branching from the base of the aorta that supply the heart muscle itself with oxygenated blood. · The heart and the cardiac cycle: pressure decides everything

Correlation

A relationship in which two variables tend to change together. It may be positive, negative or absent, and it makes no claim about which variable moved first. · Variables and controls: what an experiment can show

Cortical granules

Vesicles lying just inside the egg's cell surface membrane, whose contents are released at fertilisation and change the zona pellucida. · Gametes and fertilisation: from two cells to a blastocyst

Counter-current flow

An arrangement in which two fluids move past each other in opposite directions, so a concentration gradient is maintained along the whole length of the exchange surface. · Three other answers: tracheae, gills and stomata

Crenation

The shrinking and puckering of an animal cell when water leaves it by osmosis. · Diffusion and osmosis: movement that costs nothing

Crista

A fold of the inner mitochondrial membrane, giving a large surface area for the electron transport chain. · Cell structure: what each organelle is shaped for

Crossing over

The exchange of equivalent sections of DNA between non-sister chromatids of a homologous pair during prophase I. · Meiosis: two divisions, and why no two gametes come out the same

Cytoplasm

Everything inside the cell-surface membrane except the nucleus: the cytosol and the organelles suspended in it. · Cell structure: what each organelle is shaped for

D

Decarboxylase

An enzyme that catalyses the removal of a carbon atom from a substrate, released as carbon dioxide. · The Krebs cycle and oxidative phosphorylation

Decarboxylation

Removal of a carbon atom from a molecule, released as carbon dioxide. · Spending ATP to make ATP: glycolysis and the link reaction

Deflected succession

Succession that has been diverted from its natural course by an ongoing external factor. · Succession: how bare rock becomes woodland, and why we stop it

Degenerate

Describing a code in which most amino acids are specified by more than one triplet. · The genetic code and transcription

Degrees of freedom

A number derived from the size of the data set that decides which critical value you read from the table. · Statistics for biologists: spread, error bars and the three tests

Dehydrogenase

An enzyme that catalyses the removal of hydrogen from a substrate, passing it to a coenzyme such as NAD or FAD. · The Krebs cycle and oxidative phosphorylation

Dehydrogenation

Removal of hydrogen from a molecule, catalysed by a dehydrogenase, with the hydrogen taken up by a coenzyme. · Spending ATP to make ATP: glycolysis and the link reaction

Deletion

A mutation in which a base is lost; within a coding region, losing a number of bases not divisible by three shifts the reading of every triplet that follows. · DNA replication: one old strand in every new molecule

Denaturation

The loss of a protein's specific three-dimensional shape through disruption of the hydrogen bonds, ionic bonds and hydrophobic interactions holding it, so the function is lost, while the primary structure is unchanged. · Proteins: from one amino acid to a working shape

Dependent variable

The quantity you measure, whose value depends on the independent variable. It goes on the vertical axis. · Variables and controls: what an experiment can show

Differentiation

The process by which a cell develops the structures and characteristics needed to carry out a particular function, through some genes being expressed and others not. · Specialisation: how one genome becomes two hundred kinds of cell

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion. · Diffusion and osmosis: movement that costs nothing

Diffusion distance

The thickness of the barrier a substance has to cross, sometimes called the length of the diffusion pathway. · Surface area to volume ratio: why size limits diffusion

Digestion

The hydrolysis of large insoluble biological molecules into small soluble molecules that can be absorbed across cell membranes. · Digestion and absorption: one tube, four chemistries

Dipeptidase

An enzyme, bound to the membrane of the ileum epithelium, that hydrolyses a dipeptide into two amino acids. · Digestion and absorption: one tube, four chemistries

Diploid

Having chromosomes in homologous pairs, written 2n. · Meiosis: two divisions, and why no two gametes come out the same

Disaccharide

Two monosaccharides joined by a glycosidic bond. · Carbohydrates: from one sugar to a store of thousands

DNA methylation

The addition of methyl groups to cytosine bases; heavy methylation of a promoter reduces or prevents transcription of that gene. · Control of gene expression: which genes a cell reads

DNA microarray

A plate carrying many different DNA probes at known positions, allowing thousands of sequences to be tested for at once. · Genomes, screening and gene therapy: what a sequence is good for

DNA probe

A short single-stranded length of DNA, labelled radioactively or fluorescently, with a base sequence complementary to a target sequence. · Genomes, screening and gene therapy: what a sequence is good for

Dominant

An allele whose effect appears in the phenotype even when only one copy is present. · Monohybrid inheritance: genetic diagrams that score

Double helix

Two antiparallel polynucleotide strands wound into a spiral, bases inside and backbones outside. · DNA and RNA: four letters and a rule about pairing

E

Ecosystem

A community together with the non-living surroundings it interacts with, treated as one working unit. · Populations and their limits: what stops the numbers rising

Effector

A muscle or gland that brings about a response. · Receptors: turning a stimulus into something a neurone can carry

Elastic fibres

Fibres that stretch as the lungs inflate and recoil as the muscles relax, providing most of the force for quiet expiration. · Lungs: seventy square metres, folded into a chest

Electron transport chain

A series of carrier proteins in the inner mitochondrial membrane that pass electrons from one to the next, releasing energy used to pump protons into the intermembrane space. · The Krebs cycle and oxidative phosphorylation

Empty magnification

Enlarging an image beyond the detail its resolution contains, so nothing further becomes visible. · Microscopy: magnification, resolution and what you can trust

End-product inhibition

The inhibition of an enzyme early in a metabolic pathway by the final product of that pathway, so that the rate of the pathway matches demand. · Inhibition: how cells, poisons and medicines all turn enzymes down

Endangered species

A species at high risk of extinction in the wild, as assessed against criteria such as the IUCN Red List's. · Conservation: keeping a species where it lives, or somewhere else

Endocrine gland

A ductless gland that secretes hormones directly into the blood. · Hormones: a message broadcast in the blood that only some cells can read

Endocytosis

The bulk transport of material into a cell by invagination of the cell surface membrane to form a vesicle; requires ATP. · Active transport, co-transport and moving things in bulk

Endopeptidase

An enzyme that hydrolyses peptide bonds within a polypeptide chain, producing shorter chains and so more ends. Pepsin, trypsin and chymotrypsin. · Digestion and absorption: one tube, four chemistries

Environmental variation

Differences between individuals caused by the conditions they have experienced, which are not passed to offspring. · Natural selection: variation, selection and allele frequency

