All living organisms are made up of minute units, the cells which are the smallest entities that can be called living. 
 Each cell is made up of protoplasm with a nucleus and bounded by plasma membrane with or without a cell wall. 
 All cells are basically alike in their structure and metabolic activities. 
Function of an organism is the sum total of activities and interaction of its constituent cells. 
All living cells arise from pre-existing cells by division. The cell is the fundamental unit of structure and function in all living organisms. The activity of an organism depends on the total activity of independent cells. Energy flow metabolism occurs within cells.