Cells of Living Organisms (With Diagram)

In this essay we will discuss about Cells of the Living Organisms. After reading this essay you will learn about:- 1. Subject Matter of the Cell 2. Historical Accounts of Cell 3. Cell Size 4. Cell Shape 5. Structure of the Cell 6. Structure of Prokaryotes.

Contents:

1. Subject Matter of Cell
2. Historical Accounts of Cell
3. The Cell Size
4. The Cell Shape
5. Essay on the Structure of Cell
6. Essay on the Structure of Prokaryotes

Essay # Subject Matter of the Cell:

All the living bodies are made up of protoplasm. The fabric of life (protoplasm) is seldom found in large masses but is found in the form of discrete structures. These minute morphological bodies are known as the cells.

The cells are small structural units of the body just as the bricks are the units of structure of a house. The cell can be defined as “a unit of biological activity delimited by a semi-permeable membrane and capable of self-reproduction in a medium free of other living systems.”

The cell is an independent functional unit in all the living organisms. In the unicellular organism the single celled body is highly differentiated physiologically and it performs all the vital activities, such as nutrition, respiration, excretion, reproduction and so on. In the multicellular organisms the life activities are accomplished by highly specialised groups of innumerable cells.

Thus the vital processes of the body are the sum of the co-ordinated functions of its cells.

Essay # Historical Accounts of Cell:

Though the speculation about cell can be traced back to the age of Greek philosophers, the science of cells began to develop only after the discovery of compound microscope by Jansens at the end of the sixteenth century.

The first treatise in microscopy was given by Robert Hooke, the curator of experiments for the Royal Society [Fig. 1.1 (a)]. He published a purely descriptive account of the cell, a hitherto unexplored field of life science. He observed plant cells for the first time in 1665 using the newly invented primitive model of compound microscope.

Hooke, observed a thin section of cork under the microscope and found that the bottle cork was not a homogeneous compact material but consisted of a great many small empty boxes or cavities bounded by definite walls [Fig. 1.1(b)] Hooke applied the name cells to those boxes (Latin cellula = a little room).

He published his observations in his book ‘The Micrographia’. Later he observed the cells in fresh vegetable material and found that the cavities of the living cells were filled with “juice” but he could not notice the mystery of protoplasm.

After Robert Hooke, a great curiosity developed in scientists and more details about cells became available. Anton Van Leeuwenhoek, the famous Dutch microscopist, observed for the first time green coloured bodies in the cells of plant. These are now known as chloroplasts.

Two contemporaries, an English physician Nehemiah Grew and an Italian Professor Marcello Malphigi, continued their observations on the internal structure of plants and described in great detail the different types of cells in stem but they also could not notice the importance of protoplasm.

Corti (1772) and Fontana (1781) emphasised the presence of semi-liquid substance in the plant cells and Dutrochet (1824) noticed it in the animal cells. In 1835, a French scientist Dujardin proposed the term sarcode for the living content of the cell. The Bohemian physiologist, Purkinje (1839) introduced the term protoplasm for the living substance of the cell.

With further accumulation of knowledge about cell structure and function the complex nature of living content of the cell became more clear.

The various components of protoplasm were recognised one by one. In 1831 Robert Brown, an English Botanist, noticed the general occurrence of a definite spherical body in each of the skin cells of orchid leaf and named it ‘areola’. This was later designated as nucleus (Latin word which means kernel).

Shortly after, Mathias Schleiden found a small spherical body inside the nucleus and gave the name nucleolus to it. In 1846, Hugo Von Mohl introduced a distinction between protoplasm and cell sap and following him, Kolliker (1862) applied the name cytoplasm to the living material surrounding the nucleus.

Thus the nucleus and the cytoplasm together form the protoplasm. All these facts became a part of scientific thinking when two German biologists, named Schleiden (botanist) and Theodore Schwann (zoologist), formulated their well-known “Cell Theory” (1938-39).

