The Cell Theory

APPENDIX
1
The Cell Theory
The following four points will re-acquaint
you with the fundamental principles of the
cell theory:
1. The cell is the basic structural unit of
living things. This means that every
living thing, no matter how large, is
composed of cells.
2. The cell is the basic functional unit of
living things. This means that the life
functions of an organism are carried out
at the cellular level.
The first two points of the cell theory were
well accepted by 1840. The third, which was
first stated by 1858, was not fully understood or
accepted until 1864. That was when Louis
Pasteur demonstrated in a series of experiments
that living organisms only arise from living
organisms. Since the time of Greek philosopher
and scientist Aristotle (384–322 B.C.E.), it had
been commonly believed that living things could
arise spontaneously from non-living matter.
3. All cells are derived from pre-existing
cells. This means that life does not
arise spontaneously; it arises from
pre-existing cells.
4. In a multicellular organism, the activity
of the entire organism depends on the
total activity of its independent cells.
Background
The cell theory developed in a climate of great
scientific activity during the 1800s. Improved
glass-making technology had made it possible
to manufacture reliable compound microscopes
on a large scale and at a reasonable price. As a
result, hundreds of curious naturalists began
peering into the microscopic world. The
development of stains for specimens improved
the ability of scientists to make observations,
and the theories and discoveries made in other
scientific disciplines spurred on the quest to
learn more about cells. In 1808, John Dalton
(1766–1844) published his atomic theory,
which described matter as being made up of
numerous, tiny units called atoms. This idea
suggested a new line of investigation. What if
living things were also made of small units?
Investigating this question revealed that
plants and animals are made of cells and that
these cells have many features in common. This
is why biologists say that the cell is the
unifying structure that links all life — despite
the cell’s great diversity of form.
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MHR • Appendix 1
The first recorded cell observations (shown here) were
reported in 1665 by Robert Hooke (1635–1703). To
obtain them, Hooke used a double-lens compound light
microscope. Its optical design was similar to the optical
design of the microscopes in your school laboratory.
The dead cork cells that Hooke observed were hollow,
and the rectangular shape of their walls reminded him
of the small cells (sleeping rooms) in a monastery.
Biologists have used the word “cell” ever since.
The first observations of a living cell were made
sometime before 1674. In that year, Anton von
Leeuwenhoek (1632–1723) accurately drew and
described such organisms as bacteria for the first time.