WHAT ARE CELLS? 
The idea that every living thing is 
made up of cells emerged from an 
encounter between two German 
scientists in 1838. Nearly two cen- 
turies earlier, in 1665, the English 
physicist Robert Hooke looked at a 
sliver of cork through a microscope 
lens and noticed some "pores" or 
"cells" in it. Hooke believed the 
cells had served as containers for the 
"noble juices" or "fibrous threads" of 
the once-living cork tree. He thought 
these cells existed only in plants, 
since he and his scientific contempo- 
raries had observed the structures 
only in plant material. 
During a dinner conversation in 
1 838, however, the botanist 
Matthias Schleiden, who had been 
studying plant cells, and the zoolo- 
gist Theodor Schwann, who had 
been examining the nervous tissue of 
animals, realized that the similarities 
between the structures they had been 
investigating were too strong to be 
accidental. In 1847, Schwann 
wrote a paper describing how all 
animal tissue, including bone, blood, 
skin, muscle, and glands, is com- 
posed of cells. Even sperm and 
eggs are cells. Schleiden elabo- 
rated on this idea as it applied to 
plants. A German pathologist, 
Rudolph Virchow, is given credit for 
being the first to state, in 1 858, 
what became known as the cell 
theory: "Every animal appears as 
a sum of vital units, each of which 
bears in itself the complete character- 
istics of life." 
The cell theory united plant and 
animal sciences by recognizing that 
the cell is the fundamental component 
of all living organisms, from orchids 
and earthworms to human beings. It 
provided an intellectual framework 
that revealed the hidden similarities of 
form and function in extremely diverse 
organisms, and it gave scientists a 
way of making sense put of the bewil- 
dering array of living creatures. But 
what is a cell? 
Obviously, there are major differ- 
ences among cell types. Muscle cells, 
which can contract, have to be quite 
different from liver or bone cells. 
Nerve cells have long, thin fibers that, 
in humans, might extend more than 3 
feet from the spinal cord to the toes, 
while blood cells have no projecting 
fibers at all. Plant cells have a unique 
ability to use light for energy. 
Then what do all these cells have in 
common? Discovering their shared 
properties was difficult. At first, scien- 
tists thought that the cell was just a 
blob of jelly, or some primordial soup 
enclosed in a bag. They named the 
jelly "protoplasm." For a long time 
they could not find anything in the 
protoplasm, which later became 
known as the "cytoplasm." 
Part of the difficulty in studying cells, 
of course, is due to their extremely 
small size. The cells of multicellular 
organisms are impossible to see with 
the unaided eye. Schleiden and 
Schwann, like cell biologists before 
and after, relied upon microscopes to 
enlarge the image of cells so that they 
could be studied. Microscopes em- 
ploy one or more curved lenses and a 
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