1.1
Cell Theory and the Microscope
Animatronics is the art and science of building robotic models of animals.
Animatronic animals are programmed to be as realistic as possible and can be
used in motion pictures instead of real animals, which might be injured. The
animatronic chimpanzee in Figure 1 shares many structural features with the
real chimpanzee in Figure 2. However, no matter how lifelike an animatronic
device may be, it is not alive. What characteristics distinguish a nonliving
thing, such as a mechanical chimpanzee, from a living organism, such as a
real chimpanzee? Table 1 describes four basic characteristics that distinguish
living things from nonliving things.
Table 1
Figure 1
An animatronic chimpanzee
Characteristics of Living Things
Characteristic
Example
•
Living organisms obtain and use
energy to power activities such as
movement and growth.
Animals obtain energy by eating other organisms;
plants obtain energy from light.
•
Living organisms try to maintain a
constant internal environment.
Humans keep a constant internal temperature by
shivering when it’s cold and sweating when it’s hot.
•
•
Living organisms reproduce.
Organisms produce new organisms like themselves.
Living organisms are made of cells.
Cells are the smallest living things. Organisms can be
composed of a single cell (unicellular organisms,
such as amoeba) or many cells (multicellular
organisms, such as mammals).
Cell Theory
Figure 2
A real chimpanzee
After Robert Hooke discovered cells in 1655, scientists all over the world
began to study their structures and functions. The study of cells became
synonymous with biology.
In 1838, botanist Matthias Schleiden concluded that all plants are made of
cells. In the following year, zoologist Theodore Schwann came to a similar
conclusion about animal tissues. After concluding that plants, animals, and
microorganisms were composed of cells, these two scientists stated the
following scientific law:
• All living things are composed of one or more cells.
• Cells are the basic structural and functional units of life.
About twenty years later, in 1858, Rudolf Virchow, a medical doctor who
conducted extensive studies on the causes of human disease, stated a further
law:
• All cells arise from the division of other cells.
These three statements describe the basic nature of living things, and
together are called cell theory.
6 Unit 1
NEL
Section 1.1
Observing Cells
When you look at the skin on your hand with unaided eyes, you cannot see
that it is composed of millions of tiny living cells. Observing cells is not easy.
Much of what we know about cells could not have been discovered without the
invention and development of the microscope. Early microscopes allowed
scientists to discover the cell, and modern microscopes are so powerful that
they help reveal how cells work.
Figure 3
A van Leeuwenhoek microscope
The Simple Light Microscope
The simple light microscope, like the one used by 17th-century biologist
Anton van Leeuwenhoek, was hand held and composed of a single lens
(Figure 3). Light from the sun, a candle, or a lamp was used to illuminate the
specimen viewed with the simple light microscope.
The Compound Light Microscope
The compound light microscope (Figure 4) contains two lenses, an objective
lens close to the specimen, and an ocular lens close to the observer’s eye. The
specimen is illuminated with sunlight or an electric lamp. The quality of a
microscope may be expressed in terms of magnification and resolution.
Magnification is the ability of lenses to enlarge the image of a specimen,
and depends on the power of each lens. An image magnified 10 (10 times)
by the ocular lens and 10 by the objective lens will be seen as 100 larger
than the specimen looks to unaided eyes. The total magnification of a
microscope is equal to the product of the ocular lens magnification and
the objective lens magnification. The best compound light microscopes
can magnify up to about 2000.
Resolution refers to the amount of detail seen when viewing a specimen.
A microscope with poor resolution produces a blurred image; one with good
resolution produces a clear, sharp image in which features that are close together
in the specimen appear separate and distinct.
TRY THIS activity
magnification the ability of lenses
to enlarge the image of a specimen
resolution the ability to show
objects that are close together as
separate and distinct
Resolution
A microscope is most useful when it produces highly
magnified images with good resolution. This means that a
microscope produces a large image in which components
of the specimen that are close together appear separate
and distinct. In this activity, you will resolve components of
a colour picture with a microscope.
Materials: compound light microscope, microscope slide
and cover slip, scissors, colour picture from a magazine or
a colour dot-matrix printer
1. Cut a small (1 cm 1 cm) section of a colour image
from a magazine or colour dot-matrix printout.
