Cell is the basic structural and functional unit of living organism.
Take an inner fleshy leaf of onion bulb.With the help of forceps,peel off the skin from inner side of the onion.Put this layer immediately in watch glass containing water.This will avoid folding and drying of peel.Take a glass slide,put a drop of water on it and transfer peel from watch glass to it.Put a drop of iodine solution on the piece of onion peel followed by cover slip.Observe this slide under compound microscope.
Those organism in which single cell constitute whole organism.
For Ex:Amoeba,Paramecium,Chlamydomonas etc.
Multi cellular Organism
Those organism in which many cells group together in a single body and perform different functions to form various body parts.
For Ex:Humans,Fungi,plants,animals etc.