There are different organisms. Some of them are multi cellular and some of them are uni cellular.  The process carried out by a group of cells in multi cellular organisms are carried out by one single cell in uni cellular organisms. How is this possible? 


Unicellular organism have only one cell and it's carry out all the process of that organism , but it doesn't mean that it will do the entire process that occuring in multicellular organism . Multicellular orangism are more complex than that of unicellular , therefore the Biological activities are much complex in them . But in the case of unicellular it's much simple . 
 Giving the difference between unicellular and multicellular organism , from them you can understand the complexity between those organisms.

Unicellular organism 

1.Body is made up of single cell

2.Division of labour is at the organelle level. It gives a low level of operational efficiency.

3. A single cell carries out all the life processes

4.The cell body is exposed to the environment on all sides

5.An injury of the cells can cause death of the organism.

6.A cell body cannot attain a large size because of the limit imposed by surface area to volume ratio

7.Lifespan is short due to heavy load of work.

8.Power of division is not lost

9.A well-marked capacity of regeneration is present

10.The cell has the same role for itself and the organism

Multicellular organism

1.Body is made up of numerous cells.

2.Division of labour may be at cellular, tissue, organ and organ system level.       It gives high degree of operational efficiency.

3.Different cells are specialized to perform different functions.

4.Only outer cells are specialized to face the environment. Inner cells are devoted to other functions.

5.Injury or death of some cells does not affect the organisms as the same can be replaced by new one.

6.A multicellular body can attain a large size increasing the number of small cells.

7.Lifespan is long due to limited load of work for each cell type.

8.Certain specialized cells lose power of division.

9.The capacity of regeneration decreases with increasing specialization.

10.Cells have a double role. One for themselves and other for the organism


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