Include parts of the picture containing red, green, or
blue colour.
NEL
Figure 4
A compound light microscope
2. Place the section in the centre of a microscope slide
3.
(a)
(b)
(c)
and cover with a cover slip.
Examine the image with a microscope under low,
medium, and high power.
Describe the picture’s qualities (e.g., graininess, etc.)
when observed with unaided eyes.
Describe the magnified images of the picture when
examined under low, medium, and high power. How
do the qualities of the magnified images compare
with the image viewed with unaided eyes?
Comment on the resolving abilities of your
microscope.
Cellular Biology 7
Resolution depends on the type of light used to illuminate the specimen. The best
compound microscopes, using visible light from the sun or electric lamps, cannot
resolve (distinguish) parts of a specimen that are closer together than about
200 nm (2 107 m).
The Transmission Electron Microscope (TEM)
Figure 5
Transmission electron microscope
A transmission electron microscope (TEM) (Figure 5) uses an invisible beam
of electrons. Since the electrons must pass through the specimen, the specimen
must be sliced very thin. To accomplish this, the specimen is encased in plastic
and shaved into very thin layers that are mounted onto a fine metal grid
containing holes that allow the electrons to pass through. Light microscopes
can be used to view living cells, but transmission electron microscopes can
only be used to view dead cells (Figure 6).
The first useful transmission electron microscope was designed and built
in 1937 by University of Toronto researchers James Hillier and Albert Prebus.
This microscope could magnify up to 7000. Modern transmission electron
microscopes magnify up to 5 000 000, and may resolve parts of a specimen
that are up to 0.2 nm apart (the average distance between two atoms in a solid).
The Scanning Electron Microscope (SEM)
Unlike the transmission electron microscope, the scanning electron
microscope (SEM) (Figure 7) reflects electrons from the surface of the
specimen, allowing thicker specimens to be used. In fact, the surface features
of whole animals such as small insects may be viewed using a scanning
electron microscope (Figure 8). Scanning electron microscopes use magnets
to focus a beam of electrons to a fine point. This thin stream of electrons scans
the surface of the specimen and produces a three-dimensional image of the
specimen on a television monitor. Scanning electron microscopes cannot
magnify or resolve as well as transmission electron microscopes; however, they
provide greater contrast and depth. The best SEMs magnify up to 300 000,
and have a maximum resolution of approximately 10 nm.
Figure 6
Transmission electron micrograph of
a mosquito (Anopheles) gut
DID YOU
KNOW
?
Powerful Microscopes
The most powerful transmission
electron microscope, called a fieldemission TEM, can resolve rows of
atoms 0.01 nm apart!
8 Unit 1
Figure 7
Scanning electron microscope
Figure 8
A fruit fly (Drosophila) as seen using a scanning
electron microscope (magnification 60)
NEL
Section 1.1
Section 1.1 Questions
Understanding Concepts
1. State the cell theory in your own words.
2. Which had to come first, invention of the microscope
or development of our present cell theory? Explain.
3. A virus contains a DNA molecule surrounded by a
protein shell (Figure 9). Viruses can only reproduce
when inside a living cell. Are viruses alive?
DNA
molecule in
protein shell
5. State one advantage and one disadvantage of using
a transmission electron microscope as compared
with a scanning electron microscope.
Making Connections
6. The world’s smallest microscope, called a microlens
microscope, is approximately the size of the period
at the end of this sentence.
Conduct library and/or Internet research to
answer the following questions about microlens
microscopes.
(a) What type of microscope is a microlens
microscope?
(b) What are some useful applications of a microlens
microscope?
GO
www.science.nelson.com
7. The following claims are seen on the package labels
of two microscopes sold in the hobbies department
of a toy store:
Figure 9
4. Distinguish between magnification and resolution.
NEL
Claim
Price ($)
“Magnifies up to 5000!”
69.99
“Magnifies up to 400, and resolves
to a maximum of 300 nm.”
249.95
Which microscope is most likely the better buy? Explain.
Cellular Biology 9