In 17th century, with the development of optical lenses and compound microscopes, the fact that the cell is the basic unit of life was established and became the foundation of the cell theory. Since it is essentially a statement of fact and not a theory of a debatable nature, it can be called the cell doctrine. This very idea regarding living organisms popularly known as cell theory was given by several workers including a Frenchman H.J. Dutrochet (1842) who boiled certain tissues in acid and separated them into cells.
On this basis, he thought that all animals and plants are made of cells. However, the credit for formulating the cell theory is normally given to two German biologists. The botanist M.J. Schleiden (1838) and the zoologist Theoder Schwann (1839) established the cell theory in a more definite form. They only took the old ides and observations available to them in the form of a concrete theory.
Schleiden examined a large variety of plant tissues and observed that they were composed of cells of some kind or other. So he concluded that cells are the ultimate units forming the structure of plant tissue. At the same time Schwann closely studied many types of animal cells which were found to be with nuclei but without cell wall. The cells had a very thin outer layer, which we now call plasmamembrane.
Apart from the cell wall, which is unique to plants, the inner structure of both plant and animal cells was identical. Both consisted of a nucleus and clear substance called cytoplasm around it. So he defined cell as a membrane bound, nucleus-containing structure. After a thorough examination of plant and animal cells, they proposed a hypothesis cording to which the bodies of animals and plants are composed of cells and products of cells.
The combined views put forth by schleiden and Schwann ultimately came in the form of cell theory which states that
1. All living organisms are made up of cells which are the unit of structure and function;
2. The activities of an organism are dependent on the activities of cells, individually and collectively;
3. New cells are formed by a process similar to crystal formation.
This theory, however, was not able explain how new cells are formed. Schleiden, in fact, was of the opinion that the cells were bubbed off from the centrally located nucleus. Some 20 years later, Rudolf Virchow (1855), a great German physician made another generalisation and stated that all cells arise from preexisting cells (Omnis cellula e cellula). Thus, the concept regarding the formation of new cells as stated by cell doctrine was found to be incorrect. Despite this error, the theory ranks as one of the foundation stones of molecular biology.
Years later, the discovery of mitosis (Flemming, 1880), chromosomes (Waldeyer, 1890) and fertilisation (Hertwing, 1875) etc., provided additional strength to the generalisation of the cell theory.
The modern version of the cell theory states that
1. Cells are the morphological and physiological units of all living organisms;
2. The properties of a given organism depend on those of’ its individual cells;
3. Cells originate only from preexisting cells, and continuity of cells is maintained through the genetic material;
4. The smallest unit of life is the cell.
Most generalisations have exceptions. This is also true for the cell theory. But, cell theory is applicable in one way or another to all the living organisms. The acellular forms like slime molds, coenocytic fungi and algae are found to be exceptions to cell theory. These organisms constitute a continuous mass of protoplasm, keeping a number of nuclei within. The cell theory can be applicable to them since they arise from preexisting cells and multiply during their life cycle.
Other exceptions like bacteria and blue-green algae, although devoid of true nucleus, obey cell theory in all respects. Another exception to cell theory is the virus. It lacks protoplasm and true nucleus and reproduces only when comes in contact with a suitable host. Biologists treat them as if they were individual cells and are also considered as the basic unit of life. Here, the virus reinforces the concept that the whole cell is the basic unit of heredity.
The genetic continuity of organisms include not only the cell as a whole but some of its smaller components such as chromosomes and genes present inside nucleus. The genetic material in all cases consists of nucleic acids. The basic structure of the cell membranes and their properties are also common. The mechanism of aerobic respiration is strikingly uniform. Nucleic acids and proteins follow the same principle during synthesis. These fundamental similarities speak in favour of unity among all living organisms. In 1962, Andre Lwoff, a French microbiologist, ex- pressed the cell theory in yet another form.
Instead of considering its cellularity, he described the living organisms as the aggregation of unity of plan, unity of function and unity of composition. In order to build the diversity of living systems, a large number of building blocks have been utilised. The problem of diversity of structures and functions, the problem of heredity, and the problem of diversification of species have been solved by the use of a small number of building blocks organised into specific macromolecules, which are further associated with a specific function.