The Discovery of Cells
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.
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 to stay alive, cells with a larger volume
need to carry out more chemical activity than smaller cells do.
However, for metabolic activity to take place, the cell must also
have enough surface area to allow an adequate supply of nutrients
and waste products to move in and out of the cell. Because
surface area increases at a slower rate than volume as objects get
bigger, the surface area-to-volume ratio in a cell decreases dramatically
as the cell gets larger. It turns out that a size of 10
the surface area-to-volume ratio necessary for the survival
of most cells. (
is one thousandth of a millimeter.)
Two major types of microscopes allow scientists to study
the miniature world of the cell.
The Light Microscope
Light microscopes use light and lenses to magnify their
subjects. The most common of these used in the laboratory is the
compound microscope, which creates high magnification by combining
two relatively low-power lenses. The total power of a compound microscope
is the power of the ocular lens, located in the eyepiece, multiplied
by the power of the objective lens, located near the slide. For
example, an ocular lens of 10x and an objective lens of 11x yield
a total magnification of 110x. Typical high school microscopes offer magnifications
of up to about 430x.
From time to time the SAT II Biology tests your knowledge
of the various parts of the compound microscope, usually by showing
you an image and asking you to identify the parts.
Many parts of the cell are hard to see under microscopes
because they are colorless. In order to view them, scientists sometimes
employ stains that mark various cell parts differently. One alternative
to staining is a technique called phase contrast microscopy, which uses
filters to emphasize the contrast between different parts of the
The Electron Microscope
At high magnifications, light microscopes produce blurry
images. In the 1950s, scientists invented a new type of microscope
called the electron microscope, which offers increased image clarity,
or resolving power. Electron microscopes are powerful enough to
resolve individual fats and proteins. Light microscopes are still
widely used, however, because electron microscopes are expensive
and can only be used to view matter that is not living.