What are cells?

Cells are the building blocks of all living organisms . 

• Every single living organism is made up of cells .
• The human body has from around a billion to trillions of cells .
• Similarly other organisms are also made of a large number of cells based on their complexity.
• Cells can be of various shapes and sizes based on their functions .

For example -

In the human body, the cells vary in their shape (could be polyhedral, oval, discoidal, cylindrical, spindle shaped) and size . 

• The shape of the cells is decided by the function they perform :
• White blood cells that defend the body against pathogens are Amoeboid shaped  
• Nerve cells are long and branched that conduct nerve impulses  
• Muscle cell s are  cylindrical or spindle shaped cells concerned with the movement of body parts