1. All living things are composed of cells. This means that all living organisms, from the smallest bacteria to the largest whales, are made up of one or more cells.

2. Cells are the basic unit of structure and function in living things. This means that cells are the smallest units of life that can carry out all the processes necessary for life.

3. All cells arise from pre-existing cells. This means that cells don't spontaneously appear; they are always created from other cells.

Therefore, there is no fourth component to cell theory.