Key facts

Carbohydrates and lipids contain carbon, hydrogen and oxygen. Proteins contain carbon, hydrogen, oxygen and nitrogen, and sulfur in some amino acids such as cysteine. Nucleic acids contain carbon, hydrogen, oxygen, nitrogen and phosphorus. Phospholipids, unlike triglycerides, also contain phosphorus in their phosphate head.

Monomers are the small units from which larger molecules are made; polymers are molecules made from many monomers joined by covalent bonds. Monosaccharides, amino acids and nucleotides are examples of monomers.

In the overall condensation balance, molecules join by a covalent bond and water is removed. Hydrolysis uses water to split a bond. Cellular synthesis can involve activated intermediates beyond this school-level balance.

Glycosidic bonds join sugar residues, peptide bonds join amino acids, ester bonds join fatty acids to glycerol and phosphodiester bonds join nucleotides in a polynucleotide. Disulfide bonds can link cysteine residues within proteins. All of these are covalent bonds, much stronger than the hydrogen bonds that hold chains, or parts of chains, together.

Glucose, fructose and galactose are monosaccharides. Glucose is a hexose (six-carbon) monosaccharide; ribose and deoxyribose are pentose (five-carbon) monosaccharides. Disaccharides form by condensation of two monosaccharides: maltose is glucose + glucose, sucrose is glucose + fructose and lactose is glucose + galactose.

α-glucose and β-glucose are isomers that differ in the position of the hydroxyl group on carbon 1. In the usual ring diagram, this –OH is below the ring in α-glucose and above it in β-glucose.

Starch, the storage polysaccharide of plants, is a mixture of amylose and amylopectin, both made of α-glucose. Amylose is unbranched, with α-1,4 glycosidic bonds, and coils into a compact helix; amylopectin also has α-1,6 bonds, making it branched. Branching gives more chain ends where enzymes can act at the same time, so glucose can be released quickly. Packing many glucose units into a few large, compact molecules means storage has little effect on water potential compared with free glucose, and starch is poorly soluble.

Glycogen is the storage polysaccharide of animals. It is made of α-glucose joined by α-1,4 glycosidic bonds, with α-1,6 branch points; it is more highly branched than amylopectin, giving many chain ends where enzymes can act at the same time, so glucose can be released rapidly. Packing many glucose units into a few large, compact molecules means storage has little effect on water potential compared with free glucose.

Cellulose is a structural polysaccharide of β-glucose joined by β-1,4 glycosidic bonds; alternate residues are inverted, giving straight unbranched chains. Hydrogen bonds between many parallel chains form microfibrils, which give plant cell walls tensile strength.

A triglyceride contains one glycerol joined to three fatty acids by three ester bonds, formed in condensation reactions. Complete hydrolysis uses three water molecules and gives glycerol and three fatty acids. Triglycerides are hydrophobic, so they are stored with little water, and their hydrogen-rich fatty-acid chains give a high energy yield per gram when oxidised, making them good energy stores.

A saturated fatty acid has no carbon–carbon double bonds in its hydrocarbon chain; an unsaturated fatty acid has one or more C=C double bonds. The C=O in the carboxyl group does not count.

A typical glycerophospholipid has glycerol, two fatty-acid-derived tails and a phosphate-containing head. Its polar head is hydrophilic and its hydrocarbon tails are hydrophobic.

Cholesterol is a sterol: a lipid with four fused carbon rings, a short hydrocarbon tail and a small polar –OH group, so most of the molecule is hydrophobic. It sits within the phospholipid bilayer of animal cell membranes, where it regulates fluidity and stability, and it is the precursor of steroid hormones, vitamin D and bile salts.

An amino acid has a central carbon attached to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and a variable R group. Different amino acids differ in their R groups; about 20 common amino acids are used to build proteins.

The primary structure of a protein is the sequence of amino acids in its polypeptide chain. Two polypeptides with the same amino acids in a different order have different primary structures, and the sequence determines how the protein folds.

Secondary structure is the folding of parts of a polypeptide into α-helices or β-pleated sheets, held by hydrogen bonds between groups of the peptide backbone. It is not the whole 3-D shape.

Tertiary structure is the overall 3-D shape of a single polypeptide, held by interactions between R groups: hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bonds between cysteine residues.

