Specific Organization

The cell theory, one of the fundamental concepts of biology, states that all
living things are composed of basic units called cells and of cell products.
Although organisms (living things) vary greatly in size and appearance, all
(except the viruses‘) are composed of these small building blocks. Some of
the simplest organisms, such as bacteria, are unicellular; that is, they consist of a single cell. In contrast, the body of a human or an oak tree is made
of billions of cells. In such complex multicellular organisms the processes
of the entire organism depend on the coordinated functions of the constituent cells