The cell theory

The cell Theory
   
The German botanist, M.J. Schleiden and the Zoologist Theoder Schwann first propounded the cell theory in 1839. Biologists corrected and modified the cell theory given by Schleiden and Schwann and summarized it as follows.
Statement of cell theory:
1. All living organisms are composed of small living unit called 'cells'.
2. All the cells are fundamentally similar in chemical compositions and metabolic activities.
3. The functions of an organism as a whole are the outcome of the activities and        interactions of the cells consisting the body of that organism.
4. All cells arise from pre-existing cells.
5. The cells are structural and functional unit of life.
6. The growth of organism occurs by the cell division and cellular growth in multicellular organisms while by cellular growth in unicellular organisms.

Short-comings of cell theory
   
The cell theory is more generalized and corrected. However, it does not apply for all organisms. There are some exceptions. These are:
1. Bacteria and Cyanobacteria lack true nucleus. Their nuclear material (DNA) is not enclosed by nuclear membrane but lies directly in the cytoplasm.
2. The coenocytic bodies of some lower plants like Vaucheria (an alga) and Mucor, Rhizopus (fungi) are explainable according to the cell theory because their bodies are made up of the undivided mass of protoplasm in which many nuclei remain scattered i.e. coenocytic.
3. Viruses are also not expected to be covered by the cell theory. They consist of core of nucleic acid (DNA or RNA) surrounded by a protein sheath. They are as non-living outside the living tissue.
4. The tissues like connective tissues possess the non living materials, Matrix. About matrix like material, nothing is mentioned in cell theory.
Levels of Organization

Biosphere: The sum of all living things taken in conjunction with their environment. In essence, where life occurs, from the upper reaches of the atmosphere to the top few meters of soil, to the bottoms of the oceans. We divide the earth into atmosphere (air), lithosphere (earth), hydrosphere (water), and biosphere (life).

Ecosystem: The relationships of a smaller groups of organisms with each other and their environment. Scientists often speak of the interrelatedness of living things. Since, according to Darwin's theory, organisms adapt to their environment, they must also adapt to other organisms in that environment. We can discuss the flow of energy through an ecosystem from photosynthetic autotrophs to herbivores to carnivores.

Community: The relationships between groups of different species. For example, the desert communities consist of rabbits, coyotes, snakes, birds, mice and such plants as sahuaro cactus (Carnegia gigantea), Ocotillo, creosote bush, etc. Community structure can be disturbed by such things as fire, human activity, and over-population.

Species: Groups of similar individuals who tend to mate and produce viable, fertile offspring. We often find species described not by their reproduction (a biological species) but rather by their form (anatomical or form species).

Populations: Groups of similar individuals who tend to mate with each other in a limited geographic area. This can be as simple as a field of flowers, which is separated from another field by a hill or other area where none of these flowers occur.

Individuals: One or more cells characterized by a unique arrangement of DNA "information". These can be unicellular or multicellular. The multicellular individual exhibits specialization of cell types and division of labor into tissues, organs, and organ systems.

Organ System: (in multicellular organisms). A group of cells, tissues, and organs that perform a specific major function. For example: the cardiovascular system functions in circulation of blood.

Organ: (in multicellular organisms). A group of cells or tissues performing an overall function. For example: the heart is an organ that pumps blood within the cardiovascular system.

Tissue: (in multicellular organisms). A group of cells performing a specific function. For example heart muscle tissue is found in the heart and its unique contraction properties aid the heart's functioning as a pump. .

Cell: The fundamental unit of living things. Each cell has some sort of hereditary material (either DNA or more rarely RNA), energy acquiring chemicals, structures, etc. Living things, by definition, must have the metabolic chemicals plus a nucleic acid hereditary information molecule.

Organelle: A subunit of a cell, an organelle is involved in a specific subcellular function, for example the ribosome (the site of protein synthesis) or mitochondrion (the site of ATP generation in eukaryotes).

Molecules, atoms, and subatomic particles: The fundamental functional levels of biochemistry.