A cell is the fundamental structural of all living organisms. Some cells complete organisms, such as the unicellular and protozoa; others, such as nerve, , and muscle cells, are specialized components multicellular organisms. Cells range in size the smallest bacterialike mycoplasmas, which are .1 micrometer in diameter, to the egg of ostriches, which are about 8 in diameter. Although they may differ in appearance and function, all cells a surrounding membrane and an internal, substance called the cytoplasm, the composition which differs significantly from the external of the cell. Within the cell genetic material, deoxyribonucleic acid (DNA), containing instructions for the behavior and reproduction the cell and also the chemical for the translation of these instructions the manufacture of proteins. Viruses not considered cells because they lack translation machinery; they must parasitize cells order to translate their own genetic and reproduce themselves.