Cell - The Basic Unit of Life
Science makes use of two types units. The units that are used to describe time, weight, and distance are arbitrary units accepted by man for the sake of convenience. The other type of units such as electrons, protons, neutrons, however, has a physical reality. These units can be independently demonstrated and determined by proper experimental techniques with the help of appropriate instruments.
The cell is the basic unit of life and it is a physical entity. Cells can be broken up and individual parts can be extracted for study just as a physicist breaks up atoms. The cellular fragments, thus obtained, can function for some time; they may consume oxygen, ferment sugars, and even form new molecules. But these activities of fragments of cellular material do not constitute life itself as the behaviour of sub atomic particles is not equivalent to the behaviour of an intact atom. The fragmented cellular materials cannot continue life indefinitely. The intact cell is the basic unit that can sustain life.
Discovery of the Cell
The now familiar concept that cells are the basic units of biological systems was enunciated in 1839 in the form of a hypothesis, namely the cell doctrine. However, the existence of cells was known 200 years before the formulation of the cell doctrine.
It was Robert Hooke who first observed and named the biological units known as cells. In 1665, while examining the microscopic texture of cork, Hooke noticed that cork was perforated and had compartments somewhat like a honeycomb.(Fig.2.1.1).
These structures could be compared to many little boxes separated out of one continuous long pore by certain diaphragms. Hooke called these compartments in the cork material as 'cells', meaning hollow spaces. The beginning of the study of cells was initiated in 1632 with the invention of the first simple microscope by Van Leewenhoek. In 1674 Leewenhoek was the first to observe and describe bacteria, protozoans, sperms and red blood cells. He was also the first to notice that a cell possesses a nucleus, although, he did not understand the significance of what he saw. It was Robert Brown who described the nucleus in 1831.
Fig.2.1.1. The Diagram of 'Cells' as observed by Robert Hooke in a Slice of Cork
The work of M.J. Schleiden on Plant cells and Theodor Schwann on animal cells, led to a clear definition of cell and its functions. Schleiden concluded from his observations on plants that, although each cell of every tissue he observed could be looked upon as an independent unit, it must also contribute to the life of the whole organism to which it belonged. Schwann worked on a variety of animal tissues at the same time, and came to similar conclusions. The works of both these German scientists published in 1839, stated that all living matter, from the simplest of unicellular organisms to the very complex higher plants and animals, was composed of cells, and that each cell could act independently, but also functioned as an integral part of the complete organism. These deductions formed the basis of what came to be known as the cell theory or cell doctrine which recognized the cell as the basic biological unit.
Twenty years after the postulation of the 'cell doctrine' by Schleiden and Schwann, a German physician Rudolf Virchow made another significant discovery that cells reproduced from other cells ('omniscellulae cellula': every cell is derived from a cell). Very soon biologists recognised that sperm and ova were also cells and that they united with each other during fertilization, clearly suggesting that life from one generation to another was an uninterrupted succession of cells. Boveris's experiments with egg of sea urchins and Hammerling's experiments with Acetabularia (a green alga) convincingly demonstrated the importance of the nucleus in heredity. Thus, it can be noted that the growth, development, varied modes of metabolism, inheritance, evolution, disease, ageing, and death of organisms are just various aspects of cellular behaviour.
The essential features of the cell theory as it is explained today can be summarised as follows:
- According to this theory, life exists only in cells. (Viruses are an exception to cell theory.) Thus, organisms are made of cells and it is the cells which individually or collectively regulate the activity of an organism. Cells are ultimately responsible for storing, manipulating, and expressing biological information.
- All cells come from pre-existing cells and are responsible for the continuity of life from one generation to the next. The continuity of life not only involves the cell as a whole but also some of its components, such as genes and chromosomes.
- The components of a biological cell have a close relationship between the structure and function. In other words the biochemical activities of the cell depend on the structures within the cell organized in a specific way.
Significance of the Cell Theory
The formulation of scientific theories is a slow and accumulative process as seen in the case of the enunciation of the cell theory by Schleiden and Schwann in 1839. These German scientists never discovered the cell; they were not even the first to believe in or advance the idea that plants and animals were composed of cells. However, they could take loose threads of old ideas and observations and weave them into a meaningful hypothesis. By visualising cells as both structural and functional units of organization Schleiden and Schwann defined the basic unit of life. The 'cell doctrine' that they formulated in 1899, now ranks with Darwin's theory of evolution as one of the foundation stones of modern biology.