Cell theory

1. All organisms are composed of one or more biological cells.
2. The cell is the basic unit of organization of organisms.
3. All cells come from preexisting cells.

Microscopes continued to be improved upon by scientists that studied cells. As a result, the scientists could see closer and begin to understand the internal structures of an individual cell. The internal structures of a cell are called organelles, which means "little organs." Just as our bodies are made up of organs, the "bodies" of cells are made up of organelles.

There are two basic types of cells:

• prokaryote: do not have a nucleus or membranes surrounding its organelles. Example: bacteria.
• eukaryotes: do have a nucleus and organelles with membranes surrounding them. Example: plants, animals.

See also: germ theory