Enzyme

A globular protein that acts as a biological catalyst, with an active site complementary to its substrate. · Enzymes: what a catalyst can and cannot do

Enzyme-substrate complex

The temporary structure formed when a substrate binds to the active site. · Enzymes: what a catalyst can and cannot do

Epigenetics

Heritable changes in gene expression caused by chemical modifications to DNA and to the histones associated with it, without any change to the base sequence. · Control of gene expression: which genes a cell reads

Epigenome

The full set of such chemical marks on a cell's DNA and histones at a given time. · Control of gene expression: which genes a cell reads

Ester bond

The covalent bond formed by condensation between a hydroxyl group of glycerol and the carboxyl group of a fatty acid. · Lipids: triglycerides, phospholipids and the ester bond

Ex situ conservation

Conserving a species outside its natural habitat, in a zoo, a botanic garden, a seed bank or a captive breeding centre. · Conservation: keeping a species where it lives, or somewhere else

Exchange surface

A surface across which substances move between an organism and its environment, or between two compartments within it. · Surface area to volume ratio: why size limits diffusion

Excitatory postsynaptic potential

A depolarisation of the postsynaptic membrane that makes an action potential more likely. · The synapse: transmission, and why it runs one way

Exocrine gland

A gland that secretes its product into a duct, which carries it to a surface. · Hormones: a message broadcast in the blood that only some cells can read

Exocytosis

The bulk transport of material out of a cell by fusion of a vesicle with the cell surface membrane; requires ATP. · Active transport, co-transport and moving things in bulk

Exon

A section of a gene that is represented in the mature mRNA. · The genetic code and transcription

Exopeptidase

An enzyme that hydrolyses the peptide bond at an end of a chain, removing a single amino acid. Carboxypeptidases work from the end carrying the free carboxyl group, aminopeptidases from the amine end. · Digestion and absorption: one tube, four chemistries

Exponential growth

Growth in which the population multiplies by a constant factor per unit time, so the number added per unit time keeps increasing. · Populations and their limits: what stops the numbers rising

Extinct in the wild

A species surviving only in cultivation, captivity or as a population well outside its past range. · Conservation: keeping a species where it lives, or somewhere else

Extrinsic protein

A protein on one surface of the bilayer that does not enter the hydrophobic core. · The fluid mosaic model: what a membrane is made of

Eyepiece graticule

A ruled scale in the eyepiece whose divisions have no fixed length until it is calibrated. · Microscopy: magnification, resolution and what you can trust

F

FAD

A hydrogen-carrying coenzyme like NAD, but one that hands its electrons to the transport chain further along, so it yields less ATP per molecule. · The Krebs cycle and oxidative phosphorylation

Fermentation

The anaerobic conversion of pyruvate to lactate or to ethanol and carbon dioxide, regenerating oxidised NAD. · Anaerobic respiration and the respiratory quotient

Final electron acceptor

The molecule that takes electrons from the end of the chain. In aerobic respiration it is oxygen, which combines with protons to form water. · The Krebs cycle and oxidative phosphorylation

Fitness

The contribution an individual makes to the gene pool of the next generation, relative to other individuals in the population, usually counted as offspring that themselves survive to reproduce. · Natural selection: variation, selection and allele frequency

Founder effect

The reduced and unrepresentative genetic diversity of a population established by a small number of individuals separated from a larger population. · Resistance, drift and the evidence: selection where you can watch it happen

Frame shift

The displacement of the reading frame caused by insertion or deletion of a number of bases that is not a multiple of three. · Mutation and cancer: what goes wrong, and where it goes wrong

G

Gene

A sequence of DNA bases coding for a polypeptide or a functional RNA. · The genetic code and transcription

Gene expression

The process by which the information in a gene is used to produce a functional product, usually a polypeptide. · Control of gene expression: which genes a cell reads

Gene flow

The movement of alleles between populations, by migration of individuals or of gametes, which keeps their gene pools similar. · Speciation: how one gene pool becomes two, and why the line is genuinely fuzzy

Gene pool

All the alleles of all the genes present in a population at a given time. · Hardy-Weinberg: what alleles do in a population when nothing acts on them

Generator potential

The graded depolarisation produced in a receptor by a stimulus, whose size depends on the strength of that stimulus. · Receptors: turning a stimulus into something a neurone can carry

Genetic bottleneck

A sharp reduction in population size that leaves the survivors carrying only a fraction of the original genetic diversity. · Resistance, drift and the evidence: selection where you can watch it happen

Genetic diversity

The number of different alleles present in the gene pool of a population or species. · Measuring biodiversity: richness, evenness and the index that catches both

Genetic drift

A change in allele frequencies from one generation to the next caused by chance sampling of gametes, rather than by selection. Its effect is greatest in small populations. · Resistance, drift and the evidence: selection where you can watch it happen

Genetic variation

Differences between individuals in a population that are caused by differences in their alleles, and can therefore be inherited. · Natural selection: variation, selection and allele frequency

Genome

The complete set of DNA in a cell, including the DNA that does not code for polypeptides. · The genetic code and transcription

Genotype

The alleles an organism carries, for the gene or genes under discussion. · Monohybrid inheritance: genetic diagrams that score

Gill filament

One of the many thin projections from a gill arch, sometimes called a primary lamella. · Three other answers: tracheae, gills and stomata

Gluconeogenesis

The production of glucose from non-carbohydrate sources such as amino acids and glycerol, in the liver. Promoted by glucagon. · Blood glucose: two hormones, one liver, and two different diabetes

Glycogenesis

The synthesis of glycogen from glucose, in liver and muscle cells. Promoted by insulin. · Blood glucose: two hormones, one liver, and two different diabetes

Glycogenolysis

The hydrolysis of glycogen to glucose. Promoted by glucagon and by adrenaline. · Blood glucose: two hormones, one liver, and two different diabetes

Glycolipid

A phospholipid with a carbohydrate chain attached, acting in cell recognition and adhesion. · The fluid mosaic model: what a membrane is made of

Glycolysis

The splitting of glucose into two molecules of pyruvate in the cytoplasm, producing a net gain of two ATP and two reduced NAD, without oxygen. · Spending ATP to make ATP: glycolysis and the link reaction

Glycoprotein

A membrane protein with a carbohydrate chain attached, acting in cell recognition, adhesion or as a receptor. · The fluid mosaic model: what a membrane is made of

Glycosidic bond

The covalent bond formed by condensation between two monosaccharides, joining them through an oxygen atom. · Carbohydrates: from one sugar to a store of thousands

