What is the basic unit of all living organisms?

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The basic unit of all living organisms is cell, according to the cell theory. The simplest living organisms are the single celled organisms, also known as prokaryotes (such as bacteria, etc.). Higher life forms (such as plants, animals, human beings, etc.) are made up of many cells and are multi-cellular organisms, also known as eukaryotes. Thus, cell is the fundamental unit of life and hence cannot be further sub-divided. Also, cells only originate from pre-existing cells. 

Prokaryotes carry out all their functions using a single cell, however, eukaryotes requires much more complex systems. Cells that carry out similar functions are grouped as tissue. Tissues with similar functionalities form an organ and a number of organs working for the same goal constitute an organ system. For example, circulatory system is an organ system, heart is an organ and nerve tissues are made up of nerve cells, etc. 

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