Glossary

Temperature measured in kelvin from absolute zero. To change a Celsius temperature to kelvin, add 273.

The movement of nutrients from the intestine into the blood or lymph. Large food molecules must be digested first; small molecules such as glucose can be absorbed without being broken down.

The change in velocity per unit time. An object accelerates if its speed or direction changes.

Changing the shape of the lens in the eye to focus on objects at different distances.

The minimum energy that colliding particles need for a reaction to happen.

The part of an enzyme where the substrate fits and the reaction happens.

Cells use energy from respiration to move particles through a membrane from a lower concentration to a higher concentration.

Respiration that uses oxygen to release energy from glucose. Its products are carbon dioxide and water.

A tiny air sac in a lung where gas exchange takes place. Its thin wall and nearby capillaries let gases diffuse between air and blood.

The greatest distance a vibrating point moves from its rest position. It is measured from the middle, not from crest to trough.

Respiration that releases energy from glucose without using oxygen. It releases less energy per glucose molecule than aerobic respiration.

A signal that varies continuously. A microphone can produce an electrical signal that follows changes in sound pressure.

A blood vessel that carries blood away from the heart. Most arteries carry oxygen-rich blood, but the pulmonary artery carries oxygen-poor blood to the lungs.

The smallest particle of an element that keeps its chemical identity. It has a nucleus surrounded by electrons.

The number of particles in one mole: about 6.02 × 10²³ per mole.

A diet that supplies enough energy and the right amounts of nutrients, fibre and water for a person's needs.

An equation with the same number of atoms of each element on both sides. Atoms are rearranged during a reaction, not created or destroyed.

Forces whose resultant is zero. An object with balanced forces stays at rest or continues moving at constant velocity.

A change from liquid to gas throughout the liquid, with bubbles forming. It happens at the boiling point for the surrounding pressure.

The temperature at which a liquid boils at a particular pressure. Changing the pressure can change its boiling point.

The energy needed to break one mole of a particular bond in gaseous molecules. Breaking bonds takes in energy; making bonds releases it.

The random, jerky movement of tiny visible particles in a liquid or gas. It is caused by uneven collisions with the much smaller particles of the liquid or gas.

A very small blood vessel with a wall one cell thick. Substances pass between the blood and body tissues through its wall.

A substance that speeds up a reaction without being used up overall. It provides a reaction route with a lower activation energy.

The smallest living unit of an organism. Some organisms have one cell; others have many cells working together.

A thin boundary around a cell that controls which substances enter and leave it.

A layer outside the cell membrane that supports the cell. In plants, it is made of cellulose.

The point through which an object's weight acts. A freely hanging object settles with this point vertically below its support.

A change in which atoms are rearranged to make new substances. The new substances have different properties.

Breaking large, insoluble food molecules into smaller, soluble molecules. Enzymes carry out these reactions.

Symbols and numbers that show which elements are in a substance and their ratio. H₂O shows two hydrogen atoms for each oxygen atom.

The green pigment in chloroplasts that absorbs light energy for photosynthesis.

A structure in plant cells that contains chlorophyll and is where photosynthesis happens.

Separating substances because they travel at different speeds with a solvent through a material such as paper.

A substance made of two or more elements chemically joined in a fixed ratio. A compound has different properties from its elements.

A change in a substance's concentration from one place to another. For the same distance, a larger concentration difference gives a steeper gradient.

A change from gas to liquid when particles lose energy and come closer together.

Energy transfer through a material by interactions between neighbouring particles. In metals, moving electrons also carry energy.

A factor you keep the same so that it does not affect the comparison in an investigation.

Energy transfer by the movement of a liquid or gas. Warmer, less dense fluid rises and cooler, denser fluid moves in to replace it.

A disease in which the arteries supplying the heart muscle become narrowed or blocked. Less oxygen reaches the muscle, which can damage it.

A bond formed when atoms share a pair of electrons.

For light travelling from a material with a higher refractive index to one with a lower index, the angle of incidence that gives a refracted ray at 90° to the normal.

Making solid crystals from a solution. Removing some solvent or cooling the solution can make dissolved particles join the growing crystals.

The jelly-like material inside a cell where many chemical reactions happen.

A protein losing its normal folded shape, for example because of high temperature or extreme pH. In an enzyme, the active site changes so the substrate no longer fits and the enzyme stops working properly.

Mass per unit volume: how much mass is packed into a certain space. Divide an object's mass by its volume to find its density.