Globular proteins, such as haemoglobin, insulin and enzymes, are compact and roughly spherical; most are soluble, with hydrophilic groups on the outside, and have metabolic roles. Haemoglobin is a conjugated protein: each of its polypeptide chains carries a non-protein haem group containing iron. Fibrous proteins are long and structural: collagen has great tensile strength, elastin recoils after being stretched, and keratin makes hair and nails strong and hard and the outer layer of the skin tough and waterproof.

Quaternary structure is the arrangement of two or more polypeptide chains in one protein, such as the four chains of haemoglobin. Proteins made of a single chain have no quaternary structure.

Water is a polar molecule: the oxygen atom has a slight negative charge and the hydrogen atoms slight positive charges. Hydrogen bonds form between the oxygen of one water molecule and a hydrogen of another.

Because it is polar, water dissolves ions and many polar molecules: the slightly negative oxygen ends face positive ions and the slightly positive hydrogen ends face negative ions. Non-polar substances such as oils do not dissolve well.

Water has a high specific heat capacity, so a large energy input causes only a small temperature change, buffering organisms against temperature changes. Its high latent heat of vaporisation means evaporation removes a lot of heat, giving a cooling effect. Hydrogen bonding causes both properties.

Hydrogen bonds between water molecules make them stick together (cohesion), which allows columns of water to be pulled up xylem and gives surface tension. Adhesion is the attraction of water to other materials.

Water is a metabolite: it is a reactant in hydrolysis reactions and a product of condensation reactions. Acting as a solvent is a different role, in which water is the medium rather than a reactant.

Inorganic ions have specific roles. Hydrogen ions determine pH (the higher the H⁺ concentration, the lower the pH); phosphate groups form part of DNA, RNA and ATP; iron is part of the haem group in haemoglobin, where oxygen binds.

Enzymes are biological catalysts: they speed up reactions by lowering the activation energy, without changing the overall reaction or being used up.

Some enzymes act inside cells (intracellular), for example catalase, which breaks down hydrogen peroxide. Others are secreted and act outside the cells that make them (extracellular), for example amylase in digestion.

Each enzyme’s active site has a specific shape and chemical properties complementary to its substrate, so only particular substrates can bind. Binding forms an enzyme–substrate complex; after reaction, an enzyme–product complex forms and the product is released, leaving the enzyme free to act again.

In the earlier lock-and-key model, the active site is a fixed shape complementary to the substrate. The induced-fit model refines it: the active site changes shape slightly as the substrate binds, moulding around it and positioning groups so the reaction happens more easily. The enzyme’s amino-acid sequence does not change.

Warming increases the kinetic energy of molecules, so enzyme–substrate collisions and complexes form more often and the rate rises. Above the optimum, heat disrupts the bonds holding the tertiary structure; the active site changes shape (denaturation), so fewer enzyme–substrate complexes form and the rate falls.

Changes in pH alter the charges on R groups in the active site and on the substrate, affecting binding and catalysis. Extreme pH disrupts ionic and hydrogen bonds in the tertiary structure, so the active site changes shape and the enzyme is denatured.

At low substrate concentration, adding substrate raises the rate because more enzyme–substrate complexes form; at high concentration the active sites are all occupied and the rate levels off. With substrate in excess, increasing enzyme concentration provides more active sites and raises the rate.

A competitive inhibitor has a shape similar to the substrate and binds to the active site, so fewer enzyme–substrate complexes form. Increasing the substrate concentration reduces the effect, and the maximum rate can still be reached.

A non-competitive inhibitor binds to the enzyme at a site other than the active site, changing the enzyme’s shape so that fewer substrate molecules can be converted to product (the school model describes the active site as no longer fitting the substrate). Increasing the substrate concentration does not restore the maximum rate.

A reversible inhibitor binds temporarily, so activity can return when the inhibitor is removed. A non-reversible inhibitor binds permanently, often by covalent bonds, so activity does not return.

In end-product inhibition, the final product of a metabolic pathway inhibits an enzyme earlier in the pathway. When product is plentiful, production slows; as the product is used up, inhibition decreases. It is a form of negative feedback.

Some enzymes need a non-protein cofactor to work. Inorganic ion cofactors include chloride ions for amylase. Coenzymes are small organic cofactors, often derived from vitamins, that carry groups or electrons between reactions. A prosthetic group is a non-protein group tightly (often permanently) bound to a protein, such as haem in haemoglobin.

