GCE O/L Science key terms

The smallest unit of a living organism, bounded by a membrane and containing cytoplasm and genetic material. Some cells also contain a nucleus and specialised structures called organelles that carry out particular jobs.

A specialised structure inside a cell that performs a particular function, such as a nucleus controlling activities, chloroplasts carrying out photosynthesis, or mitochondria releasing energy during respiration.

A thin partially permeable layer surrounding a cell that controls which substances enter and leave. Water and small dissolved particles pass through it, while larger molecules are restricted.

The net movement of particles from a region of higher concentration to a region of lower concentration, caused by random motion. It needs no energy input from the cell and continues until concentrations are even.

The movement of water through a partially permeable membrane from a dilute solution to a more concentrated one. It is a passive process important for water uptake by roots and for keeping cells firm.

Movement of dissolved substances across a membrane from lower to higher concentration, using energy released by respiration. It allows cells to absorb minerals even when soil solution is more dilute than cell contents.

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

The process by which green plants use light energy to combine carbon dioxide and water into glucose, releasing oxygen. It takes place mainly in chloroplasts and stores energy in chemical bonds.

The green pigment inside chloroplasts that absorbs light energy for photosynthesis. It absorbs mainly red and blue light and reflects green light, which is why most leaves look green.

The controlled release of energy from glucose in living cells, producing carbon dioxide and water and supplying energy for life processes. Aerobic respiration uses oxygen; anaerobic respiration does not.

Tiny openings, mostly on the underside of leaves, that allow carbon dioxide to enter and oxygen and water vapour to escape. Each is controlled by two guard cells that open and close it.

A substance built up in muscles during vigorous exercise when oxygen supply cannot keep up with demand. It causes muscle fatigue and must later be broken down using extra oxygen.

Describing a process that happens without oxygen. In muscles, glucose is broken down to lactic acid; in yeast, glucose is converted to ethanol and carbon dioxide during fermentation.

The loss of water vapour from plant leaves through stomata, mainly during the day. It pulls water up the stem and cools the plant, and its rate rises with temperature, wind and dryness.

Plant tissue made of hollow dead tubes that carries water and dissolved minerals from the roots up to the leaves. Its thick walls also help support the plant.

Plant tissue of living tubes that transports sugars made in the leaves to other parts of the plant, including growing tips, roots and storage organs. Transport can occur in either direction.

The maintenance of a stable internal environment in the body, such as constant temperature, water content and blood glucose. It relies on receptors detecting change and effectors correcting it.

A control mechanism in which a detected change triggers a response that reverses that change, bringing conditions back towards normal. It underlies control of body temperature and blood glucose.

The removal from the body of waste products made by chemical reactions in cells, such as carbon dioxide from respiration and urea from the breakdown of excess amino acids.

One of the alternative forms of a gene that can occupy the same position on a chromosome. An organism inherits two alleles for each gene, one from each parent, which may be dominant or recessive.

Describing an allele whose effect appears in the phenotype even when only one copy is present. A recessive allele shows only when two copies are inherited.

The combination of alleles an organism carries for a particular gene, such as two identical alleles or two different ones. The visible result of this combination is the phenotype.

The observable characteristics of an organism, produced by its genotype interacting with the environment. Examples include eye colour, blood group and the shape of a seed.

Differences in characteristics between individuals of the same species. Variation may be inherited, caused by genes, or acquired through environmental factors such as diet and exercise.

A sequence showing how energy passes from one organism to another through feeding, beginning with a producer and ending with a top consumer. Arrows point in the direction energy flows.

An organism that makes its own food by photosynthesis, usually a green plant or alga, forming the first level of a food chain. All energy in an ecosystem ultimately comes from producers.

An organism such as a bacterium or fungus that breaks down dead material and waste, releasing nutrients back into the soil. Decomposers recycle minerals that plants need to grow.

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 arranged in shells.

An atom or group of atoms that has gained or lost electrons and so carries an electric charge. Positive ions form when electrons are lost, negative ions when electrons are gained.

The strong electrostatic attraction between oppositely charged ions, formed when a metal transfers electrons to a non-metal. Compounds with ionic bonds usually form crystals and conduct when molten or dissolved.

A bond formed when two atoms share one or more pairs of electrons, typically between non-metals. Substances with simple covalent molecules have low melting points and do not conduct electricity.

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

The sum of the relative atomic masses of all atoms in a formula unit of a compound. It is used with the mole to convert between mass and amount of substance.

The reaction between an acid and a base in which hydrogen ions and hydroxide ions combine to form water, producing a salt and water. The pH moves towards seven.

A scale from 0 to 14 measuring how acidic or alkaline a solution is. Values below 7 are acidic, 7 is neutral, and values above 7 are alkaline.

A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion, for example sodium chloride. Salts can be made by neutralising an acid with a base or carbonate.

A reaction in which a substance gains oxygen or loses electrons. In terms of electrons, oxidation and reduction always occur together in a redox reaction.

A reaction in which a substance loses oxygen or gains electrons. It is the opposite of oxidation and occurs alongside it in reactions such as the extraction of metals from their ores.

The distance travelled by an object per unit time, calculated by dividing distance by time. It is a scalar quantity, so direction is not stated.

The rate at which velocity changes with time, calculated as change in velocity divided by time taken. A negative value means the object is slowing down.

A push or pull that can change the speed, direction or shape of an object. Force is measured in newtons and has both size and direction.

The rate of flow of electric charge around a circuit, measured in amperes. A current flows only in a complete circuit and is measured with an ammeter connected in series.

The energy transferred per unit charge as charge passes between two points in a circuit, measured in volts. It is also called potential difference and is measured with a voltmeter in parallel.

A measure of how hard it is for current to flow through a component, measured in ohms. For a fixed resistor, voltage equals current multiplied by resistance.