All living organisms are cellular. Cells in different organisms exhibit close similarly in structure, molecular organization and biological activities. Thus, there exists a unique master plan of organization of all cells. All organisms have the cell as their basic structural and functional unit. Schleiden and Schwann cofounded the cell theory, according to which all living organisms are composed of one or more cells. Cells are the basic units of living organisms. All cells arise from preexisting cells.
Each cell acts as a self-contained unit. It can carry out nutrition, respiration, growth, reproduction and self regulation wherever required. Isolated living cells of multicellular organisms can grow for generations artificially. Plant cells are found to be totipotent.
A single cell performs all its life processes in unicellular organisms. The organelles it contains help the cell to carry out and regulate different cellular activities. But in multicellulars, there is division of labour and the cells interact and cooperate to perform different activities.
Cellular life needs compartmentalisation. Cell, itself is a bigger compartment bounded by plasmamembrane. In plant cells an additional cellulosic boundary called cell wall is present. Eukaryotic cells possess a number of membrane bound organelles including the nucleus. Each organelle has a specific structure and function. But prokaryotes lack this intracellular compartmentalisation.
The story of origin of life is based on presumptions. The history of earth goes back to 4.5 billion years. At the beginning, it had a reducing atmosphere. After getting energy from the sun, chemical interactions took place that led to the formation of a variety of macromolecules which ultimately formed the proteins and nucleic acids by polymerisation. Both these compounds assembled and the first living organism was created.
Miller and Urey recreated the atmosphere of the primitive earth in laboratory condition and synthesised amino acids and bases required for synthesis of proteins and nucleic acids. Subsequently, there was formation of autotrophs and heterotrophs that gave rise to plant cells and animal cells, respectively.