A cell is defined as the smallest basic unit of life as it is responsible for all the life processes. Thus they are called the structural, functional and biological units of all living beings. A cell can replicate itself independently.
Different cells combine to form tissues. Tissues are groups of cells that have a similar structure and act together to perform a specific function.
Tissues combine to form organs. An organ is a self-contained group of tissues that performs a specific function in the body. The heart, liver, and stomach are examples of organs in humans.
Organs combine to form organ system such as respiratory system, excretory system, circulatory system and reproductive system.
All the systems and functions are regulated at the cellular level and are gradually segregated to different parts such as when some cells perform a specific function, they are called as tissues and when some tissues perform a specific function, they are called as organ and when organs perform a specific function, they are called as organ system. As a result of this, it is concluded that, cells are the basic unit of every system or process, thus becoming the structural unit of living organisms.