Essay on Cell as a Self Contained Unit

The cell acts as an autonomous, self-contained unit and is capable of performing all the processes of life like metabolism, growth, reproduction and damage repair.

The entire set of information for fundamental processes of life is stored inside the cell so that the cell can itself control its own activities. The cell acts as an open system exchanging matters with its environment.

The cell exchanges gases with, and absorbs nutrients from its environment. The cell derives energy from respiration, builds up macromolecules from simpler molecules, and replaces its worn-out structures with new ones and form new cells with similar hereditary properties.

The cell controls its own physic-chemical environment by enzymes produced by it. Each cell is capable of independent existence and has a definite life span. In multi cellular organisms the cell may not have complete independence as there exists a division of labor among different cells of the organism. In such organisms, cells are differentiated into specialized cells to carry out different functions.

Totipotency

All multi cellular and sexually reproducing organisms begin their lives as single cells called zygotes formed by the union of sperms and eggs. This zygote contains all the information necessary for divisions and differentiation at the right time and right place so that a multi cellular organism is formed from this single cell.

Based on this knowledge German botanist Gottleb Haberlandt in 1902 gave the concept of totipotency in plants. Totipotency is the potential of any plant cell to regenerate the entire plant.

This concept of totipotency cannot be applied to all kinds of animal cells. The somatic cell lines in animals after their differentiation to specialized cells lose the power of division.

These cells cannot be dedifferentiated to start dividing again. But in contrast, the plant cells can exhibit totipotency when grown in artificial medium. Though the concept of totipotency was advocated by Haberlandt, he himself could not demonstrate it.

He cultured isolated palisade tissues of leaf in Rnop’s solution. The cells grew in size and lived up to a month but failed to divide.F.C.Steward at Cornell

University in 1958 placed phloem tissue of Carrot (Daucus carota) in a liquid growth medium and observed that individual cells broke away from tissue fragments. They often divided and developed into multi cellular roots.

When he placed these roots in solid growth medium they developed into entire plants. Since then totipotency has been demonstrated by several workers in many plants. Barring some animal cells this phenomenon of totipotency is restricted to plants.

Totipotency in plants has several applications like micro propagations of horticultural plants, production of disease free plants, micro forestry etc.

Tools and Techniques

Most of the cells are not visible to naked eye. Except some egg cells, all other kind of cells requires the help of microscope to observe them.

The sub cellular fractions are to be first isolated and then studied either through biochemical analysis or under microscope. Therefore, it is important to have a preliminary idea about the principle of microscopy and the techniques on how the sub cellular fractions are isolated.

Microscope

This is an instrument for observing magnified image of minute objects with a greater resolution. Robert Hooke and Antonie Von Leuwenhoek used glass lenses to magnify small cells. Gradually qualities of lenses were improved to give greater magnification. Later, instead of single lens, combinations of lenses were used in microscopes to obtain many stage magnifications. Further developments resulted in the use of different wavelengths of light other than visible light in microscope for greater clarity.

The latest development in this field is the invention of electron microscope that uses electron rays. The microscope that uses visible light for viewing is known as light microscope.

Any light microscope where a single lens is used for a single stage magnification is known as simple microscope and where a combination of two lenses are used for two stage magnification of the object is known as compound microscope.

Compound Microscope

The light compound microscope uses visible light to observe the magnified image of the object. The maximum magnification possible under a compound microscope is about 2000 times. In this microscope the object is magnified in tw6 stages.

Principle: Compound Microscope has a combination of two biconvex lenses for magnification: objective and eye piece. The object on a glass slide is placed under the objective for the first stage magnification.

The distance between the objective and eye piece is so adjusted that the magnified image formed by the objective falls within the focus of eye piece. The eye piece further magnifies the image formed by the objective in the second stage magnification. The final image after the two stage magnification is viewed through the eye piece.