The theory at present suggests that:

(i) All the living organisms are composed of cells and cell products,

(ii) There are considerable similarities in the chemical constitution and metabolic activities of all the cells, and

(iii) The total activities of an organism is the sum of the activities and interactions of its independent cell units.

Virchow (1855) stated the fact that all the cells in the body arise by divisions of pre-existing ones. Mechanism of cell division was established by the end of 1870. In the year 1861, Schultze also supported the cell theory and put forth the protoplasm doctrine. Hanstein (1880) introduced the term protoplast to the organised discrete mass of protoplasm and stressed that the term protoplast be used instead of the term cell.

By continuous observations more parts of the cell came to light. These parts included the vacuoles, mitochondria, Golgi bodies, etc.

The cell theory was severely criticised and the concepts were considerably refined by some eminent scientists. The discovery of virus particles in the recent years has raised a problem before the biologists as to whether these particles are living or non-living.

A virus can be defined as a self-reproducing biological unit which does not possess a finite semi-permeable membrane and is capable of self-reproduction only when it is present within a living cell.

Outside the living cell, it is just like non-living particle. If they are regarded as living then the cell theory cannot be applied to them because their bodies are in the form of very minute particles and are not cellular.

The viruses are formed of nucleic acid and proteins, the key compounds invariably present in all the cells (fig 1.2).

These particles have been interpreted in various ways by different biologists According to them:

(a) Viruses are living chemicals.

(b) Viruses evolved from cellular forms through degeneration and they became reduced to particle size through parasitism.

(c) Viruses are the primitive organisms which could not reach to cellular state.

Besides viruses, there also exist some forms that appear intermediate between viruses and cells. These systems, called mycoplasma and rickettsias appear to have membranes but are incapable of growing outside living cells. It is likely, however, that they have evolved from more independent bacteria which have been converted to parasitism and have lost some of their biochemical potentialities.

The second important objection to cell theory is that it does not explain coenocytic bodies of some lower plants, as for example, Vaucheria, Mucor, Rhizopus, etc. According to the cell theory, a typical cell of the body is the unit mass of protoplasm consisting of two distinct components namely the nucleus and the cytoplasm (Figs. 1.3 and 1.4).

This definition, however, does not seem proper in those cases in which the body is coenocytic. The coenocytic body is a mass of living substances with numerous nuclei retained inside the surrounding wall. Thus it would be difficult to define the cell as the basic unit in such living bodies.

Essay # Cell Size:

In scientific works the cells are measured in terms of metric system, i. e., in centimeters and millimeters. When measuring plant or animal cells under microscope, the units micron (Greek, μ = 1/1000 or 0.001 mm and millimicron (mµ = 1/1000 µ) are used.

Further subdivision of millimicron into units often gives the Angstrom unit (Å). This unit corresponds to one-tenth of a millimicron (Å = 0.1 mµ,. This is used to measure sub-microscopic structures revealed with electron microscope and to determine the wavelengths of radiations.

The sizes of cells vary widely. Some cells, e.g., root hairs and fibre cells of cotton and other plants are visible to unaided eyes. In certain monocots and in the family urticaceae the fibre cells measure approximately 2 to 56 cms in length. The bacterial cells which are probably the smallest (approximately 0.2-5.0 µ) lie at the lowest limits of microscopic resolution.

On volume basis, one of the largest cells among the animals is the yolk cell of the ostrich egg which is approximately 8 cms in diameter. Thus the volume of ostrich egg yolk is about fifteen million billion times greater than that of a bacterial cell measuring 0.2 µ.

The nerve cells of some mammals measure a meter or more in length. Most cells in both plants and animals have average dimensions from 0.5 µ to 20 µ. The size of cell is directly related to the increase or decrease of its content.

Essay # Cell Shape:

In general, the cell shape varies greatly from tissue to tissue. It may be fixed or changeable; spherical or oval; flat or elongate; spindle shaped or globose; cylindrical or polyhedral and so forth. These different shapes of the cells are related to the particular functions they perform.