Goblet cell

A cell in the airway epithelium that secretes mucus, which traps dust, pollen and bacteria. · Lungs: seventy square metres, folded into a chest

GP

Glycerate 3-phosphate, a three-carbon compound; the first stable product of fixation. · The Calvin cycle: three steps, and the arithmetic behind them

Granum

A stack of thylakoids, giving a large area of membrane within a small volume. · Chloroplasts: catching light and turning it into ATP

H

H zone

The lighter region at the centre of the A band, containing thick filaments only. · Sliding filaments: shortening a muscle without shortening anything

Habitat

The place where an organism lives: a rock pool, a hedgerow, the gut of a cow. · Populations and their limits: what stops the numbers rising

Haemoglobinic acid

Haemoglobin that has taken up hydrogen ions, buffering the contents of the red blood cell. · Haemoglobin: an S-shaped curve and everything it explains

Haemolysis

The bursting of a red blood cell when water enters it by osmosis. · Diffusion and osmosis: movement that costs nothing

Haploid

Having one chromosome from each homologous pair, written n. · Meiosis: two divisions, and why no two gametes come out the same

Herd immunity

The protection of unvaccinated individuals that results when a large enough proportion of a population is immune for transmission not to be sustained. · Antibodies, vaccination and the four kinds of immunity

Heterozygous

Carrying two different alleles of a gene. · Monohybrid inheritance: genetic diagrams that score

Histone acetylation

The addition of acetyl groups to histone tails, which reduces their positive charge, loosens the chromatin and increases transcription. · Control of gene expression: which genes a cell reads

Homeostasis

The maintenance of a stable internal environment within narrow limits, despite changes in the external environment. · Hormones: a message broadcast in the blood that only some cells can read

Homologous pair

Two chromosomes of the same size carrying the same genes at the same loci, one inherited from each parent. Not the same thing as a pair of chromatids. · The cell cycle: growth, copying, and one division that changes nothing

Homologous structures

Structures with the same underlying plan, inherited from a common ancestor, whatever they are now used for. · Classification and phylogeny: sorting by ancestry, not by looks

Homozygous

Carrying two identical alleles of a gene. · Monohybrid inheritance: genetic diagrams that score

Horizontal gene transfer

Passing genes between bacteria that are not parent and offspring, most often on a plasmid transferred by conjugation. · Resistance, drift and the evidence: selection where you can watch it happen

Hormone

A chemical messenger, secreted by an endocrine gland into the blood, that acts on target cells carrying a complementary receptor. · Hormones: a message broadcast in the blood that only some cells can read

Hybridisation

The binding of a probe to a complementary single-stranded target sequence by hydrogen bonds between complementary bases. · Genomes, screening and gene therapy: what a sequence is good for

Hydrogen bond

A weak attraction between a slightly positive hydrogen atom of one molecule and a slightly negative atom of another. · Water and inorganic ions: the properties life depends on

Hydrolysis

A reaction that breaks a covalent bond between two molecules using a molecule of water. · Carbohydrates: from one sugar to a store of thousands

Hydrolysis of ATP

The breaking of the bond to the terminal phosphate by addition of water, catalysed by ATP hydrolase, giving ADP and inorganic phosphate and releasing about 30.5 kJ per mole. · Spending ATP to make ATP: glycolysis and the link reaction

Hydrophilic

Attracted to water: charged or polar, so it interacts with water molecules. · Lipids: triglycerides, phospholipids and the ester bond

Hydrophobic

Not attracted to water: non-polar, so it is excluded from water rather than dissolving in it. · Lipids: triglycerides, phospholipids and the ester bond

Hydrostatic pressure

The physical pressure the blood exerts against the capillary wall, pushing fluid out through the gaps between endothelial cells. · Blood vessels and tissue fluid: what leaks out and what comes back

Hypothalamus

The region of the brain that monitors the blood and controls the pituitary gland. · Hormones: a message broadcast in the blood that only some cells can read

I

I band

The light band, containing thin filaments only, spanning a Z line. · Sliding filaments: shortening a muscle without shortening anything

In situ conservation

Conserving a species in its natural habitat, so that the community and the conditions it is adapted to are conserved with it. · Conservation: keeping a species where it lives, or somewhere else

Incipient plasmolysis

The point at which the pressure potential of a plant cell has just fallen to zero, so its water potential equals its solute potential. · Diffusion and osmosis: movement that costs nothing

Independent variable

The quantity you change on purpose, and the only one that should differ between your treatments. It goes on the horizontal axis. · Variables and controls: what an experiment can show

Indirect active transport

Transport that is driven by a gradient which was itself established using ATP elsewhere in the cell. Glucose uptake in the ileum is the standard example. · Digestion and absorption: one tube, four chemistries

Initial rate

The rate of reaction at time zero, found from the gradient of the tangent to a progress curve at the origin. · Factors affecting enzyme rate: temperature, pH and concentration

Intrinsic protein

A protein embedded within the bilayer, often spanning it completely. · The fluid mosaic model: what a membrane is made of

Intron

A non-coding section of a gene, transcribed into pre-mRNA and then removed before the mRNA leaves the nucleus. · The genetic code and transcription

K

Krebs cycle

A closed series of reactions in the mitochondrial matrix in which an acetyl group is combined with a four-carbon acceptor and completely oxidised, releasing carbon dioxide and reducing coenzymes, and regenerating the acceptor. · The Krebs cycle and oxidative phosphorylation

L

Lamella

One of the thin plates set at right angles along a gill filament, one cell thick and richly supplied with blood; the actual exchange surface. · Three other answers: tracheae, gills and stomata

Latent heat of vaporisation

The energy needed to change one kilogram of a liquid into a gas without a change in temperature. · Water and inorganic ions: the properties life depends on

Leading zeros

Never significant. 0.0043 has two significant figures, because the zeros only locate the decimal point. · Handling data: units, scales, rates and uncertainty

Limiting factor

The factor that, of all those affecting a rate, is the one restricting it at that moment. · Factors affecting enzyme rate: temperature, pH and concentration

Link reaction

The oxidative decarboxylation of pyruvate in the mitochondrial matrix, producing carbon dioxide, reduced NAD and a two-carbon acetyl group carried by coenzyme A. · Spending ATP to make ATP: glycolysis and the link reaction

Lipid

A biological molecule that is largely non-polar, insoluble in water and soluble in organic solvents such as ethanol. · Lipids: triglycerides, phospholipids and the ester bond

Loading

Association of oxygen with haemoglobin, at the gas exchange surface. · Haemoglobin: an S-shaped curve and everything it explains