A nucleotide is made of a pentose sugar, a phosphate group and a nitrogen-containing base. The sugar is deoxyribose in DNA and ribose in RNA.

DNA contains the bases adenine (A), thymine (T), cytosine (C) and guanine (G). RNA contains A, C and G, with uracil (U) instead of thymine.

DNA and RNA are polynucleotides. Nucleotides are joined by condensation reactions forming phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next, giving a sugar–phosphate backbone.

DNA usually consists of two complementary polynucleotide strands running in opposite (antiparallel) directions, held together by hydrogen bonds between bases and wound into a double helix. RNA is usually a single strand.

In DNA, adenine pairs with thymine and guanine pairs with cytosine, by hydrogen bonds between bases on opposite strands. This complementary base pairing holds the two strands together.

In semi-conservative replication, the two DNA strands separate and each acts as a template for a new complementary strand. Each new DNA molecule contains one original strand and one new strand.

In DNA replication, DNA helicase separates the two strands by breaking the hydrogen bonds between bases. Free nucleotides pair with the exposed bases, and DNA polymerase joins adjacent nucleotides into a new strand, using each original strand as a template.

ATP (adenosine triphosphate) is a nucleotide derivative made of adenine, ribose and three phosphate groups; ADP has two phosphate groups. Adenine plus ribose is adenosine.

ATP hydrolase catalyses the hydrolysis of ATP to ADP and inorganic phosphate (Pi), releasing energy that is used in coupled energy-requiring reactions. The phosphate can be added to other compounds (phosphorylation), making them more reactive, as when glucose is phosphorylated at the start of glycolysis.

ATP is resynthesised by a condensation reaction between ADP and inorganic phosphate, catalysed by ATP synthase, using energy from respiration or photosynthesis. ATP is the immediate energy source for cellular work such as active transport, muscle contraction and synthesis of molecules, and is used and remade continually.

The nucleus is surrounded by a nuclear envelope with pores, through which molecules such as mRNA leave. It contains chromosomes of linear DNA associated with proteins, and a nucleolus that makes ribosome subunits. Some specialised mature cells, such as mammalian red blood cells, lack a nucleus.

The cytoskeleton is a network of protein fibres, including microfilaments and microtubules, in the cytoplasm. It gives the cell mechanical strength and shape, holds some organelles in position, provides tracks for moving vesicles and organelles, and enables cell movement.

Centrioles are a pair of cylinders of microtubules at right angles in animal cells; they help organise the microtubules that separate chromosomes during cell division. Motile cilia and eukaryotic flagella are projections that typically contain a ring of nine pairs of microtubules around a central pair. Flagella move whole cells, and cilia can move a cell or move substances such as mucus across the cell surface.

Rough ER has ribosomes on its cytosolic surface and supports production and processing of proteins for secretion or membranes. Smooth ER lacks attached ribosomes and contributes to lipid synthesis.

The Golgi apparatus is a stack of flattened membrane sacs that modifies, sorts and packages proteins and lipids, for example adding carbohydrate to make glycoproteins. Vesicles carry the products to the cell-surface membrane or to lysosomes.

Ribosomes are made of RNA and protein and are the site of protein synthesis. They are found free in the cytoplasm or attached to rough ER, and are not surrounded by a membrane. Prokaryotic ribosomes are smaller than the ribosomes in eukaryotic cytoplasm.

Lysosomes are membrane-bound vesicles containing hydrolytic enzymes. They digest worn-out organelles and material taken into the cell, such as pathogens engulfed by phagocytes; the membrane keeps the enzymes separate from the cytoplasm.

Mitochondria have a double membrane; the inner membrane is folded into cristae, around a matrix. They are the site of the later stages of aerobic respiration and produce most of the cell’s ATP, and contain their own DNA and ribosomes.

Chloroplasts are the site of photosynthesis in plant and algal cells. They have a double-membrane envelope around the stroma, which contains thylakoid membranes stacked into grana. Not all plant cells contain chloroplasts.

Plant cells have a cellulose cell wall outside the cell-surface membrane, which supports the cell. A large vacuole surrounded by the tonoplast stores cell sap; when water enters, the protoplast presses the cell-surface membrane against the wall, making the cell turgid.