Mechanical force also determines the shapes of cells to some extent, e.g., the cells from undifferentiated tissue are polyhedral in shape because of their mutual pressure. Regardless of their shapes, all the cells tend to be spherical if they are freed from various restraining factors.

The shape of the newly developed cell is also dependent upon the mother cell, plane of cell division and times of division the mother cell has undergone. On maturity the cells generally become surrounded by a rigid or semi-rigid non-living membrane which also stabilizes their shapes.

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Essay # Structure of the Cell:

Before the discovery of electron microscope, most of our knowledge regarding the internal structure of cell was based on the observations made with the help of light microscope. In recent years the electron microscope has been used in the study of cell. The term “ultrastructure” refers to the fact that the cell structures accounted for have been analysed with the help of electron microscope.

The electron microscope makes use of a beam of light and is not in principle very different from the light microscope. For electron microscopic studies the tissues are fixed in osmium oxide (OsO₄), formaldehyde solutions etc., dehydrated, embedded in liquid plastics (usually the mixture of butyl methyl methacrylate) and sectioned with ultratome (section thickness about 100 Å).

The electron microscope magnifies the image upto 3, 00,000 or more times the actual size of the object on a fluorescent plate resembling that of a television tube. Photographs are made for further studies. One drawback with this apparatus is that it is difficult to examine materials in living state.

The detailed general reviews on the electron microscopy of plant cells have been given by Muihlethaler (1957), Buvat (1958) and many other workers.

There are two well recognized basic plans of cellular organization.

These are:

(i) Simple prokaryotic plan

(ii) Complex eukaryotic plan

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Essay # Structure of Prokaryotes:

The organisms in which nucleus is not bounded by a definite nuclear membrane are called prokaryotes. Among the prokaryotes are the viruses, Pleuropnemonia like organisms (PPLO), bacteria and blue- green algae. Prokaryotes are very small (generally from 0.5 µ to 3 µ) and exhibit no differentiation into nucleus and cytoplasm. In the eukaryotic organisms the nucleus is bounded by a definite nuclear membrane.

Prokaryotic and eukaryotic cells differ fundamentally in following points:

1. Nuclear Membrane:

As mentioned previously, prokaryotic cells exhibit no differentiation into nucleus and cytoplasm, i.e., cytoplasmic and nuclear material remains intermingled. Eukaryotic cells have definite nuclear membrane.

2. Chromosomes:

In prokaryotes the genetic information is located in a single chromosome which is a circular DNA. DNA of prokaryote lacks the basic proteins called histones. In eukaryotic cells DNA of chromosome is complexed with histones.

3. Absence of Mitotic Apparatus and Nucleoli:

The cell division in prokaryotes is not typically mitotic nor does meiosis take place -n the real sense.

4. Cytoplasmic Organelles:

Prokaryotic cells lack in clearly defined membrane limited organelles such as endoplasmic reticulum, golgi complex, mitochondria, lysosomes, centrioles and chloroplasts. The enzymatic functions of mitochondria are carried out by plasma membrane which becomes infolded at several points.

In eukaryotic cells, definite membrane bound structures like the endoplasmic reticulum, golgi complex, mitochondria, chloroplasts, lysosomes and centrioles are present.

5. Cell Wall:

The cell wall of prokaryote contains amino sugars, muramic acid and other carbohydrates. In eukaryotes the cell wall, when present, does not contain these substances.

6. Plasma Membrane:

Plasma membrane in prokaryotes forms intrusions in the cytoplasm called mesosomes. In eukaryotes mesosomes do not exist.

7. Flagella:

Flagella or cilia found in some prokaryotes do not have 9 peripheral and 2 central fibrils (9 + 2 configuration). In eukaryotes flagella and cilia exhibit a definite structural plan, having 9 peripheral fibrils and two central fibrils.

8. Photosynthetic Apparatus:

In prokaryotes the chlorophyll, when present, is associated with free lamella; and the lamellae are not enclosed by a membrane. In eukaryotes the pigmented lamellae are found in distinct chloroplast.