Lymph

Tissue fluid that has drained into the lymphatic system; similar to tissue fluid but containing lymphocytes and more fatty acids after a meal. · Blood vessels and tissue fluid: what leaks out and what comes back

Lymph node

A swelling in a lymph vessel where lymphocytes collect and pathogens in the lymph are removed. · Blood vessels and tissue fluid: what leaks out and what comes back

Lymphocyte

A white blood cell carrying receptors of a single shape, produced in bone marrow; T cells mature in the thymus and B cells in the bone marrow. · The specific response: selected, cloned, remembered

Lysosome

A vesicle containing hydrolytic enzymes, bounded by a single membrane, which digests worn-out organelles and engulfed material. · Cell structure: what each organelle is shaped for

M

Magnification

How many times larger the image is than the actual object. A ratio, with no units. · Microscopy: magnification, resolution and what you can trust

Mass transport system

A system in which substances are moved in bulk, in a fluid driven by a pump, over distances too great for diffusion to serve. · The heart and the cardiac cycle: pressure decides everything

Mean

The total divided by the number of readings. Uses every value, and is what almost every biological calculation wants. · Statistics for biologists: spread, error bars and the three tests

Median

The middle value when the readings are put in order. Unmoved by a single extreme value, which is why it is used for skewed data such as incomes, reaction times or the number of parasites per host. · Statistics for biologists: spread, error bars and the three tests

Memory cell

A long-lived cell produced by clonal expansion that does not secrete antibody, but divides rapidly into plasma cells if the same antigen is met again. · The specific response: selected, cloned, remembered

Messenger RNA (mRNA)

A single-stranded copy of one gene, made in the nucleus and carrying codons to a ribosome in the cytoplasm. · DNA and RNA: four letters and a rule about pairing

Metabolic rate

The rate at which an organism uses energy, and so the rate at which it must be supplied with oxygen and relieved of carbon dioxide. · The heart and the cardiac cycle: pressure decides everything

Metastasis

The spread of cells from a malignant tumour through the blood or lymph to form secondary tumours. · Mutation and cancer: what goes wrong, and where it goes wrong

Mitotic index

The proportion of cells in a sample that are visibly in mitosis. · The cell cycle: growth, copying, and one division that changes nothing

Mode

The most common value. The only one of the three that works for categories such as blood group or flower colour. · Statistics for biologists: spread, error bars and the three tests

Monomer

A small molecule from which a larger one is built by joining many of them together. · Carbohydrates: from one sugar to a store of thousands

Monosaccharide

A single sugar unit, the monomer from which disaccharides and polysaccharides are built. · Carbohydrates: from one sugar to a store of thousands

Multipotent

Able to develop into a limited range of cell types. Found in adult stem cells, such as those in bone marrow which produce the blood cells. · Specialisation: how one genome becomes two hundred kinds of cell

Mutation

A change in the base sequence of DNA. · DNA replication: one old strand in every new molecule

Mycorrhiza

A mutualistic association between a fungus and a plant root, in which the fungus increases the surface area for absorbing water and mineral ions and receives organic compounds in return. · Nutrient cycles: nitrogen, phosphorus and eutrophication

Myogenic

Contracting from within the muscle itself, without needing a nerve impulse to initiate the beat. · The heart and the cardiac cycle: pressure decides everything

N

Natural selection

The process by which individuals with phenotypes better suited to the environment reproduce more successfully, so the alleles responsible become more frequent. · Natural selection: variation, selection and allele frequency

Negative feedback

A control mechanism in which a deviation from the norm triggers a response that reverses the deviation. · Hormones: a message broadcast in the blood that only some cells can read

Nephron

The functional unit of the kidney: a renal capsule and its tubule, running from the cortex into the medulla and back. · The kidney: filter everything, then take almost all of it back

Neurotransmitter

A chemical released from a presynaptic neurone that diffuses across the cleft and binds to receptors on the postsynaptic membrane. · The synapse: transmission, and why it runs one way

Niche

The role a species plays in its community: all the conditions it can tolerate and all the resources it uses, including where it lives, what it eats, what eats it and when it is active. · Populations and their limits: what stops the numbers rising

Non-competitive inhibitor

A molecule that binds to a site other than the active site, altering the enzyme's tertiary structure so that the active site is no longer complementary to the substrate. · Inhibition: how cells, poisons and medicines all turn enzymes down

Non-overlapping

Describing a code in which each base is part of one triplet only and triplets are read in sequence. · The genetic code and transcription

Non-self

Antigens not present on the organism's own cells: pathogens, transplanted tissue, cancerous cells with altered surface proteins, and toxins. · The specific response: selected, cloned, remembered

Non-specific defence

A defence that acts against any pathogen, without recognising which one it is, and acts at the same speed every time. · Pathogens and non-specific defences

Nuclear envelope

The double membrane surrounding the nucleus, continuous with the endoplasmic reticulum and perforated by nuclear pores. · Cell structure: what each organelle is shaped for

Nucleoid

The region of a prokaryotic cell occupied by its circular DNA, with no surrounding membrane. · Prokaryotic cells and viruses: less machinery, same problems

Nucleolus

A dense region within the nucleus where ribosomal RNA is made and ribosomes are assembled. · Cell structure: what each organelle is shaped for

Nucleotide

A phosphate group, a pentose sugar and a nitrogenous base joined together. · DNA and RNA: four letters and a rule about pairing

Null hypothesis

A statement that there is no significant difference, no significant association or no significant correlation between the variables, and that any difference observed is due to chance. · Statistics for biologists: spread, error bars and the three tests

O

Oestrogen response element

A specific base sequence near a target gene to which an oestrogen-receptor complex binds, increasing the rate of transcription. · Control of gene expression: which genes a cell reads

Oncogene

A mutated or over-expressed proto-oncogene whose product stimulates division continuously. · Mutation and cancer: what goes wrong, and where it goes wrong

Oncotic pressure

The inward pull created by plasma proteins, which stay in the capillary and lower the water potential of the blood relative to the tissue fluid. Also called colloid osmotic pressure. · Blood vessels and tissue fluid: what leaks out and what comes back

Operculum

The bony flap covering the gills of a bony fish, acting as a valve and part of the pump that maintains the flow of water. · Three other answers: tracheae, gills and stomata

Optimum

The value of a variable at which an enzyme's rate is highest. · Factors affecting enzyme rate: temperature, pH and concentration

Organ

A structure made of several different tissues working together to perform a particular function. · Specialisation: how one genome becomes two hundred kinds of cell

