The cell theory is an important unifying principle of biology. There are three components of the cell theory:
Cells are the fundamental units of life.
All living organisms are composed of cells.
All cells come from preexisting cells.
To the original cell theory, first stated in 1838, should be added:
Modern cells evolved from a common ancestor.
Cells contain water and other small and large molecules, which we examined in Chapters 2, 3 and 4. Each cell contains at least 10,000 different types of molecules, most of them present in many copies. Cells use these molecules to transform matter and energy, to respond to their environments, and to reproduce. The cell theory has three important implications:
Studying cell biology is in some sense the same as studying life. The principles that underlie the functions of the single cell of a bacterium are similar to those governing the approximately 60 trillion human cells of your body.
Life is continuous. All the cells in your body came from a single cell, a fertilized egg (zygote). That zygote came from the fusion of two cells, a sperm and an egg, from your parents. The cells of your parents’ bodies were all derived from their parents, and so on back through generations and evolution to the initial cell.
The origin of life on Earth was marked by the origin of the first cells (see Chapter 4).
Even the largest creatures on Earth are composed of cells, but the cells themselves are usually too small for the naked eye to see. Why are cells so small?