CELL The smallest unit of living matter that can exist by itself is the cell. Some plants and animals consist of only a single cell. Others are composed of many billions of cells.
Cells exist in a variety of shapes and sizes.
They may, for example, be cube-shaped or flat. Scientists who study cells have determined that a single cell may be as large as a tennis ball or so small that thousands would fit on the period at the end of this sentence. While normally cells function with great efficiency, they are subject to various disorders that result in disease. The size of cells is usually measured in microns. A micron is a millionth of a meter, and about 25,000 microns equal one inch.