All organisms are composed of cells. In
this topic, we will have a close look at cells and learn something about
their internal structure and functions.
The Cell Theory
The cell theory cannot be credited to
any particular person. It is the result of research and writings of many
biologists.
Lorenz Oken
(1805): A German scientist believed that all living beings originate from or
consist of vesicles or cells.
Robert Hooke
(1665)
He observed a thin slice of cork under
his microscope. He saw tiny spaces with walls around them. He gave them the
Latin name cellulae (meaning small rooms), hence the origin of the biological
term cell. He published his work in micrographia.
Jean Baptist
de-Lamarck (1809)
He was a French biologist. He expressed
that nobody can have life, if its constituent parts are not cellular tissue or
are not formed by cellular tissue.
Henry Dutrochet (1824)
He was a French biologist. He boiled
some plant material in H NO3. The plant material broke up into tiny pieces.
Under the microscope the pieces were seen as box like units. ‘The heavy walled
structure was called cell wall.
Robert Brown
(1831)
He was a Scottish scientist. He
discovered nucleus in the cells of orchids (or-kids).
Dujardin (1855)
He was a French scientist. He reported
that the cells are not empty structures. He discovered that they are filled
with a thick jelly like fluid.
Matthias Schleiden
(1838)
He was a German botanist. He came to
the conclusion that all plants are composed of cells.
Theoder Schwann
(1839)
He was a German zoologist. He came to
the conclusion that animals also consist of cells.
Rudolf Virchow (1855)
He was a German biologist. He revealed
that the cells come from other living things or cells. To put in Virchow’s
words:
“Omnis cellula e cellula”.
Louis Pasteur
(1862)
A French biologist. He experimentally
proved that micro-organisms, i.e. bacteria could be formed only from existing
bacteria.
August Weismann
(1880)
According to him all presently living
cells have a common origin because they have basic similarities in structure
and molecules”.
From all these information scientists
were led to one of the most important concept in biology. This concept is the
cell theory. The three most important postulates of the cell theory are:
1. The cell is the unit of structure
and function of all living things.
2. All cells come from former cells, as
a result of cellular reproduction.
3. All living things are composed of
cell or cells and cell products.
Early Work with
Microscope
Probably the most basic tool of the
biological studies is the microscope (micro, tiny). With the help of microscope
we observe the things that we cannot see with naked eyes. There are many types
of microscopes. Here the basic working principle of compound microscope and
electron microscope will be discussed.
Compound Microscope
To magnify any
object two lenses are used so it is called a compound microscope. A mirror
focuses light from the room up to the eye from beneath the specimen, or a lamp
is used for illumination, so the compound microscope is also known as compound
light microscope. The compound light microscope is used to see cells and can
magnify them up to 1500 times. Special stains are used to visualize particular
structure in a cell.
Electron Microscope
It is like an
upside down light microscope. The radiation enters at the top and the specimen
is viewed at the bottom. The electron microscope does not use light to
visualize specialize specimens but instead uses a fine beam of electrons
transmitted to a specimen in a vacuum. The transmitted electrons, after partial
absorption by the object, are focused by magnets to form the image of the
specimen. Electrons have a shorter wave length than visible light; this
difference gives the electron microscope a greater resolution power than the
compound light microscope. Therefore, specimens can be magnified more highly
with the electron microscope and still transmit a clear image. A variety of
techniques can be used to prepare specimens. There are two types of electron
microscopes
(a) Transmission
electron microscope.
(b) Scanning
electron microscope.
(a) Transmission Electron Microscope
In it the electron
beam is transmitted through the specimen before viewing. Only very thin
sections of material can be observed because electrons are easily absorbed by
larger objects. Electrons cannot be seen with the human eyes, so the image is
seen on a fluorescent screen. A photograph taken with the electron microscope
is called an electron micro-graph. The TEM has high resolution power. As the
specimen is viewed in a vacuum so the specimen must be dry.
(b) Scanning Electron Microscope
In it the electron
beam is scanned to and fro across the specimen and electrons reflected from the
surface are collected. These are used to form a TV-like image on a cathode ray
tube. Much larger samples can be shown than with a TEM, but resolution is less
than the TEM.
Resolution and Magnification
Cells are so small
that we cannot see them with naked eye. Most animal and plant cells are between
10 and 30 micrometers. When two objects are closer together, about 100
micrometers the two light beams fall on the same “detector” cells at the rear
of the eye. When the two dots are farther apart than 100 micrometers, the beam
falls on different cells, only then our eyes resolve them-tell that they are
two objects and not one. Resolution is defined as the minimum distance that two
points can be separated and be distinguished as two separate points. One way to
increase resolution is to increase magnification to make small object seen
larger. Robert Hooke and Antony Van Leeuwenhoek were able to see small cells by
magnifying i.e. enlarging the size, so the cells appeared larger than 100
micrometer limit imposed by the structure of human eye. We may explain
resolution citing another example. If we take a photograph and keep on
enlarging the size, we will not eventually be able to see the atom.
Magnification can be increased but resolution of the photograph remains the
same.
The resolution is much greater in electron microscope. The resolution of an electron microscope is about 0.5mm in practice, compared with 200 mm for the light microscope. Electron microscope is used in research laboratories and for any diagnosis of tumor, cancer etc. in hospitals. The compound microscope is used for all practical purposes in schools, colleges, universities, research laboratories, pathological and histological diagnosis and studies in medical colleges and hospitals.
Technique to Isolate Cell Components
A cell can be
studied under a light microscope or an electron microscope. What is observed is
a qualitative picture of the location of all constituents at the moment the
cells were killed, so, modern technique to study cell and their organelles has
been developed. It is called cell fraction technique.
Cell fraction technique
By this technique
cells are broken, open, and their internal components are separated, for study
under conditions that allow organelles to remain able to perform their
specialized functions.
Homogenization
The first step in
cell fraction is to disrupt the cells and release its components without
significantly altering their structure and activity.
Process of homogenization
In this process a
glass tube fitted with a plastic or glass pestle is used to break open cell
after homogenization, and cell components such as nucleus and mitochondrion etc.,
can be isolated by differential centrifugation.
Centrifugation
It is the process
to separate substances on the basis of their densities.
Differential centrifugation
In this separation
technique centrifugal force is used. A container fitted with a mixture of
particles is spun in a centrifuge (an apparatus for centrifugation) to drive
the heavier particles to the bottom of the container. The heavier particles
form a sediment or pellet, while the light particles remain suspended in the
surrounding solutions or supernatant.
Density gradient centrifugation
In this technique
a homogenate is placed on top of a special medium e.g. sucrose solution that
progressively increases in concentration and, therefore, in density also, from
the top to the bottom of the container, when this sucrose gradient is
centrifuged at high speed each particle in the homogenate will move down in the
tube and will come to rest at the point in the gradient where a density equals
that of the sucrose solution. Thus nuclei will be seen from a cell homogenate
at a lower point in the gradient than the mitochondria which are less dense.
All these techniques have been used in developing our present knowledge of
cells and their organelle.
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