Organ system

A group of organs working together to perform an overall life function. · Specialisation: how one genome becomes two hundred kinds of cell

Organelle

A structure inside a cell with a distinct function, usually but not always surrounded by its own membrane. · Cell structure: what each organelle is shaped for

Osmoregulation

The control of the water potential of the blood and body fluids. · The kidney: filter everything, then take almost all of it back

Osmosis

The net movement of water molecules from a solution of higher water potential to a solution of lower water potential, through a partially permeable membrane. · Diffusion and osmosis: movement that costs nothing

Oxidation

In respiration, most easily read as the loss of hydrogen. A substrate that hands hydrogen to NAD has been oxidised; the NAD has been reduced. · Spending ATP to make ATP: glycolysis and the link reaction

Oxidative phosphorylation

The formation of ATP from ADP and inorganic phosphate using energy from the electron transport chain, with oxygen as the final electron acceptor. · The Krebs cycle and oxidative phosphorylation

P

Partial pressure of oxygen

The contribution oxygen makes to the total pressure of a gas mixture, measured in kPa; a measure of how much oxygen is present. · Haemoglobin: an S-shaped curve and everything it explains

Passive

Requiring no energy from ATP; driven by the kinetic energy of the particles themselves. · Diffusion and osmosis: movement that costs nothing

Pathogen

An organism that causes disease in its host. · Pathogens and non-specific defences

Percentage saturation

The proportion of the oxygen-binding sites on haemoglobin that are actually occupied. · Haemoglobin: an S-shaped curve and everything it explains

Permeability

How readily a membrane allows a substance to cross it. · The fluid mosaic model: what a membrane is made of

Phagocytosis

The engulfing and hydrolysis of a pathogen or other foreign material by a phagocyte, forming a phagosome that fuses with a lysosome. · Pathogens and non-specific defences

Phagosome

The vesicle formed when a phagocyte's cell-surface membrane closes around an engulfed pathogen. · Pathogens and non-specific defences

Phenetic classification

Grouping organisms by observable similarity, without regard to how they are related. · Classification and phylogeny: sorting by ancestry, not by looks

Phenotype

The observable characteristics of an organism, produced by its genotype interacting with its environment. · Monohybrid inheritance: genetic diagrams that score

Phosphodiester bond

The covalent bond joining the phosphate group of one nucleotide to the sugar of the next, formed by condensation. · DNA and RNA: four letters and a rule about pairing

Phospholipid

A lipid in which one of the three fatty acids of a triglyceride is replaced by a phosphate group, giving a hydrophilic head and two hydrophobic tails. · The fluid mosaic model: what a membrane is made of

Phosphorylation

The addition of a phosphate group to a molecule. The phosphate released from ATP is often added to another molecule, making it more reactive. · Spending ATP to make ATP: glycolysis and the link reaction

Photoionisation

The loss of an electron from a chlorophyll molecule after it absorbs light energy, leaving the molecule positively charged. · Chloroplasts: catching light and turning it into ATP

Photolysis

The splitting of water using light energy, giving electrons, protons and oxygen. · Chloroplasts: catching light and turning it into ATP

Photophosphorylation

The synthesis of ATP from ADP and inorganic phosphate using energy that originally came from light. · Chloroplasts: catching light and turning it into ATP

Photosystem

A cluster of pigment molecules held in a thylakoid membrane, funnelling absorbed energy to a single chlorophyll a molecule at its reaction centre. · Chloroplasts: catching light and turning it into ATP

Phylogenetic classification

Grouping organisms so that each group contains a common ancestor and all of its descendants. · Classification and phylogeny: sorting by ancestry, not by looks

Phylogeny

The evolutionary history of a group: which lineages split from which, and in what order. · Classification and phylogeny: sorting by ancestry, not by looks

Physical digestion

The mechanical breakdown of food into smaller pieces, increasing the surface area available to digestive enzymes without hydrolysing anything. · Digestion and absorption: one tube, four chemistries

Pioneer species

The first species to colonise a bare surface, adapted to survive there when almost nothing else can. · Succession: how bare rock becomes woodland, and why we stop it

Plagioclimax

A community held at a stage short of its climax by a repeated interference, usually human or from grazing animals. · Succession: how bare rock becomes woodland, and why we stop it

Plasma cell

A differentiated B cell that secretes large quantities of a single antibody into blood and lymph. Most survive only a few days; a minority settle in the bone marrow and secrete for years. · The specific response: selected, cloned, remembered

Plasmid

A small circular molecule of DNA, separate from the main chromosome, which replicates independently and often carries genes such as antibiotic resistance. · Prokaryotic cells and viruses: less machinery, same problems

Plasmolysis

The pulling away of the cell surface membrane from the cell wall as water leaves a plant cell. · Diffusion and osmosis: movement that costs nothing

Plateau

The region of a rate graph where increasing the factor on the horizontal axis no longer increases the rate, because something else has become limiting. · Limiting factors and primary productivity

Pluripotent

Able to develop into any cell type in the body, but not into the placenta. Found in embryonic stem cells in the blastocyst. · Specialisation: how one genome becomes two hundred kinds of cell

Polar molecule

A molecule with an uneven distribution of charge, so that one part is slightly negative and another slightly positive. · Water and inorganic ions: the properties life depends on

Polarised

Describes a membrane with a potential difference across it; at rest the axon membrane is polarised with the inside negative. · The action potential: the same size, however hard you push

Polymer

A large molecule made of many similar monomers joined together. · Carbohydrates: from one sugar to a store of thousands

Polynucleotide

A chain of nucleotides joined by phosphodiester bonds between the sugar of one and the phosphate of the next. · DNA and RNA: four letters and a rule about pairing

Polypeptide

A chain of amino acids joined by peptide bonds. · Proteins: from one amino acid to a working shape

Polysaccharide

Many monosaccharides joined by glycosidic bonds into one large molecule. · Carbohydrates: from one sugar to a store of thousands

Polyspermy

Fusion of more than one sperm with a single egg, which gives a cell with too many chromosome sets to develop. · Gametes and fertilisation: from two cells to a blastocyst

Population

A group of organisms of the same species living in the same place at the same time and able to interbreed. · Hardy-Weinberg: what alleles do in a population when nothing acts on them

Positive feedback

A mechanism in which a deviation from the norm triggers a response that increases the deviation. · Hormones: a message broadcast in the blood that only some cells can read