A typical bacterial cell has a cell wall containing peptidoglycan (murein), a cell-surface membrane, cytoplasm, small ribosomes and chromosomal DNA in a nucleoid, usually a single circular molecule. It lacks a membrane-enclosed nucleus and membrane-bounded organelles. A few bacteria lack walls, and structures such as plasmids, capsules and flagella vary between bacteria.

Viruses are acellular particles. A virus has a DNA or RNA genome inside a protein capsid, and some have a lipid envelope; attachment proteins on the surface bind receptors on host cells. Viruses have no ribosomes and replicate only inside host cells.

Specialised cells are adapted for particular functions. A tissue is a group of cells, often of one main type and sometimes with extracellular material, working together to perform a function; an organ contains different tissues working together, and an organ system is a group of organs working together.

Magnification is how many times larger the image is than the real object (image size ÷ actual size, in the same units). Resolution is the minimum distance at which two points can be distinguished as separate. Enlarging an image does not improve its resolution.

Electrons have a much shorter wavelength than light, so electron microscopes have much higher resolution. A transmission electron microscope (TEM) passes electrons through a very thin specimen, showing internal structure; a scanning electron microscope (SEM) scans the surface, giving a 3-D surface image. Electron microscopes need a vacuum and preparation kills the specimen, so specimens are non-living, and preparation can introduce artefacts. Light microscopes can show living cells; any colour in an electron micrograph is added afterwards.

Stains increase the contrast between transparent structures and their surroundings, so cells and their parts can be seen more clearly; different stains can pick out different structures. Staining does not enlarge the specimen or change the magnification.

In cell fractionation, tissue is homogenised in a cold, isotonic, buffered solution: cold slows enzymes that damage organelles, isotonic prevents osmotic damage and a buffer keeps pH stable. Differential centrifugation then spins the homogenate at increasing speeds; the largest, heaviest organelles, such as nuclei, sediment first, and the supernatant is spun again at higher speed to pellet smaller organelles such as mitochondria.

Cell membranes are a phospholipid bilayer, with hydrophilic heads facing outwards and hydrophobic tails inwards, in which proteins are embedded. Many components move sideways (fluid), and the proteins are scattered like a mosaic. Cholesterol among the phospholipid tails regulates membrane fluidity and stability. Glycoproteins and glycolipids carry surface carbohydrate used in cell recognition and signalling. Membranes separate the cell and its compartments from their surroundings and control exchange, and membrane receptor proteins bind specific signalling molecules such as hormones.

Diffusion is the net movement of particles down a concentration gradient, from a higher to a lower concentration; it is passive. Individual molecules keep moving randomly in both directions, so at equilibrium the opposing movements are equal rather than stopped.

Facilitated diffusion is the passive movement of substances down their concentration gradient through channel or carrier proteins in the membrane. It needs no ATP, and the proteins are specific; at high concentration gradients its rate levels off, because it is limited by the number of channel or carrier proteins.

Osmosis is the net movement of water through a partially permeable membrane from a higher to a lower water potential; it is passive. At the same (atmospheric) pressure, pure water has the highest water potential, 0 kPa, and adding solute lowers it, so solutions have negative water potentials.

In a solution with a higher water potential than the cell, a plant cell takes up water by osmosis and becomes turgid; its wall stops it bursting. In a solution with a lower water potential than the cell, it loses water, and the protoplast can pull away from the wall (plasmolysis). Animal cells have no wall, so they swell and may burst (lyse) in a solution with a higher water potential than the cell and shrink in one with a lower water potential.

Small, non-polar (lipid-soluble) molecules cross the phospholipid bilayer more easily than ions or large polar molecules, so size, charge and lipid solubility all affect permeability. High temperatures and solvents such as ethanol disrupt membrane structure and increase permeability.

Active transport moves substances against their concentration gradient, using ATP and carrier proteins. ATP hydrolysis changes the shape of the carrier so it moves the molecule across the membrane.

In co-transport, one solute moving down its concentration gradient drives another solute’s uphill movement through the same carrier protein. In ileal epithelial cells, sodium entering down its gradient carries glucose in with it; a sodium–potassium pump using ATP keeps intracellular sodium low, so the co-transporter itself need not hydrolyse ATP.

In endocytosis, the cell-surface membrane folds around material and pinches off as a vesicle, bringing it into the cell. In exocytosis, a vesicle fuses with the cell-surface membrane and releases its contents outside. Both are active processes that need ATP.

