Protoplasm is packaged into a unit called a cell. A cell is considered to be the basic unit of life. An organism may be a single cell (a unicellular organism) or it may be composed of a few to many cells (multicellular organisms). Unicellular organisms carry out all the activities that define life. In simple multicellular organisms, each cell carries out all the life activities much like the unicellular organism. In contrast, complex multicellular organisms have cellular specialization. Different cell types are responsible for different life activities, and the cooperation of these cells gives rise to the complex organism.

Euglena: Unicellular Organism:

Porphyra: Simple Multicellular Organism:

Complex Organism Showing Cellular Specialization