Post-translational modification

Changes made to a polypeptide after translation, such as cutting out sections or adding carbohydrate or lipid groups, carried out in the endoplasmic reticulum and the Golgi apparatus. Folding is not one of them: it begins on the ribosome and is finished in the endoplasmic reticulum. · Translation: turning a message into a working protein

Posterior pituitary

The lobe that stores and releases ADH and oxytocin, both made in the hypothalamus. · Hormones: a message broadcast in the blood that only some cells can read

Potency

The range of cell types a stem cell is still able to become. · Stem cells: what a cell can still become, and what fixes it

Pre-mRNA

The immediate product of transcription in a eukaryote, containing both exons and introns. · The genetic code and transcription

Precise

Repeat readings close to one another. Precision says nothing at all about being right. · Variables and controls: what an experiment can show

Primary structure

The sequence of amino acids in a polypeptide, held by peptide bonds. · Proteins: from one amino acid to a working shape

Prokaryotic cell

A cell with no nucleus and no membrane-bound organelles, whose DNA is a single circular molecule free in the cytoplasm. · Prokaryotic cells and viruses: less machinery, same problems

Promoter

A region of DNA at the start of a gene to which RNA polymerase and transcription factors bind. · Control of gene expression: which genes a cell reads

Prosthetic group

The non-protein component; in haemoglobin, the haem group with its iron(II) ion. · Haemoglobin: an S-shaped curve and everything it explains

Protein

One or more polypeptides folded into a specific three-dimensional shape. · Proteins: from one amino acid to a working shape

Proto-oncogene

A normal gene whose product stimulates cell division in response to a growth signal. · Mutation and cancer: what goes wrong, and where it goes wrong

Purkyne fibres

Conducting fibres running from the bundle of His up through the ventricle walls, so contraction begins at the apex. · The heart and the cardiac cycle: pressure decides everything

Q

Quaternary structure

The arrangement of two or more polypeptides, and any non-protein groups, in a functioning protein. · Proteins: from one amino acid to a working shape

R

Randomisation

Assigning subjects or sample positions by chance rather than by choice, so that variables nobody thought of tend to balance out between the groups on average. · Variables and controls: what an experiment can show

Randomised controlled trial

Participants are allocated by chance, so the groups are balanced on every variable, including the ones nobody thought to measure. · Diet, energy and how to read a health claim

Receptor

A cell or structure that detects one kind of stimulus and converts it into a change in membrane potential. · Receptors: turning a stimulus into something a neurone can carry

Recessive

An allele whose effect appears only when two copies are present. · Monohybrid inheritance: genetic diagrams that score

Recombinant

A chromatid, or an offspring, carrying a combination of alleles that neither parental chromosome carried. · Meiosis: two divisions, and why no two gametes come out the same

Recombinant DNA

DNA made by joining together sequences from two different organisms. · Recombinant DNA technology: isolating, inserting and expressing a gene

Reduction division

A division that halves the chromosome number. Meiosis I is one; meiosis II is not. · Meiosis: two divisions, and why no two gametes come out the same

Reflex

A rapid, automatic response to a stimulus, involving a fixed pathway of neurones and no conscious decision. · Receptors: turning a stimulus into something a neurone can carry

Reflex arc

The pathway of neurones taken by a reflex. In the examined withdrawal reflex: receptor, sensory neurone, relay neurone, motor neurone, effector. · Receptors: turning a stimulus into something a neurone can carry

Relative risk

The risk in the exposed group divided by the risk in the unexposed group; 1 means no association. · Diet, energy and how to read a health claim

Repeatable

The same person, using the same method and the same apparatus, gets the same results again. · Variables and controls: what an experiment can show

Reproducible

Somebody else, or the same person using different apparatus or a different method, gets the same results. · Variables and controls: what an experiment can show

Reproductive isolation

The state in which gene flow between two populations has stopped, because of some feature of the organisms or of where and when they breed. · Speciation: how one gene pool becomes two, and why the line is genuinely fuzzy

Residual volume

The volume left in the lungs after the hardest possible breath out, about 1.2 dm³. It cannot be measured with a spirometer, because it never leaves the lungs. · Lungs: seventy square metres, folded into a chest

Resolution

The smallest change an instrument can register: 0.01 g on a three-decimal-place balance, 1 mm on a metre rule. · Variables and controls: what an experiment can show

Resting potential

The potential difference across the membrane of a neurone that is not conducting an impulse, about −70 mV inside relative to outside. · The action potential: the same size, however hard you push

Reverse transcriptase

An enzyme carried by retroviruses such as HIV which catalyses the synthesis of DNA from an RNA template. · Prokaryotic cells and viruses: less machinery, same problems

Ribosomal RNA (rRNA)

RNA which, with proteins, forms the two subunits of a ribosome and catalyses the formation of peptide bonds. · DNA and RNA: four letters and a rule about pairing

Ribosome

A structure of ribosomal RNA and protein, in two subunits, with no membrane, which is the site of translation. 80S in eukaryotic cytoplasm, 70S in prokaryotes and in mitochondria and chloroplasts. · Cell structure: what each organelle is shaped for

RNA interference

The inhibition of gene expression by small RNA molecules binding to complementary mRNA. · Control of gene expression: which genes a cell reads

Root hair cell

An epidermal cell of the root extended into a long narrow projection, giving a large surface area for the absorption of water and mineral ions. · Xylem and the transpiration stream: the pull comes from the top

Rough endoplasmic reticulum

Flattened membrane sacs with ribosomes attached, which make and transport proteins for export or for membranes. · Cell structure: what each organelle is shaped for

Rubisco

RuBP carboxylase, the enzyme that joins carbon dioxide to RuBP. Slow, easily out-competed by oxygen, and probably the most abundant protein on Earth. · The Calvin cycle: three steps, and the arithmetic behind them

RuBP

Ribulose bisphosphate, a five-carbon compound; the molecule that accepts carbon dioxide. · The Calvin cycle: three steps, and the arithmetic behind them

S

Saprobiont

An organism that obtains its energy from dead material by secreting enzymes onto it and absorbing the soluble products. · Nutrient cycles: nitrogen, phosphorus and eutrophication

Sarcomere

The functional unit of a myofibril, running from one Z line to the next. · Sliding filaments: shortening a muscle without shortening anything

Sarcoplasmic reticulum

The membrane system inside a muscle fibre that stores and releases calcium ions. · Sliding filaments: shortening a muscle without shortening anything

Secondary structure

The regular coiling or pleating of the backbone into an α-helix or β-pleated sheet, held by hydrogen bonds between the C=O of one peptide group and the N–H of another. · Proteins: from one amino acid to a working shape