A way to identify an organism by choosing between two descriptions at each step. Each choice leads to another pair or to a name.

Particles move randomly. Diffusion is their overall movement from where their concentration is higher to where it is lower.

Particles move randomly. Diffusion is their overall movement from where their concentration is higher to where it is lower.

A signal sent using separate levels, usually represented by 0 and 1. Small amounts of interference can be removed when the signal is regenerated.

Separating substances by boiling a liquid and then cooling its vapour to collect a liquid. Dissolved solids stay behind when a solvent is distilled.

A circulation in which blood passes through the heart twice on one complete journey: once on the way to the lungs and once on the way to the body.

In a closed system, the forward and reverse reactions happen at the same rate. Reactions keep happening, but the amounts of reactants and products stay constant.

A muscle or gland that produces a response. A muscle may contract, or a gland may release a substance.

The fraction of the input energy or power that becomes useful output. Multiply this fraction by 100 to give a percentage.

A wave that can travel through a vacuum. All electromagnetic waves travel at the same speed in a vacuum, but their frequencies and wavelengths can differ.

A very small particle with one negative charge. Electrons occupy shells around an atom's nucleus.

A substance made of only one type of atom. It cannot be broken down into simpler substances by chemical reactions.

A reaction that takes in energy from the surroundings. The surroundings usually get cooler.

A protein that speeds up a chemical reaction in living things without being used up. Each enzyme works on particular substances.

A state in which there is no resultant force and no resultant turning effect on an object. It does not speed up or start turning.

A change from liquid to gas at the liquid's surface. It can happen below the boiling point as some particles escape.

A liquid cools when its faster-moving particles escape from the surface. The particles left behind have less average kinetic energy, so the temperature falls.

The removal of waste made by chemical reactions in the body, substances in excess and toxic substances. Removing undigested food is egestion, not excretion.

A reaction that transfers energy to the surroundings. The surroundings usually get warmer.

How much longer a spring becomes when it is stretched. Subtract the original length from the stretched length.

Material in plant food that humans cannot digest. It helps food move through the gut and helps prevent constipation.

The liquid that passes through the filter paper during filtration.

Separating an insoluble solid from a liquid using a filter. The liquid passes through while the solid stays behind.

For a converging lens, the distance from the lens to the point where rays parallel to its principal axis meet.

Coal, oil or natural gas, formed from ancient living material over millions of years. Burning it transfers energy from a chemical store.

Separating a mixture of liquids with different boiling points using a fractionating column. Repeated evaporation and condensation improve the separation.

The number of complete waves passing a point each second. Frequency is measured in hertz; 1 Hz means one wave each second.

A state of matter with no fixed shape or volume. Its particles move freely and spread out to fill their container.

The movement of oxygen and carbon dioxide between an organism and its surroundings. In the lungs, these gases diffuse between the air in the alveoli and the blood.

The force per unit area caused by gas particles hitting a surface. More frequent or harder collisions can increase the pressure.

A large network of atoms joined by covalent bonds. Diamond and graphite have this kind of structure.

Energy an object has because of its position in a gravitational field. Lifting an object increases this energy.

The substance in red blood cells that binds to oxygen and carries it around the body.

The number of heartbeats in one minute. It usually rises during exercise to supply working muscles with more oxygen and glucose.

Keeping the body's internal conditions near their normal values. Blood glucose concentration and body temperature are two examples.

A chemical messenger made by a gland and carried in the blood. It changes the activity of cells that can respond to it.

The change in an object's momentum. For a constant force, impulse is force multiplied by the time it acts.

The factor you deliberately change in an investigation to find out what effect it has.

A hormone made by the pancreas that lowers blood glucose concentration when it is too high.

The total energy of an object's particles, from their motion and from the forces between them. Heating can increase this energy without always raising the temperature.

An atom or group of atoms with an electric charge because it has lost or gained electrons.

The strong attraction between oppositely charged ions.

The coloured part of the eye that changes pupil size and controls how much light enters.

Atoms of the same element with different numbers of neutrons. They have the same number of protons.

The energy an object has because it is moving. A greater mass or speed gives it more kinetic energy.

The point beyond which a spring's extension is no longer proportional to the applied force.

A factor that holds back the rate of a process because there is too little of it. Increasing a different factor will not help until this shortage is reduced.

The reactant that runs out first and limits how much product can form.

