I. Cell Theory
A. Robert Hooke was the first to observe cells in 1665, naming the shapes he saw in cork
cellulae. This comes down to us as cells.
B. Cell theory is the unifying foundation of cell biology
1. The cell theory was proposed to explain the observation that all organisms are
composed of cells.
2. In its modern form, cell theory includes the following three principals:
a. All organisms are composed of one or more cells, and the life
processes of metabolism and heredity occur within these cells.
b. Cells are the smallest living things, the basic units of organization of all
c. Cells arise only by division of a previously existing cell.
C. Cell size is limited
1. The rate of diffusion is affected by a number of variables, including surface
area available for diffusion, temperature, concentration gradient of diffusing
substance, and the distance over which diffusion must occur.
2. Larger cells need to synthesize more macromolecules, have a correspondingly
higher energy requirements, and produce a greater quantity of waste.
3. The rate at which this transport occurs depends on both the distance to the
membrane, as well as the area of membrane available. For this reason, an
organism made up of many relatively small cells has an advantage over one
compound of fewer, larger cells.
4. The advantage of small cell size is readily apparent in terms of the
. As a cell’s size increases, its volume increases much
more rapidly than its surface area.
5. The cell surface provides the only opportunity for interaction with the
environment, because all substances enter and exit a cell via this surface.
D. Microscopes allow visualization of cells and components
1. The reason we can’t see such small objects is the limited resolution of the
is defined as the minimum distance two points can be
apart and still be distinguished as two separated points.
, which operate with visible light, use two
magnifying lenses to achieve very high magnification and clarity.
3. Microscopes that magnify in stages using several lenses are called
. They can resolve structures that are separated by at
least 200 nanometers (nm).
4. Electrons have a much shorter wavelength, and an
employing electron beams, has 1000 times the resolving power of a light
Transmission electron microscopes
, so called because the electrons used
to visualize the specimens are transmitted through the material, are capable of
resolving objects only 0.2 nm apart – just twice the diameter of a hydrogen atom!
6. A second kind of electron microscope, the
scanning electron microscope
beams the electrons onto the surface of the specimen. The electrons reflected
back from the surface, together with other electrons that the specimen itself emits
as a result of the bombardment, are amplified and transmitted to a screen, where
the image can be viewed and photographed.
E. All cells exhibit structural similarities