Living things

Fish and plants in a streamFish 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.

Paramecium, a protist, observed through a microscopeParamecium, a protist, observed through a microscope


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 tigerBengal 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 leafCells 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 cellCross-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 eukaryotesThe 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.

 Chris Jarvis

The oldest living animal is a marine bivalve, Arctica islandica, whose common name is the ocean quahog. These animals are reported to be more than 500 years old.

An ancient stretch of giant seagrass in the Mediterranean Sea is thought to be the oldest living organism on Earth. Posidonia oceanica reproduces by cloning (making identical copies of itself). Calculating the plant's annual growth rate, the meadow, which extends from Spain to Cyprus, a distance of about 3500 km (more than 2000 miles), must be between 80,000 and 200,000 years old.

According to a 2011 estimate, there are 8.7 million species—excluding bacteria and other micro-organisms. Of these, 7.7 million are animals. However only 1.2 million of these species (animals, plants, fungi and protists) have been named and described.

Viruses are not considered to be organisms, because they cannot grow or reproduce by themselves.

Eukaryote comes from the Greek eu ("well formed") and karyon ("nut" or "kernel"), referring to the nucleus.

Archaea, microscopic organisms that live, for example, in hot springs or near hydrothermal vents in the oceans, are similar to bacteria. Like bacteria, they are single-celled micro-organisms that have no cell nucleus. But they are considered different enough to be classed in a separate domain.

An elephant has an estimated 6500 million million cells.

The smallest cell belongs to kinds of bacteria called mycoplasma. These cause a type of pneumonia in humans. The mycoplasma cells are each one ten thousandth of a millimetre wide.

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