Selection pressure

An environmental factor that causes individuals with certain phenotypes to survive and reproduce more successfully than others. · Natural selection: variation, selection and allele frequency

Selective reabsorption

The return of useful substances from the filtrate to the blood, by transport proteins in the tubule wall. · The kidney: filter everything, then take almost all of it back

Self

Antigens present on an organism's own cells, which its lymphocytes do not respond to. · The specific response: selected, cloned, remembered

Self-renewal

Division that produces at least one daughter cell identical to the parent stem cell, so the population of stem cells is maintained. · Stem cells: what a cell can still become, and what fixes it

Semi-conservative replication

Replication in which the two strands of the parent molecule separate and each acts as a template, so every daughter molecule contains one original strand and one newly synthesised strand. · DNA replication: one old strand in every new molecule

Semilunar valves

The valves at the base of the aorta and the pulmonary artery, preventing backflow into the ventricles. · The heart and the cardiac cycle: pressure decides everything

Seral stage

One identifiable community in the sequence; the whole sequence is a sere. · Succession: how bare rock becomes woodland, and why we stop it

Sieve plate

The perforated end wall between two sieve tube elements, through which the sap passes. · Phloem and translocation: pumped at the ends, flowing in the middle

Sieve tube element

A living cell of the phloem, joined end to end with others into a tube; at maturity it has no nucleus, few organelles and only a thin layer of cytoplasm around the edge. · Phloem and translocation: pumped at the ends, flowing in the middle

Significance level

The probability threshold, conventionally 0.05, below which the null hypothesis is rejected. · Statistics for biologists: spread, error bars and the three tests

Significant figures

The digits in a measurement that carry information about its size, counted from the first non-zero digit. · Handling data: units, scales, rates and uncertainty

Silent mutation

A base substitution that produces a triplet coding for the same amino acid, so the primary structure of the polypeptide is unchanged. · Mutation and cancer: what goes wrong, and where it goes wrong

Simple diffusion

Diffusion directly through the phospholipid bilayer, available to small non-polar molecules such as oxygen and carbon dioxide. · Diffusion and osmosis: movement that costs nothing

Sinoatrial node

The patch of tissue in the right atrium wall that sets the rhythm of the heart. · The heart and the cardiac cycle: pressure decides everything

siRNA

Small interfering RNA: short double-stranded RNA fragments, one strand of which guides a protein complex to a complementary mRNA so that the mRNA is broken down and cannot be translated. · Control of gene expression: which genes a cell reads

Smooth endoplasmic reticulum

Membrane sacs without ribosomes, which synthesise lipids and steroids and store calcium ions. · Cell structure: what each organelle is shaped for

Smooth muscle

Involuntary muscle in the airway wall; its contraction narrows the lumen, and it is the tissue that contracts inappropriately during an asthma attack. · Lungs: seventy square metres, folded into a chest

Specialised cell

A cell whose structure is adapted to one particular function. · Specialisation: how one genome becomes two hundred kinds of cell

Speciation

The formation of a new species from an existing one, through reproductive isolation followed by genetic divergence of the isolated populations. · Speciation: how one gene pool becomes two, and why the line is genuinely fuzzy

Species

A group of organisms with similar characteristics that can interbreed to produce fertile offspring, and which are reproductively isolated from other such groups. · Speciation: how one gene pool becomes two, and why the line is genuinely fuzzy

Species diversity

Richness and evenness together, usually expressed as an index. · Measuring biodiversity: richness, evenness and the index that catches both

Species evenness

How equally the individuals are shared out between those species. · Measuring biodiversity: richness, evenness and the index that catches both

Species richness

The number of different species in a community. · Measuring biodiversity: richness, evenness and the index that catches both

Specific heat capacity

The energy needed to raise the temperature of one kilogram of a substance by one kelvin. · Water and inorganic ions: the properties life depends on

Specificity

The property of an enzyme that it catalyses only one reaction, or one class of reaction, because only a substrate complementary to its active site can bind. · Enzymes: what a catalyst can and cannot do

Spindle

The framework of protein microtubules built across the cell before the chromosomes are moved, running from pole to pole and attaching to every centromere; shortening the fibres is what separates the chromatids. · The cell cycle: growth, copying, and one division that changes nothing

Splicing

The removal of introns from pre-mRNA and the joining of exons to produce mature mRNA. · The genetic code and transcription

Squamous epithelium

An epithelium of very flattened cells, giving a wall a fraction of a micrometre thick; the lining of an alveolus. · Lungs: seventy square metres, folded into a chest

Stage micrometer

A slide bearing a scale of known length, used to calibrate an eyepiece graticule for one particular objective lens. · Microscopy: magnification, resolution and what you can trust

Stem cell

An unspecialised cell that can divide repeatedly to produce more cells like itself, and can differentiate into one or more specialised types. · Stem cells: what a cell can still become, and what fixes it

Steroid hormone

A lipid-soluble hormone that crosses the cell-surface membrane by simple diffusion and acts on an intracellular receptor. · Control of gene expression: which genes a cell reads

Stimulus

A detectable change in the internal or external environment. · Receptors: turning a stimulus into something a neurone can carry

Stoma

A pore in the epidermis, bounded by two guard cells, through which gases and water vapour pass. · Transpiration: the price of keeping the stomata open

Stroma

The fluid surrounding the thylakoids, containing the enzymes of the light-independent stage, along with DNA and 70S ribosomes. · Chloroplasts: catching light and turning it into ATP

Substitution

A mutation in which one base is replaced by another. · DNA replication: one old strand in every new molecule

Substrate

The molecule on which an enzyme acts. · Enzymes: what a catalyst can and cannot do

Substrate-level phosphorylation

The direct transfer of a phosphate group from a substrate molecule onto ADP, as opposed to ATP made using a proton gradient. · Spending ATP to make ATP: glycolysis and the link reaction

Succession

The change over time in the species occupying an area, in which each community alters the environment and makes it more suitable for the community that follows. · Succession: how bare rock becomes woodland, and why we stop it

Surface area to volume ratio

The exchange surface available per unit of volume; it falls as an object gets larger. · The heart and the cardiac cycle: pressure decides everything

Symplast pathway

Movement of water through the cytoplasm of the cells, passing from one to the next through plasmodesmata. · Xylem and the transpiration stream: the pull comes from the top

Synapse

The junction between the end of one neurone and another neurone or an effector cell, across which chemical transmission occurs. · The synapse: transmission, and why it runs one way

