Cell Theory :
The German botanist M.J. Schleiden (1838) and the zoologist T.S. Schwann (1839) established the cell theory in definite form. These two investigators are regarded as the founders of cell theory which postulates that in both plants and animals, living matter exists in the form of essentially similar unit masses or cells. Both of them, postulated that the cell is the basic structural and functional unit of cell in all living organisms. The simple, basic and formal biological generalization is known as cell theory or cell doctrine. The cell theory announced by them gave a great impetus to cell research and forms an important land-mark in the history of the biology.
Schwan studied animal tissues and his work with connective tissues and his work with connective tissues such as bone and cartilage led him to modify the evolving cell theory to include the fact that living things are composed of both cells and the products or secretions of the cells.
In 1839, J.E. Purkinje coined the term protoplasm for the formative substance found in the cell of animal embryo and Von Mohl in 1846, applied the name protoplasm to the contents of embryonic cells of the plant. Protoplasm is the physical basis of life and is divided into two parts, the central part, the nucleus or karyon and the peripheral extra-nuclear part of cytoplasm. The nucleus contains chromosomes (Waldeyer, 1888) on which the hereditary units, the factors or the electron microscope has helped in understanding the exact arrangement of macromolecules of the various protoplasmic compounds.
The modern version of the cell theory states that :
i) All living organisms are made up of cells.
ii) Cell is the basic structural and functional unit of life.
iii) Cells contain genetic information which is passed on form cell to cell during cell division.
iv) All cells arise form pre-existing cells. (Rudolf Virchow 1858).
v) Cells are self-duplicating, self-contained units and some cells at totipotent.
This clearly indicates that the organisms grow from a single cells. When a cell divides its nucleus divides first into two nuclei. The chromosomes (bearing the genes) split lengthwise and each gene is thus duplicated. Thus exact replicas of the original nucleus is produced. Then extra-nuclear protoplasm, called cytoplasm divides into two parts to form two cells. The body of unicellular organism is comprised of a single cell. In multicellular organisms single celled zygote undergoes several mitotic cell divisions to produce large number of cells giving rise to tissues. Tissues, in turn, combine to form organs. Various organs combine to form a system while all systems make up a multicellular organism.
There are some exceptions to cell theory like, viruses do not have cellular organization, Bacteria and blue-green algae are prokaryotes and they do not process true cellular structures. Fungi like Mucor, Rhizopus and alga like Vaucheria are tubular and multinucleated. These organisms have bodies containing undivided mass of protoplasm.
All cells have certain common characteristics such as each cell is enclosed by a plasma membrane made up of lipo-proteins. All cells have protoplasm consisting of water and various organic and inorganic substances. Each living cells possesses genetic material which directs and controls the cellular functions. Proteins synthesis is an important metabolic function. The protein produced within a cell determines the form and structure of a cell. A cell is an independent unit by itself and has a capacity to duplicate. All cells arise only by division of pre-existing cells. Genetic information is passed on form one generation to the next by the cells through genes. The basic composition of protoplasm of different cells remains same in all organisms.
Living organisms are grouped into two main categories, the Prokaryotes and Eukaryotes. The prokaryotes have simple cellular organisation while eukaryotes exhibit high degree of organization.