Cell Theory
Cells are the smallest units of life.
Cell theory was developed over time using contributions from a number of different
scientists.
Cell Theory
Cell theory is one of the foundations of biology. There are six major tenets of cell theory:
1. The cell is the basic building block in all living things.
2. All organisms are made up of one or more cells.
3. Similar cells from similar species are essentially the same in chemical composition.
4. All cells come from the division of pre-existing cells.
5. Cells contain genetic material that is passed down to daughter cells during cell division.
6. Energy flow (metabolism) occurs within cells.
Scientific Contributions to Cell Theory
The timeline shown below lists some of the
scientists that contributed to the cell theory.
A few years later, another German scientist named
Rudolf Virchow observed cells dividing to produce new
cells. Based on his observations, Virchow proposed
that all cells were the product of cell division. With
this modification, cell theory became widely accepted
and has changed only slightly from the original theory.
Cells
All living organisms are made of at least one cell.
Cells Vary in Shape and Size
All living organisms are made
of cells. Although most cells are very
small, they do vary in shape and size.
Some very simple cells may be only one
to ten micrometers in diameter, animal
cells tend to be approximately ten to
thirty micrometers in diameter, and
plant cells are generally ten to one hundred micrometers in diameter.
Most cells are too small to be seen with the naked eye, so scientists must use
microscopes to view and study cells.
History of the Microscope and the Cell
The microscope played a pivotal role in the
development of cell theory. Born in 1635 in England,
Robert Hooke contributed to many different fields of
science and technology. He built a compound
microscope—a microscope that used two lenses. With
this, Hooke became the first to observe what he would
later name cells. He
published his
observations in 1665 in
the popular
book Micrographia.
This image of cork cells is taken from Micrographia by Robert Hooke.
Image is courtesy of Wikipedia
Anton van Leeuwenhoek, born in Holland in 1632, was a
contemporary of Robert Hooke. The Dutch scientist became
very skilled at grinding lenses and was able to build simple
microscopes that could magnify objects up to 200 times
while still giving a clear image. Because of this, van
Leeuwenhoek is considered by many to be the father of microscopy.
Though the first microscopes were invented in the previous century, van Leeuwenhoek's
models were such a vast improvement over earlier microscopes that some people
attribute the invention of the microscope to him. He was the first to observe and describe
bacteria as well as protists. A replica of one of Leeuwenhoek's microscopes is shown
below.
Image courtesy of Wikipedia
Using Compound Light Microscopes
Compound microscopes are light microscopes that use a
series of lenses to focus light emitted from a source at the
bottom of the microscope up through a sample and to the
eye. One lens is built into each eyepiece. These lenses are
called ocular lenses. Generally the magnification of the
ocular lenses is 10x. Compound microscopes also have a
series of lenses, known as objectives, that are mounted on
a rotating nosepiece located just above the microscope's
stage.
Most compound microscopes have four different objective
lenses—usually 4x, 10x, 40x, and 100x. As indicated in
the diagram, the shortest lens has the lowest magnification and the longest one has the
highest.
As shown in the diagram to the right, the
main parts of a compound light microscope
include the course adjustment knob, the fine
adjustment knob, the objective lenses, and the
stage. When operating a compound
microscope, the microscope should always be
set to the lowest power objective lens
first. Then, a slide can be added and the
coarse adjustment knob can be used to
focus. Once the desired object is in focus,
higher power objective lenses can be used and
the focus can be further adjusted using the
fine focus knob, if needed.
Since light is transmitted through the sample to the eye, samples must be thin enough to
allow light to pass through them. This is also why specimens are prepared on glass slides
prior to viewing. Dyes can be added to the samples so that certain structures are
emphasized. Some samples look better when a drop of water is placed on the microscope
slide. This is called a wet mount. The proper procedure for making a wet mount is: place
a sample on a slide; use an eyedropper to place a drop of water on the sample; place one
end of a cover slip on the slide and slowly lower the other end (try to avoid making air
bubbles during this step); and, if necessary, remove excess water by holding a paper
towel up to the edge of the cover slip, or add more water by placing another drop next to
the cover slip.
Never use the coarse focus knob or replace slides while the high power objective lens is
selected. Because the high power lens is so close to the slide, either of these actions could
result in the lens being scratched or the slide being cracked.