Cell factories

The cells in a drop of blood contain a different sort of molecular factory, the mitochondrion, which combines food with oxygen to extract useful energy.These factories exist in plant and animal cells today but may once themselves have been free-living cells.By three billion years ago, a number of one-celled plants had joined together,perhaps because a mutation prevented a single cell from separating after splitting in two.The first multicellular or ganisms had evolved. Every cell of your body is a kind of commune, with once free-living parts all banded together for the common good. And you are made of a hundred trillion cells.