A state of matter with a fixed volume that takes the shape of its container. Its particles are close together but can move past one another.

A wave in which vibrations are parallel to the direction the wave travels. Sound in air is one example.

How many times larger an image is than the real object. Divide the image size by the real size, using the same units.

A measure of the amount of matter in an object. It is measured in kilograms and does not change just because the object moves to a different planet.

A change from solid to liquid. Energy is taken in as particles become able to move past one another.

The curved surface of a liquid in a container. For water in a measuring cylinder, read the bottom of the curve at eye level.

The strong attraction between positive metal ions and electrons that can move through the metal.

Two or more substances together without being chemically joined. Their amounts can vary, and physical methods can separate them.

An amount containing about 6.02 × 10²³ particles, such as atoms, molecules or ions. Always say which particles you are counting.

A separate group of two or more atoms joined by covalent bonds, with no overall electric charge. Its atoms can be the same element, as in O₂, or different elements, as in H₂O.

The turning effect of a force about a pivot. Multiply the force by the perpendicular distance from the pivot to the force's line of action.

Mass multiplied by velocity. Momentum has a direction, so direction matters when adding the momenta of different objects.

Light of a single frequency. In visible light, this corresponds to a single colour.

A control process that detects a change and produces a response that reduces that change. It helps keep internal conditions near their normal values.

A particle in an atom's nucleus with no electric charge. Most atoms have neutrons, but an ordinary hydrogen atom does not.

An energy resource that is used faster than it is replaced. Fossil fuels and nuclear fuels are examples.

An imaginary line at right angles to a surface at the point where a ray meets it. Angles of incidence and reflection are measured from this line.

The total number of protons and neutrons in an atom's nucleus. It is also called the mass number.

The part of a plant or animal cell that contains genetic material and controls the cell's activities.

An individual living thing, such as a bacterium, a plant or an animal.

The overall movement of water through a partially permeable membrane from higher water potential to lower water potential. With ordinary dilute solutions at the same pressure, water moves from the more dilute side towards the more concentrated side.

An error caused by looking at a scale from the side. Put your eye level with the mark to avoid it.

A membrane that lets some particles pass through but stops others. Water can pass through it even when some dissolved substances cannot.

A way to explain matter using tiny particles. How the particles are arranged and move helps explain the properties of solids, liquids and gases.

The unit of pressure. One pascal is one newton of force acting normally over an area of one square metre.

The amount of product actually obtained divided by the theoretical yield, multiplied by 100. Compare amounts using the same units.

The time taken for one complete oscillation. A pendulum's period is the time for one whole swing out and back.

Plant tissue that carries sucrose and amino acids from parts that release them (sources) to parts that use or store them (sinks).

Plants use light energy to make glucose from carbon dioxide and water. Oxygen is released, and chlorophyll absorbs the light.

A change that does not make a new substance, such as melting or dissolving.

Breaking food into smaller pieces without changing its molecules. Chewing is an example; the smaller pieces give enzymes more surface to act on.

How high or low a sound seems. A higher frequency gives a higher pitch.

The liquid part of blood. It carries blood cells and dissolved substances, such as glucose, carbon dioxide and urea.

A small piece of a cell that helps blood clot and stops blood escaping from a damaged vessel.

The rate of energy transfer or the rate of doing work. One watt means one joule transferred each second.

Force per unit area. The same force produces a greater pressure when it acts over a smaller area.

An enzyme that breaks down proteins into smaller molecules. During digestion, protein is eventually broken down into amino acids.

A large molecule made from amino acids. Proteins are needed for growth and repair; enzymes are proteins too.

A particle in an atom's nucleus with one positive charge. The number of protons decides which element the atom is.

Motion with no fixed direction or repeated path. Gas particles keep changing direction when they collide.

How quickly reactants are used up or products are made. A rate compares the amount changed with the time taken.

An image formed where light rays actually meet. It can be projected onto a screen.

A cell or group of cells that detects a stimulus. For example, light receptors in the eye detect light.

A sugar, such as glucose, that gives a positive Benedict's test. When heated with Benedict's solution, it can produce a coloured solid.

A quick, automatic response to a stimulus. You do not have to decide to make it happen.

A change in a wave's speed as it enters another material. Its direction usually changes too, except when it enters along the normal.

A number that compares the speed of light in a vacuum with its speed in a material. A larger refractive index means light travels more slowly in that material.

