Sep 2, 2012

The Cell


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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