The cell cycle consists of interphase and mitosis (followed by cytokinesis). Interphase has three stages: G1 (growth), S (DNA replication) and G2 (further growth and preparation for division).

Mitosis separates the sister chromatids of each replicated chromosome into two genetically identical daughter nuclei, assuming accurate copying and no mutation. It is used for growth, repair and asexual reproduction.

In prophase, chromosomes condense, the nuclear envelope breaks down and the spindle forms. In metaphase, chromosomes line up at the equator, attached to spindle fibres at their centromeres. In anaphase, sister chromatids separate and are pulled to opposite poles. In telophase, nuclear envelopes re-form around the two groups of chromosomes. Cytokinesis then divides the cytoplasm.

Checkpoints in the cell cycle normally stop a cell moving on to the next stage until conditions are right. If this regulation is lost, cells can go on dividing and form a tumour, which may be benign or malignant (cancerous). Fast growth of normal tissue is not, on its own, cancer. Many cancer treatments, such as chemotherapy, kill or slow fast-dividing cells, so they can also harm healthy cells that divide quickly.

Meiosis follows one round of DNA replication with two nuclear divisions, producing four haploid cells from one diploid cell. Homologous chromosomes separate in meiosis I and sister chromatids separate in meiosis II.

A diploid cell has two sets of chromosomes; a haploid cell has one set. DNA replication doubles the DNA content but does not change the number of sets.

Prokaryotic cells divide by binary fission: the circular DNA and any plasmids replicate, then the cytoplasm divides into two daughter cells, each with one copy of the circular DNA and a variable number of plasmids.

A stem cell is an undifferentiated cell that can divide to renew itself and can give rise to specialised cells. During differentiation, cells express different subsets of their genes, so they make different proteins while keeping the same genome.

Some uses of stem cells are established: stem-cell (bone-marrow) transplants restore blood-forming cells in leukaemia and some other blood disorders. Others are still being developed, such as replacing damaged or lost cells to repair tissues, including in neurological conditions such as Parkinson’s disease; how well transplanted cells integrate with tissue varies. Stem cells are also used in research, for example on how cells develop.

In mammalian development, totipotent cells can give rise to the embryo and its supporting extraembryonic tissues. Pluripotent cells can give rise to all cell types of the body (embryo proper) but not the full set of extraembryonic tissues. Multipotent cells form a limited range of related cell types; unipotent cells form a single type.

An antigen is a molecule, usually a protein, that the immune system can recognise. Antigens occur on cells and pathogens, and some are soluble, such as toxins. Non-self antigens trigger an immune response; the body normally does not respond to its own (self) antigens.

Non-specific defences, such as skin, mucus and phagocytosis, act in the same way against any pathogen. The specific immune response uses lymphocytes that recognise particular antigens, and it produces immunological memory.

A phagocyte binds and engulfs a pathogen into a vesicle called a phagosome. Lysosomes fuse with it and their hydrolytic enzymes destroy the pathogen. Phagocytes can then present antigen fragments at their surface to helper T cells.

Helper T cells bind to antigens presented by other cells and release chemical signals that stimulate B cells to divide, cytotoxic T cells to act and phagocytes to engulf pathogens. They coordinate the response but do not make antibodies.

Cytotoxic T cells recognise antigens presented on the surface of infected or abnormal body cells and kill those cells. They kill infected cells rather than engulfing pathogens or making antibodies.

In clonal selection, an antigen binds the few lymphocytes that already have complementary receptors. These cells divide rapidly by mitosis (clonal expansion), producing many identical cells specific to that antigen.

Activated B cells divide to form plasma cells, which secrete large amounts of a specific antibody, and memory B cells, which remain in the body and respond rapidly to a later infection by the same antigen.

The first response to an antigen (primary response) is slow. Memory cells formed during it persist, so a later exposure to the same antigen produces a faster and larger secondary response.

A typical antibody is a Y-shaped protein (a conventional monomer) made of four polypeptide chains: two heavy and two light, held together by disulfide bonds. Each monomer has two antigen-binding sites in the variable regions at the tips, complementary to a specific antigen; the rest is the constant region. Antibodies bind antigens to form antigen–antibody complexes, and agglutination clumps bacteria together for phagocytosis.

Antibodies can act as opsonins, coating pathogens so phagocytes recognise and engulf them more easily; as agglutinins, cross-linking pathogens into clumps; and as antitoxins, binding toxins so they cannot act on cells.

