What are theories of a Cell? from chapter 8 Biology Class 11
In this article, we will discuss ‘Cell Theories’ from Unit 3 – Chapter 8 Biology Class 11.
Modern Version of Cell Theory
According to this theory-
- All living organisms (animals, plants or microbes) are made up of one or more cells or cell products.
- All metabolic reactions in unicellular and multicellular organisms take place inside the cells.
- Cells originate from other cells, i.e. no cells can originate spontaneously or de novo, but comes into being only by cell division and duplication of already existing cells.
- The smallest clearly defined unit of life is CELL.
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The exception to Cell Theory
Cell theory does not have universal application, this means, there are certain living organisms which do not have true cells. A true cell has following characteristics-
- A set of genes (the functional region of DNA) which constitutes the blueprints for regulating cellular activities and makes new cells.
- A plasma membrane that permits controlling the exchange of matter and energy from outside.
- Metabolic machinery for sustaining life activities such as growth, reproduction, and repair of cells.
Viruses do not possess the plasma membrane and metabolic machinery, but they have set of genes/ hereditary material either RNA or DNA. They also have a definite genetically determined macromolecular organization, the capacity of auto-reproduction and capacity of mutation in their genetic material. Thus they can only reproduce inside the host cells (animals, plants or bacteria) and use their machinery for their own multiplication. Outside the body of an organism, viruses are non-living particles.
There are certain other organisms such as protozoan Paramecium, the fungus Rhizopus, and the alga Vaucheriawhich do not have true cells because they have bodies containing an undivided mass of protoplasm which lacks cell-like organization and has more than one nucleus. This raised the question, whether the cell is a basic unit of structure in them?
Vaucheria Different types of Viruses
Figure: Organisms forming an exception to cell theory
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Up to the middle of the nineteenth century, great emphasis was laid on the cell wall and not to the cellular contents. But soon, some cell biologists stated to recognize the importance of “juicy” contents of the cells. Different scientists suggested different names for this juicy content of the cell, finally in 1892; O. Hertwig put forward the protoplasm theory.
According to this theory, all living matter, out of which animals and plants are formed, is the protoplasm. The cell is an accumulation of living substance or protoplasm which is limited in space by an outer membrane and has a nucleus. The protoplasm which is inside the nucleus is called nucleoplasm and that exists between nucleus and plasma membrane is called cytoplasm.
The twentieth century has witnessed great advancement in cell biological knowledge due to the following reasons-
- The increased resolving power of instrumental analysis due to the introduction of electron microscopy and X-ray diffraction techniques.
- The convergence of cytology with other fields of biological research, especially genetics, physiology, biochemistry.
- New histochemical, cytochemical and immunocytochemical (using antibodies to localize antigen) techniques have been developed to detect various molecular components of the cell.
- Cellular components have been separated using ultracentrifugation.
- The structure of plasma membrane has been studied by techniques of freeze-fracturing and freeze-etching.
Due to development of various improved techniques in the study of cells, the validity of the cell theory and protoplasm theory has become vague. Therefore another theory has been proposed which is known as Organismal Theory.
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According to this theory, the body of all multicellular organisms is a continuous mass of protoplasm which remains divided into small centers, called cells, for various biological activities. Thus a multicellular organism is a highly differentiated protoplasmic individual, which differs from unicellular protozoa in size and degree of differentiation of protoplasm. The differentiation involves separation of protoplasm into small semi-independent compartments called Cells. Even the embryological development of a multicellular individual includes growth and internal differentiation of a single protoplasmic individual (egg). But the organismal theory also fails to explain the position of viruses.
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