THE EARTH IS HOME TO MILLIONS OF DIF- ferent kinds of organisms, from slime molds to sheep, blue whales to naked mole rats. Yet the diversity of size and structure evident in all of these organisms emerges from one fundamental building block: the cell. A cell is a small, membrane-bound compartment capable of performing all the basic functions of life. Since all living organisms, from the simplest to the most complex, are composed of cells, understanding how cells work is integral to understanding larger life processes—not to mention that the cell and its structure are common topics tested by the SAT II Biology.

Most cells are too small to be observed with the naked eye. For this reason, even the existence of cells escaped notice until scientists first learned to harness the magnifying power of lenses in the second half of the seventeenth century. At that time a Dutch clothing dealer named Antonie van Leeuwenhoek (1632–1723) fashioned extraordinarily accurate single-lens microscopes. Gazing into the lens of these microscopes, he discovered single-celled organisms, which he called “animalcules” and which, today, we call bacteria and protists.

Englishman Robert Hooke (1635–1703) expanded on Leeuwenhoek’s observations with the newly developed compound microscope, which uses two or more aligned lenses to increase magnification while reducing blurring. When Hooke turned the microscope on a piece of cork, he noticed that the tiny, boxlike compartments of the wood resembled the cells of a monastery. The term “cell” was born.
Cell Theory Emerges
As microscope technology improved, scientists were able to study cells in ever-greater detail. Hooke had no way to tell if cells were living things, but later researchers who could see the nucleus and the swirling motion of the cytoplasm were convinced that cells were indeed alive. By 1839, enough evidence had accumulated for German biologists Matthias Schleiden and Theodore Schwann to proclaim that cells are “the elementary particles of organisms.” But many researchers still did not believe that cells arose from other cells until 1855, when famous German pathologist Rudolph Virchow pronounced, “All cells come from cells.” Nearly 200 years after the discovery of cells, the observations of Virchow, Schleiden, and Schwann established the cell theory:
All living things are made of cells.
All cells arise from preexisting cells.
These two tenets made clear that the cell is the fundamental unit of life.
Cell Size
Cells could not be studied until the microscope was developed because they are very small. This fact raises two questions: why are cells so small, and why are living things made up of millions of tiny cells?
Cells are small because their surface area and volume must be balanced. In order