MATSEC SEC Core Science key terms

The smallest unit of life that can carry out all the processes needed to stay alive. Organisms may be single cells or built from many cells working together.

A thin, flexible layer surrounding the cytoplasm of a cell. It controls which substances enter and leave, letting small molecules such as oxygen pass while restricting others.

The structure inside most plant and animal cells that contains genetic material in the form of chromosomes. It directs the cell's activities, including growth and division.

The jelly-like substance filling the inside of a cell, outside the nucleus. Most chemical reactions of the cell, such as respiration, take place in it.

A green structure found in plant cells that contains chlorophyll. It absorbs light energy so the cell can make glucose by photosynthesis.

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.

The diffusion of water across a partially permeable membrane, from a dilute solution to a more concentrated one. It explains how plant roots absorb water and why cells shrink in strong salt solutions.

A protein that speeds up a specific chemical reaction without being used up itself. Enzymes work best at a particular temperature and pH and are destroyed if heated too much.

The process by which green plants use light energy to combine carbon dioxide and water into glucose, releasing oxygen. It stores energy in chemical form and is the basis of most food chains.

The chemical breakdown of glucose in cells to release energy, used for processes such as movement and growth. Aerobic respiration uses oxygen; anaerobic respiration does not and releases less energy.

The breakdown of large food molecules into smaller, soluble ones that can be absorbed into the blood. It happens mechanically in the mouth and chemically by enzymes in the gut.

Tiny finger-like projections covering the inner surface of the small intestine. They greatly increase the surface area for absorbing digested food and contain blood vessels to carry it away.

One of many tiny air sacs in the lungs where gas exchange takes place. Its thin wall, large total surface area and rich blood supply let oxygen enter the blood and carbon dioxide leave it.

The loss of water vapour from the leaves of plants, mainly through small pores called stomata. It pulls water up the stem and cools the plant.

Small openings, mostly on the underside of leaves, that allow gases to move in and out. Guard cells open and close them to control water loss and carbon dioxide uptake.

A tissue of hollow tubes in plants that carries water and dissolved minerals from the roots upwards to the leaves. Its walls are strengthened with a substance called lignin.

A tissue of tubes in plants that transports dissolved sugars made in the leaves to other parts of the plant, such as growing shoots and roots, for use or storage.

A sequence showing how energy passes from one organism to another through feeding, beginning with a producer. The arrows point in the direction the energy flows.

An organism that makes its own food, usually by photosynthesis, and so starts a food chain. Green plants and algae are the main examples in most ecosystems.

An organism such as a bacterium or fungus that breaks down dead material and waste, returning nutrients to the soil. Decomposers keep ecosystems supplied with minerals.

A thread-like structure in the nucleus made of DNA, carrying genes in a fixed order. Humans have 23 pairs in body cells, and sex cells carry one of each pair.

A short section of DNA that carries the instructions for one characteristic or for making one protein. Genes are passed from parents to offspring and may exist in different forms called alleles.

One of the different versions of a gene that can occupy the same position on a chromosome. An organism inherits one allele from each parent for each gene.

Describing an allele whose effect shows in the organism even when only one copy is present. The characteristic it controls masks the effect of a recessive allele.

Describing an allele whose effect appears only when two copies are inherited, one from each parent. A single recessive allele is hidden by a dominant one.

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.

A pure substance made of only one kind of atom, which cannot be split into simpler substances by chemical means. Examples include oxygen, iron and carbon.

A substance formed when two or more different elements are chemically joined in fixed proportions. Its properties differ from those of the elements it contains.

A group of two or more atoms held together by chemical bonds. Molecules may be of an element, such as oxygen, or of a compound, such as water.

An atom or group of atoms that has gained or lost electrons, giving it a positive or negative charge. Ions carry electric current in solutions and in molten salts.

A change in which atoms are rearranged to form new substances, with the total mass staying the same. Signs include a new colour, gas release, heat change or a solid forming.

A starting substance in a chemical reaction that is used up as the reaction proceeds. Reactants are written on the left of a word or symbol equation.

A new substance formed during a chemical reaction. Products are written on the right of an equation and have the same atoms as the reactants, just arranged differently.

The principle that the total mass of the reactants equals the total mass of the products in a chemical reaction, because no atoms are created or destroyed.

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

A substance that releases hydrogen ions in water, has a pH below 7 and turns blue litmus red. It reacts with bases to form a salt and water.

The reaction between an acid and a base to form a salt and water, bringing the pH closer to 7. It is used to treat acid indigestion and to adjust soil acidity.

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

The force that opposes motion when two surfaces slide or rub against each other, converting some energy to heat. It can be reduced with lubricants or smooth surfaces.

The force of gravity acting on an object's mass, measured in newtons. On Earth it is roughly ten times the mass in kilograms, so it changes with gravity while mass does not.

The ability to do work or cause a change, measured in joules. It exists in forms such as kinetic, potential, thermal, chemical and electrical, and is conserved in transfers.

The energy an object has because of its movement. It depends on the object's mass and on the square of its speed, so doubling speed quadruples it.

The movement of energy from one store to another, for example from chemical energy in food to kinetic energy in a runner. Some energy is usually transferred to the surroundings as heat.

The transfer of heat through a solid by particles passing on vibrations, without the material itself moving. Metals conduct well because they also have free electrons.

The transfer of heat in liquids and gases by the movement of warmer, less dense fluid rising and cooler, denser fluid sinking. It sets up circulating currents.

The transfer of energy by waves, such as infrared waves from a hot object, which can travel through a vacuum. Dark, dull surfaces absorb and emit it better than shiny ones.

The rate of flow of electric charge around a circuit, measured in amperes. It flows only in a complete circuit and is the same at every point in a series circuit.

The energy transferred per unit charge between two points in a circuit, measured in volts. It is provided by cells or batteries and used up by components.

A measure of how much a component opposes the flow of current, measured in ohms. For a fixed resistor, increasing the voltage increases the current in proportion.

A circuit in which components are connected in a single loop, so the same current passes through each. The total resistance adds up and one broken link stops everything.

A longitudinal wave of compressions and rarefactions travelling through a material medium. It cannot pass through a vacuum, and its pitch depends on frequency.

The number of complete waves passing a point each second, measured in hertz. Higher frequency means higher pitch in sound and a different colour in light.

The distance between two matching points on consecutive waves, such as two crests, measured in metres. It relates to wave speed and frequency.

The bouncing back of light or another wave when it meets a surface. The angle of incidence equals the angle of reflection, both measured from the normal.

The bending of light as it passes from one material to another where its speed changes. It explains why a straw looks bent in water and how lenses focus light.