THE CELL: DOWN TO BASICS
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What do a two-foot-long neuron, a two pound ostrich egg, and .2micrometer-long bacteria all have in common? Each is a single cell. Cells are the
basic unit of structure and function in living things, the smallest things that can
perform all of the functions of life.
A neuron is also
a single cell,
and may stretch
up to two feet in
length.
An ostrich egg,
which may
weigh up to two
pounds, is
actually a single
cell.
Aside from the occasional ostrich egg or nerve cell, most cells are very
small and can't be seen without magnification. As a result, our knowledge of
cells has grown as technology has allowed us to see them. The art of
glassmaking in the 1600s led to the creation of the first simple microscope
lenses, making it possible to see cells up close and personal for the first time.
Robert Hooke, an English scientist, first described cells in 1665. When he
looked at a very thin slice of cork through his microscope, Hooke noticed that
the cork was porous and comprised of many tiny square boxes that reminded
him of the small rooms in a monastery. He called them cells, and the name has
stuck, even to today.
In 1675, a Dutch lens maker named Antonie van Leeuenhoek described
the first living cells. In the 150 years after his work, microscope lenses
improved and scientists were able to observe and understand more parts of the
cell. In the mid-19th century, three different scientists working separately each
published important conclusions about cells.
In 1838, Dutch botanist Matthias Schleiden concluded that all plants are
composed of cells. One year later, a German zoologist by the name of Theodor
Schwaan postulated that animals are also composed of cells. And in 1855,
Rudolph Virchow, a German doctor, asserted that all cells must come from other
cells by the process of cell division. The work of these three scientists was
combined into what is now known as the CELL THEORY .
The cell theory consists of three basic points:
1. All living things are made of cells.
2. The cell is the smallest living thing that can perform all the functions of life.
3. All cells must come from preexisting cells.
Because cells are the most basic unit of life that all organisms share,
understanding cells is the key to learning about life itself.
A side-by-side view of an animal and plant cell. Plant cells contain special organelles called
chloroplasts and an outer covering called a cell wall. Animal cells lack both of these structures.
Most plant and animal cells range in size between 10 and 50 micrometers.
The longest cells are nerve cells that can sometimes extend several feet. Their
function is to carry electrical impulses through the body quickly. Compare these
to prokaryotic bacteria, which, at .2 micrometers in length, are the smallest
cells.
Some cells can live independently of other cells. These are unicellular
organisms and can grow, feed, reproduce, and respire completely on their own.
Other cells are highly specialized and can function only in association with many
others as part of a multicellular organism.