ABSTRACT
The cell is the structural and functional unit of all living organisms. It is the building block of life. There are two types of cells, prokaryotic and eukaryotic cells. Prokaryotic cells are singletons without a nuclear membrane, whereas eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. All cells share the following activities: reproduction by cell division, use and production of enzymes and other proteins coded by DNA genes, and response to external and internal stimuli such as changes in temperature, pH, or nutrient levels. Living cells are composed of a wide array of compounds, and physiological or chemical reactions that occur simultaneously.