Active immunity is produced when a person’s own immune system makes antibodies and memory cells, after infection (natural) or vaccination (artificial); because memory cells are made, it lasts much longer than passive immunity, although it can wane. Passive immunity is the transfer of ready-made antibodies, across the placenta or in breast milk (natural) or by injection (artificial); it is short-lived, with no memory cells.

A vaccine introduces antigens (for example in killed or weakened pathogens, or isolated antigens), or instructions for cells to make an antigen, triggering a primary immune response without causing the disease. Memory cells formed then give a faster, larger secondary response on later exposure to the same antigen.

If a pathogen’s antigens change shape, existing memory cells and antibodies may no longer recognise them. A person can then be infected again, and new vaccines may be needed.

When enough of a population is immune, transmission falls, which indirectly protects people who are not immune by limiting spread. Herd immunity does not transfer antibodies between people or guarantee protection, and the coverage needed depends on the pathogen, the vaccine and contact patterns.

In an autoimmune disease, the immune system attacks the body’s own cells or tissues (self antigens), through T cells, antibodies or both. Rheumatoid arthritis, in which the immune system attacks the joints, is an example.

HIV has a lipid envelope with attachment proteins, a capsid containing two RNA molecules, and enzymes including reverse transcriptase. It binds CD4 on helper T cells; reverse transcriptase makes DNA from the viral RNA; this viral DNA is inserted into the host DNA and used to make new viral RNA and proteins, which assemble into new virus particles that bud from the cell. Loss of helper T cells weakens the immune system, increasing opportunistic infections.

Monoclonal antibodies come from a single clone of B cells, so all bind the same antigen site. They can target drugs to particular cells or detect antigens. In an ELISA, an antibody linked to an enzyme binds the target, and the enzyme’s substrate gives a coloured signal; washing removes unbound antibody.

Pathogens include bacteria (causing, for example, tuberculosis and potato ring rot), viruses (for example HIV, influenza viruses and tobacco mosaic virus), fungi (causing, for example, athlete’s foot and black sigatoka) and protoctists (for example Plasmodium, which causes malaria, and the organism causing potato/tomato late blight).

Pathogens spread in different ways. Tuberculosis can spread through the air in droplets; malaria is usually transmitted by infected female Anopheles mosquitoes carrying Plasmodium; tobacco mosaic virus (TMV) can spread mechanically in contaminated sap on hands or tools. Transmission can be direct (contact or droplets) or indirect (through a vector such as a mosquito, spores carried by wind or water, or contaminated material). Climate and living conditions affect how readily pathogens spread.

The skin and mucous membranes form barriers to pathogens. Mucus traps microorganisms, cilia sweep it away from the lungs, and reflexes such as coughing and sneezing expel it.

When the skin is broken, platelets release substances that trigger a cascade forming fibrin threads, which trap platelets and blood cells in a clot that seals the wound. Inflammation follows: local blood vessels dilate and become more permeable, so phagocytes and other defences reach the site, and the tissue is then repaired.

Plants resist pathogens with physical barriers such as waxy cuticles and cell walls, and respond to infection by depositing callose to block spread and producing antimicrobial chemicals. These defences reduce, but do not always prevent, infection.

Antibiotics act on structures and processes that bacteria have, such as cell-wall synthesis or bacterial ribosomes. Viruses such as HIV have none of these and replicate inside host cells, so antibiotics are ineffective against them.

Antibiotics kill or inhibit bacteria by acting on bacterial structures or processes, such as cell-wall synthesis or bacterial ribosomes. Because viruses lack these targets, antibiotics do not treat viral infections.

Penicillin interferes with bacterial cell-wall construction in susceptible, growing bacteria. Human cells have no peptidoglycan wall, so this target is absent from them; susceptibility differs between bacterial strains.

In a bacterial population, a mutation or acquired gene can make some bacteria resistant to an antibiotic. When the antibiotic is used, susceptible bacteria die and resistant ones survive and reproduce, so resistance becomes more common. The antibiotic does not cause the mutation.

Many medicines originate from organisms: penicillin came from a mould and paclitaxel from the Pacific yew. A natural extract still needs testing for safety and effectiveness. Personalised medicine uses an individual’s biological differences, such as genetic markers, to choose treatment. Conserving biodiversity keeps species and chemicals that may lead to future medicines.