The objective lens has shorter focal length than the eye piece lens. The eye piece is fixed at the distal end of the body tube of the microscope. Two or three objectives of different magnifying powers are attached to a rotary disc like nose piece placed at the proximal end of the body tube so that the nose piece can be rotated to bring a particular objective to use.

The distance between the objective and eye piece is so adjusted that the Focus or Focal point (F_e) of eye piece is within the Focus (F_O) and Center of curvature (2F) of the objective. The object is placed within Focus and Center of curvature of objective for first stage magnification .A real, inverted and magnified image is formed within the

Focus and Optical Center of the second lens (eye piece).Then eye piece forms a virtual, erect and highly magnified image. Thus the final image of the object is virtual, inverted and highly magnified.

The total magnification by a compound microscope is a result of the combined magnifying power of the objective and eye piece.

The total magnification, M = Magnifying power of objective, Mo x magnifying power of the eyepiece, Me Another important aspect of any microscope is its resolving power or resolution. Resolution is defined as the minimum distance two points can be apart and still can be distinguished as two separated points. Human eye can resolve two points when they are at least 100 micrometers (o.imm) apart.

When two points are less than 100 micrometers apart, light reflected from each point strikes the same “detector” cell at the rear of the eye. So the eye perceives the two points as a single point. The resolution power of a microscope is expressed in Abbe’s Formula: d =0.61 l\ n Sin a

d =linear distance between two points resolved by the microscope.

l= wave length of the light used.

n = refractive index of the medium between the object and the objective lens,

N Sin a = Numerical aperture or NA.

The value of 0.61 is derived from the computation of a number of complex trigonometric ratios.

a= 1\2 angle formed by the cone of the reflected light entering the objective. The lesser the value of l the more is the resolving power of the microscope. For normal light lis the wave length of green light, i.e., 0.6 micro meters which is the average wave length of visible range i.e. from 0.4 micro meters to 0.7 micro meters.

One way of increasing the resolution of microscope is to use light of shorter wave length as in the case of U.V. microscope or X-Ray microscope.

The medium between the object and objective lens is air having Refractive Index of one. In the case of Oil Immersion lens’ Sedar Wood oil is used as the medium whose Refractive Index is more than one and this increases the resolution of the microscope. The half angle a is always less than 90. Hence Sina is always less than one. The value of half angle depends on the diameter of the objective lens.

Thus the resolving power of a microscope depends on the composite effect of the light quality, medium and the diameter of the objective lens. Depending upon the type of visible light used the resolving power of a light microscope ranges from 0.2 mm -0.4 mm. The resolving power of microscope also depends upon the numerical aperture of the condenser if any.

Electron Microscope

Even the most powerful compound microscope cannot resolve many objects within a cell. The plasma membrane, about 5~7nm (nanometer) thick cannot be resolved by any compound microscope because when two objects are closer than a few hundred nanometer, the reflected light beams overlap.

As a result, the clarity of the image is lost. To increase the resolution one way is to use a beam of electrons replacing a beam of light. A microscope using a beam of electrons is known as electron microscope.

The electron microscope has a 1000 times more powerful resolution than a light microscope. It can resolve objects o.inm or 1 A° apart. Boris and Ruska in 1940 developed electron microscope for the first time. Since then this has been greatly improved.

In transmission electron microscope a beam of electrons of very high voltage of about 100,000V is passed through the specimen. The electrons that pass through are used to. Form an image.

Those areas of specimen that scatter electrons appear dark. Then heavy metal staining can enhance the images. It has a resolution power of 0.2 nanometer (twice the diameters of hydrogen atom). Study of living cells cannot be done through this microscope because; very high voltage used may kill the cells.

The scanning electron microscope was developed by Knoll and his co-workers in i960. Here electron beams are reflected back from the surface of the specimen, together with other electrons that the specimen itself emits as a result of the bombardment.

The reflected beams are amplified and transmitted to a screen where the image can be viewed as a photograph. Scanning electron microscope yields three dimensional images and has greatly improved our understanding of many biological fine structures.