Cells are classified by their shapes and sizes and by the relative complexity of their biochemical processes, as well as their genetic content.
Insert a semantically appropriate sentence at the beginning of the following text so that the topic is introduced smoothly.
All living organisms are composed of cells, which are microscopic and usually visible only under a microscope. Cells are classified by their shapes and sizes and by the relative complexity of their biochemical processes, as well as their genetic content.