Botswana BGCSE Science Double Award (0569) key terms

The thin outer layer of a cell that controls which substances enter and leave. It is partially permeable, allowing small molecules such as oxygen and water through while restricting others.

The random movement of particles from a region of higher concentration to a region of lower concentration, continuing until the concentrations are equal. It needs no energy input from the cell.

The movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution. It explains why plant cells gain or lose water in different surroundings.

A protein made by living cells that speeds up a chemical reaction without being used up. Each enzyme works best at a particular temperature and pH and acts on one specific substrate.

The process in green plants that uses light energy to combine carbon dioxide and water into glucose, releasing oxygen as a by-product. Chlorophyll in chloroplasts absorbs the light needed.

The green pigment found in chloroplasts that absorbs mainly red and blue light for photosynthesis. Light reflected by it gives leaves their familiar green appearance.

The release of energy from glucose inside living cells, using oxygen in aerobic respiration and producing carbon dioxide and water. Anaerobic respiration releases less energy and forms lactic acid or ethanol.

The firm, swollen state of a plant cell when water has entered its vacuole and pushed the cytoplasm against the cell wall. Loss of turgor makes plants wilt.

One of many tiny air sacs in the lungs where gas exchange takes place. Each has a thin, moist wall and a rich blood supply so oxygen and carbon dioxide diffuse quickly.

Finger-like projections lining the small intestine that greatly increase the surface area for absorbing digested food. Each contains blood vessels and a lacteal to carry nutrients away.

The keeping of a stable internal environment within narrow limits, for example constant body temperature and blood glucose. Nervous and hormonal control systems detect changes and correct them.

A chemical messenger produced by a gland and carried in the blood to a target organ, where it alters that organ's activity. Insulin and adrenaline are examples.

A control mechanism in which a change from the normal level triggers a response that reverses the change. It keeps conditions such as temperature and blood sugar steady.

A microorganism, such as a bacterium, virus or fungus, that causes disease when it enters the body. The body defends itself with skin, mucus, white blood cells and antibodies.

A protein produced by white blood cells in response to a specific antigen on a pathogen. It attaches to that antigen and helps destroy the invading organism.

The introduction into the body of weakened or harmless forms of a pathogen so that antibodies and memory cells are made. Later infection is then resisted quickly.

A sequence showing how energy passes from one organism to the next through feeding, beginning with a producer. Arrows point in the direction in which energy flows.

An organism, usually a green plant, that makes its own food by photosynthesis and so starts every food chain. It supplies energy to all the consumers that follow it.

An organism such as a bacterium or fungus that breaks down dead material and waste, returning mineral nutrients to the soil. Decomposers keep nutrients circulating in an ecosystem.

The enrichment of water with nutrients, often from fertiliser runoff, which causes rapid algal growth. Decay of the algae then removes oxygen and kills fish and other animals.

The protection and careful management of natural resources and wildlife so that habitats and species survive for the future. It includes laws, reserves and sustainable use of land.

The smallest particle of an element that can take part in a chemical reaction. It consists of a nucleus of protons and neutrons surrounded by electrons.

An atom or group of atoms that has gained or lost electrons, giving it an overall electric charge. Positive ions form when electrons are lost and negative ions when they are gained.

The strong electrostatic attraction between oppositely charged ions, formed when electrons are transferred from a metal to a non-metal. Ionic compounds have high melting points and conduct when molten.

A bond formed when two atoms share one or more pairs of electrons, usually between non-metals. Covalent substances are often gases, liquids or soft solids with low melting points.

The amount of a substance that contains 6.02 x 10^23 particles, equal to its relative formula mass in grams. It allows masses of reactants and products to be compared.

A starting substance in a chemical reaction that is used up as new substances are formed. Its mass, together with that of the other reactants, equals the mass of the products.

A reaction that releases energy to the surroundings, usually as heat, so the temperature rises. Burning fuels and neutralisation are common examples.

A substance that releases hydrogen ions in water, turns blue litmus red and reacts with bases to form a salt and water. Acids have a pH below seven.

A substance that reacts with an acid to form a salt and water only. Soluble bases are called alkalis and have a pH above seven.

The reaction between an acid and a base that produces a salt and water, bringing the pH towards seven. It is used to treat acid spills and to relieve indigestion.

The breaking down of an ionic compound by passing an electric current through it when molten or in solution. Positive ions go to the cathode and negative ions to the anode.

A rock containing enough of a metal compound, usually an oxide or sulphide, to make extracting the metal worthwhile. Ores are mined and then reduced or electrolysed.

A reaction in which a substance loses oxygen or gains electrons. In metal extraction, ores such as iron oxide are reduced with carbon to obtain the metal.

The single force that has the same effect as all the forces acting on an object combined. It determines whether the object speeds up, slows down or stays at constant velocity.

The rule that the acceleration of an object is directly proportional to the resultant force on it and inversely proportional to its mass. It is written as force equals mass times acceleration.

The distance between two neighbouring points in phase on a wave, such as from one crest to the next. It is measured in metres and relates to frequency and speed.

The number of complete waves passing a point each second, measured in hertz. A higher frequency means a shorter wavelength for a wave of the same speed.

The change in direction of a wave as it passes from one medium into another of different speed. Light bends towards the normal when entering a denser medium.

The statement that the current through a metallic conductor is directly proportional to the potential difference across it, provided temperature stays constant. It gives voltage equals current times resistance.

A circuit in which components are connected one after another so that the same current flows through each. The total resistance is the sum of the individual resistances.

The time taken for half the unstable nuclei in a radioactive sample to decay. It may range from fractions of a second to millions of years and is unaffected by temperature.

A quantity in an experiment that can change or be changed. Independent variables are set by the experimenter, dependent variables are measured, and others must be controlled.

An experiment in which only the independent variable is changed while all other variables are kept the same. This makes the results valid and allows a reliable conclusion.

How close a measured value is to the true value. It is improved by using suitable instruments, avoiding parallax error and repeating readings where possible.