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In biology, the basic unit of a living organism. It is the smallest unit capable of independent existence. In organisms, other than the smallest ones, the body of the organism is made up of several cells or many cells. A single cell, therefore, is the smallest unit that shows characteristic features of life, such as reproduction, growth, respiration, response to environmental stimuli, and the ability to take in mineral salts. Viruses are particles that are not cells. A virus can only reproduce by ‘taking over’ a cell from another organism. This cell often dies as a result of making many new virus particles.

Some organisms are composed of only one cell, including many bacteria and some fungi, such as yeast. Single-cell organisms are termed unicellular, while plants and animals which contain many cells are termed multicellular organisms. Organisms such as human beings consist of billions of cells. In organisms made of many cells, groups of cells are specialized to carry out specific functions and are organized into tissues and organs.

Cells always have a cell membrane around them and cytoplasm inside, and normally a nucleus. There are differences between plant cells and animal cells; for example, plant cells have a cell wall made of cellulose outside their cell membrane. This helps to explain why plants look so different from animals.

In plants and animals, cells divide by mitosis or by meiosis when the organism carries out sexual reproduction. In both forms of cell division, the chemical carrying inherited information, DNA, has to be copied before division. Rarely, mistakes occur, causing mutations, but it is normally done accurately. Copying the DNA results in the duplication of structures called chromosomes in the nucleus. In cell division, the duplicated chromosomes are separated from each other into daughter cells. New cells produced by mitosis are needed to replace cells that die and some cells live only a short time, such as white blood cells which live for only a few days. Within the human body, about 3 billion cells die every minute. Many cells may be lost in normal activities – for example, human skin cells are constantly being worn off and have to be replaced by new cells produced by mitosis. The new cells needed during growth are also produced by mitosis.

In respiration a cell uses chemicals that it has taken in, as nutrients, and breaks them down to release energy that the cell can use for its life processes.

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