All organisms are built from cells. All animal tissues including human are also organised from collections of cells. Thus cell is the fundamental unit of life. If cell dies, tissue dies and it cannot function.
Modern cell theory can be divided into the following fundamental statements:
• Cells make up all living matter
• All cells arise from other cells
• The genetic information required during the maintenance of existing cells and the production of new cells passes from one generation to the other next generation
• The chemical reactions of an organism that is its metabolism, both anabolism and catabolism, takes place in the cells.
Types of Cells
In general two types of cells exist in nature. They are: 1. Prokaryotic cells 2. Eukaryotic cells
1. Prokaryotic Cells
Typical prokaryotic cells (Greek: Pro-before and karyonnucleus) include the bacteria and cyanobacteria. Most studied prokaryotic cell is Escherichia coli (E. coli).
• It has a minimum of internal organisation and smaller in size
• It does not have any membrane bound organelles.
• Its genetic material is not enclosed by a nuclear membrane
• Its DNA is not complexed with histones. Histones are not found in prokaryotic cells
• Its respiratory system is closely associated with its plasma membrane and
• Its sexual reproduction does not involve mitosis or meiosis.
2. Eukaryotic Cells
The eukaryotic cells (Greek: Eu-true and karyon-nucleus) include the protists, fungi, plants and animals including humans. Cells are larger in size
• It has considerable degree of internal structure with a large number of distinctive membrane enclosed having specific functions
• Nucleus is the site for informational components collectively called chromatin
• Sexual reproduction involves both mitosis and meiosis
• The respiratory site is the mitochondria
• In the plant cells, the site of the conversion of radiant energy to chemical energy is the highly structural chloroplasts.