Cell

The cell is the structural and functional unit of life. Every living organism -- whether a single-celled microbe or a large multicellular plant or animal -- is built of cells, and a cell on its own is capable of independent existence and can carry out all the basic functions of life.

Seeing cells: why microscopes matter

Cells are far too small to see with the naked eye in most cases, so different levels of magnification reveal different levels of detail. A simple microscope (a single strong lens) can magnify an image 50 to 100 times, enough to make out larger cells. A compound light microscope, which is what is normally used in a school or college laboratory, uses a beam of light passed through two or more lens systems and can magnify 1000 times or more, letting us see individual cells clearly. To look inside a cell and resolve its organelles, an electron microscope is needed, which can magnify up to about 500,000 times.

Shape and size of cells

There is no single, typical shape for a cell -- cells can be spherical, rectangular, flattened, polygonal, oval, triangular, conical or columnar, among other shapes, depending on the organism and the cell's job. Cell size also varies enormously. Some cells are far too small to see unaided, some are barely visible, and a few are large enough to see with the naked eye. Mycoplasma has the smallest known cell, only about 0.3 micrometres long; a typical bacterial cell is 3 to 5 micrometres; while the largest single cell known is the ostrich egg, at nearly 15 centimetres. The longest cells in the body, meanwhile, are nerve cells, which can stretch a long distance without being especially wide.

How cell theory developed

The idea that living things are made of cells goes back to the work of the German botanist Matthias Schleiden (1838), who examined many kinds of plants and concluded that plant tissues are built from different types of cells, and the British zoologist Theodore Schwann (1839), who proposed that cells are bounded by a thin membrane and also recognised the cell wall as a feature unique to plant cells. Together, their observations led to the proposal that both animals and plants are made up of cells and the products cells make. However, this early cell theory did not explain where new cells come from. It was Rudolf Virchow, in 1855, who first explained that new cells arise only by division from pre-existing cells -- summarised in the Latin phrase Omnis cellula-e-cellula (every cell from a cell). As research in cytology advanced further, the theory was refined into what is now called the Modern Cell Theory.

Postulates of the Modern Cell Theory

• All living organisms are made up of cells.
• Living cells arise only from pre-existing cells.
• A cell is the structural and functional unit of life.
• The combined activities of a living thing's cells are what produce the activity of the whole organism.
• Cells carry out the transformation of energy.
• Cells contain nucleic acids -- DNA and RNA -- located in the nucleus and the cytoplasm.

Totipotency

Totipotency is the capacity of a living, nucleated cell to differentiate and divide so as to give rise to any other cell type of the organism, and thereby build an entire new organism. A cell can do this because its nucleus carries the organism's entire genetic information. Embryonic animal cells that show this ability are called stem cells, and because they can, in principle, be directed to form almost any tissue, they have major and growing medical applications, including possible treatments for a range of diseases.