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Every organism has trillions of cells, but an entire organism begins from a single cell, a fertilized egg known as a zygote. During cleavage this fertilized egg multiplies, thereby increasing the number of cells as the development occurs. One substantial single-celled zygote proliferate into numerous compact cells. ![Screenshot 2021-04-16 154157.png](https://resources.cdn.yaclass.in/e9ade472-6154-444e-aa7e-f7f38f85f2bb/Screenshot20210416154157w600.png)
The number of cells varies in all living organisms. Based on the number of cells, the organisms are classified into two groups, namely unicellular & multicellular.
Important!
Wonder facts:
- We have approximately, \(37.2\) \(trillion\) cells; each of these cells varies in shapes and sizes.
- In our human body, \(1\) \(million\) cells die every second.
Unicellular organisms:
The organisms consist of only one cell are called unicellular organisms. In a unicellular organism, a single cell has to perform all the life processes like respiration, digestion, absorption, excretion, growth and development as there is no division of labour.
Example:
Bacteria, amoeba, plankton, paramecium, euglena and chlamydomonas. ![Simple_diagram_of_bacterium_(en).svg](https://resources.cdn.yaclass.in/f7c2808a-47ac-4d63-9cf8-7a4e982b0678/Simplediagramofbacteriumen.svg)
Bacteria
![Screenshot 2021-04-16 154157.png](https://resources.cdn.yaclass.in/e9ade472-6154-444e-aa7e-f7f38f85f2bb/Screenshot20210416154157w600.png)
Unicellular organisms
Reference:
https://commons.wikimedia.org/wiki/File:Simple_diagram_of_bacterium_(en).svg