T

Target cell

A cell with receptors complementary to a particular hormone, and therefore the only kind of cell that responds to it. · Hormones: a message broadcast in the blood that only some cells can read

Taxon

Any one group in that hierarchy: Mammalia, Primates and Homo are all taxa. · Classification and phylogeny: sorting by ancestry, not by looks

Taxonomy

The naming and classifying of organisms into a hierarchy of groups. · Classification and phylogeny: sorting by ancestry, not by looks

Template strand

A strand of DNA whose base sequence determines the sequence of the new strand built against it. · DNA replication: one old strand in every new molecule

Tendinous cords

Inelastic strands running from the atrioventricular valve flaps to papillary muscles projecting from the ventricle wall. The muscles tense as the ventricle contracts, so ventricular pressure cannot turn the flaps inside out; the pressure does the opening and shutting, the cords only stop the flaps going too far. · The heart and the cardiac cycle: pressure decides everything

Tertiary structure

The overall three-dimensional fold of a single polypeptide, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between side chains. · Proteins: from one amino acid to a working shape

Threshold

The membrane potential, around −55 mV, that must be reached before an action potential is triggered. · Receptors: turning a stimulus into something a neurone can carry

Thrombosis

The formation of a blood clot inside a blood vessel, most often on a ruptured atheroma, which may block the vessel completely. · Cholesterol, atheroma and what a risk factor means

Thylakoid

A flattened, membrane-bound sac inside a chloroplast; the membrane holds the pigments, the electron carriers and ATP synthase. · Chloroplasts: catching light and turning it into ATP

Tidal volume

The volume of air moved in or out in one normal breath at rest, about 0.5 dm³ in an adult. · Lungs: seventy square metres, folded into a chest

Tissue

A group of similar cells working together to perform a particular function. · Specialisation: how one genome becomes two hundred kinds of cell

Tissue fluid

The fluid surrounding cells, formed by ultrafiltration from capillaries; it is very poor in plasma protein, though not literally protein-free, and has no red blood cells and no platelets. · Blood vessels and tissue fluid: what leaks out and what comes back

Totipotent

Able to develop into any cell type in the organism and into the extra-embryonic tissues such as the placenta. Found in the zygote and the cells of the first few divisions. · Specialisation: how one genome becomes two hundred kinds of cell

TP

Triose phosphate, a three-carbon sugar phosphate; the product of reducing GP, and the molecule that either leaves the cycle or rebuilds RuBP. · The Calvin cycle: three steps, and the arithmetic behind them

Trailing zeros after a decimal point

Always significant. 4.20 has three, and says the hundredths digit was measured. · Handling data: units, scales, rates and uncertainty

Transcription factor

A protein that binds to a specific base sequence in DNA, usually in or near a promoter region, and so increases or decreases the rate of transcription of that gene. · Control of gene expression: which genes a cell reads

Transducer

Something that converts energy from one form into another; a receptor converts the energy of a stimulus into a change in membrane potential. · Receptors: turning a stimulus into something a neurone can carry

Transfer RNA (tRNA)

A small folded RNA of about 80 nucleotides with an anticodon at one end and a binding site for a specific amino acid at the other. · DNA and RNA: four letters and a rule about pairing

Transgenic organism

An organism containing DNA transferred into it from another species. · Recombinant DNA technology: isolating, inserting and expressing a gene

Translocation

The transport of assimilates, mainly sucrose, through the phloem from a source to a sink. · Phloem and translocation: pumped at the ends, flowing in the middle

Transpiration

The loss of water vapour from the aerial parts of a plant, mainly through the stomata, by evaporation from the mesophyll cell walls followed by diffusion out of the leaf. · Transpiration: the price of keeping the stomata open

Transpiration stream

The continuous movement of water from the soil, through the root and up the xylem, to replace the water lost from the leaves. · Transpiration: the price of keeping the stomata open

Triglyceride

One glycerol molecule joined to three fatty acids by three ester bonds. · Lipids: triglycerides, phospholipids and the ester bond

Tumour suppressor gene

A gene whose product slows the cell cycle or triggers apoptosis; loss of both alleles removes that control. · Mutation and cancer: what goes wrong, and where it goes wrong

Turgid

A plant cell whose contents press against the wall, giving a positive pressure potential. · Diffusion and osmosis: movement that costs nothing

U

Ultrafiltration

Filtration under pressure through a partially permeable barrier, separating small molecules from large ones. · The kidney: filter everything, then take almost all of it back

Ultrastructure

The detail of a cell's internal structure as revealed by an electron microscope. · Cell structure: what each organelle is shaped for

Unloading

Dissociation of oxygen from haemoglobin, at respiring tissue. · Haemoglobin: an S-shaped curve and everything it explains

V

Vaccine

A preparation containing antigens from a pathogen, given to produce a primary immune response and memory cells without causing the disease. · Antibodies, vaccination and the four kinds of immunity

Valid

A measurement is valid if it measures what it claims to measure; an investigation is valid if its design allows its conclusion to follow. A confounding variable destroys validity however careful the measuring was. · Variables and controls: what an experiment can show

Variable region

The part of an antibody whose amino acid sequence differs between antibodies, forming the antigen-binding site. · Antibodies, vaccination and the four kinds of immunity

Ventilation rate

The number of breaths taken per minute, about 12 to 15 at rest. Also called breathing rate. · Lungs: seventy square metres, folded into a chest

Vertical gene transfer

Passing genes from a parent cell to its daughter cells by binary fission. · Resistance, drift and the evidence: selection where you can watch it happen

Virus

An acellular infectious particle consisting of nucleic acid within a protein capsid, sometimes with a lipid envelope, which can only replicate inside a host cell. · Prokaryotic cells and viruses: less machinery, same problems

Vital capacity

The largest volume that can be breathed out after the deepest possible breath in; roughly 4.6 dm³ in a young adult man and 3.1 dm³ in a young adult woman. · Lungs: seventy square metres, folded into a chest

Voltage-gated channel

A channel protein that opens or closes in response to a change in the potential difference across the membrane. · The action potential: the same size, however hard you push

W

Water potential (ψ)

The tendency of water molecules to move out of a solution, measured in kPa. Pure water at atmospheric pressure has a water potential of 0 kPa; adding any solute makes it negative. · Diffusion and osmosis: movement that costs nothing

Z

Zona pellucida

The glycoprotein coat outside the egg's cell surface membrane, carrying the receptors a sperm binds to. · Gametes and fertilisation: from two cells to a blastocyst