Fish and plants in a stream Living things are called organisms. We know if something is an organism, rather than non-living, from several features. First, an organism grows and develops at some stage, usually changing its shape and getting bigger. Second, life processes happen inside the organism that change chemical substances from one form to another and which use up energy. Third, an organism must take in raw materials for its growth and also take in energy to power its life processes. Fourth, an organism reproduces—it produces more of its own kind.
Organisms come in many shapes and sizes. Some are too small to see except with a microscope, like an amoeba in the mud. Some are quite small and seen better with a magnifying lens, like a ladybird. Some are gigantic, like the sequoia tree. Some, like the dinosaurs, lived in prehistoric times but no longer survive. We humans are also living things.
In any particular environment such as, for example, rainforest or grassland, there is a wide variety of organisms. A measure of this variety is called biodiversity.
Bengal tigerOrganisms can be classified, or put into groups, according to their similarities, relationships and ancestry. Most scientists work on the basis that organisms are divided into three domains: archaea, bacteria and the eukaryotes (see below). Each of these domains is then divided into further groups, or ranks, of decreasing size: kingdom, phylum, class, order, family, genus and species. Any organism, for example a tiger, can be classified accordingly.
Cells in a leaf
All living organisms, from minute bacteria to giant coniferous trees, are made of cells. Most contain millions, sometimes even billions of them. Each cell is microscopic in size, but they combine to form all parts of the organism. The cells are the building blocks of any living thing, in the same way that bricks fit together to make buildings. Cells come in many different shapes and sizes, but they have the same basic internal structure.
Cross-section through a typical cell
Inside a cell
Every cell is surrounded by a thin, sieve-like barrier, called a cell membrane. This controls which substances enter or leave the cell. Inside the membrane, there is a mass of jelly, called cytoplasm, with various even smaller parts called organelles, or “little organs”, suspended in it.
Mitochondria are small and sausage-shaped. They break up substances such as sugar (glucose) to release energy for use in the cell. A network of membranes, called the endoplasmic reticulum, is the cell’s "factory", where various substances and products, including fats and enzymes, are made.
The biggest organelle in the cell is the nucleus, a dark lump that lies at the cell’s centre. It contains chromosomes, into which DNA is tightly wound. It acts like a computer programme that controls the workings of the cell.
The four kingdoms of eukaryotes
Eukaryotic organisms, or eukaryotes, have one or more cells. Unlike archaea or bacteria (the prokaryotes), their cells contain a nucleus and organelles. Eukaryotes are divided into four kingdoms: protists, fungi, plants and animals.
Protists are blob-like single cells, but they differ from bacteria in that they each contain a nucleus. Fungi, such as mushrooms, toadstools and moulds, get their energy by dissolving or digesting dead and dying organisms into simple nutrients, which they absorb through their body surface. Plants obtain their energy from sunlight. Animals live by consuming or eating other living things.
Consultant: Chris Jarvis