The average mass of an element's atoms compared with one twelfth of the mass of a carbon-12 atom. The average takes account of the different amounts of its isotopes.

The sum of the relative atomic masses of all the atoms shown in a formula. It has no unit.

An energy resource that is replenished naturally as it is used, such as sunlight or wind. Renewable does not mean it has no environmental effects.

The process that produces new organisms of the same kind.

The solid left on the filter paper after filtration.

Chemical reactions in cells that break down nutrient molecules and release energy. Breathing moves air into and out of the lungs; respiration releases energy in cells.

The single force that has the same effect as all the forces on an object together. Add forces with their directions taken into account.

A reaction that can happen in both directions. Products can react to form the starting substances again.

In chromatography, the distance a substance travels divided by the distance the solvent front travels. Measure both distances from the starting line.

A solution that cannot dissolve any more of a particular solute at that temperature.

The ability to detect and respond to changes inside or outside an organism. A plant growing towards light is one example.

A state of matter with a fixed shape and volume. Its particles are close together and vibrate around fixed positions.

A substance that dissolves in a solvent. In salt water, the salt is the solute.

A mixture formed when a solute dissolves in a solvent. The dissolved particles are spread throughout the solvent.

The liquid that dissolves a solute to make a solution. In salt water, water is the solvent.

The furthest point reached by the solvent during chromatography. Mark it before the solvent evaporates.

A group of organisms that can reproduce with one another to produce fertile offspring. Fertile offspring can also reproduce.

The energy needed to raise the temperature of 1 kg of a substance by 1 °C. Different substances need different amounts of energy.

The distance travelled per unit time. For example, 5 m/s means travelling 5 metres each second.

A measure of a spring's stiffness. Where force is proportional to extension, divide force by extension to find the spring constant.

A large carbohydrate made from many glucose units joined together. Plants use it to store glucose; iodine solution turns blue-black when starch is present.

A change that an organism detects and responds to, such as a bright light or a hot surface.

A tiny pore in a leaf through which gases pass. Guard cells control how far it opens; the plural is stomata.

The substance that an enzyme acts on. It fits into the enzyme's active site.

The constant velocity reached by a falling object when air resistance balances its weight. The resultant force and acceleration are then zero.

The greatest amount of product predicted from the starting amounts in a reaction. The amount actually collected may be smaller.

An increase in the size of a substance when its temperature rises. Its particles usually move further apart; the particles themselves do not grow.

Energy transfer by electromagnetic waves, mainly infrared at everyday temperatures. It can travel through a vacuum, so no material is needed between the objects.

Complete reflection of light back into a material. It happens only towards a material with a lower refractive index and with an incidence angle greater than the critical angle.

The movement of sucrose and amino acids in the phloem from sources to sinks. Sources release these substances; sinks use or store them. Stored food can supply a growing shoot.

The loss of water vapour from a plant's leaves. Water evaporates inside the leaf and the vapour diffuses out through the stomata.

A wave in which vibrations are at right angles to the direction the wave travels. Ripples on water are a familiar example in the wave model.

Sound with a frequency above 20 000 Hz, beyond the upper limit of typical human hearing.

A waste substance made in the liver when excess amino acids are broken down. The kidneys remove it from the blood, and it leaves the body in urine.

A structure that stops blood flowing backwards. Heart valves keep blood moving through the heart in the right direction.

A blood vessel that carries blood towards the heart. Many veins have valves that stop blood flowing backwards.

Speed in a particular direction. Velocity changes if the speed or the direction changes.

Moving air into and out of the lungs. This replaces air so that gas exchange can continue.

An animal with a backbone. Mammals, birds, reptiles, amphibians and fish are vertebrates.

A small finger-like projection from the lining of the small intestine. Villi provide a large surface for absorbing digested nutrients.

An image formed where light rays appear to come from, although they do not actually meet there. It cannot be projected onto a screen.

A measure that helps predict which way water will move. Across a partially permeable membrane, water moves overall from higher water potential to lower water potential. At the same temperature and pressure, a dilute solution has a higher water potential than a more concentrated solution.

The distance between two neighbouring points at the same stage of a wave, such as one crest and the next.

The force of gravity on an object. It is measured in newtons and depends on the local gravitational field strength.

Energy transferred when a force moves an object. Work is the force multiplied by the distance moved in the direction of that force.

Plant tissue that carries water and mineral ions from the roots to the rest of the plant. It also helps support the plant.