The simplest cells are not much more than microscopic pools of DNA (deoxyribonucleic acid) and enzymes wrapped in a thin shell called a membrane.
A cell’s DNA is a sort of code that programs the cell to operate—like software. Combinations of proteins form enzymes that carry out functions such as metabolism, the production and release of energy, through chemical reactions.
The membrane simply holds the DNA and enzymes together in a single unit. These cells are called prokaryotes.