Atoms are the smallest unit of the chemical elements.
Molecules are formed of atoms and are found in all the matter, non-living and living, and they make up the most fundamental structures of living beings.
The molecules further combine together to form a cell, which is the basic unit of life.
The tissue is formed of cells, which functions together to do a certain task. For example, connective tissue, muscle tissue, and neural tissue.
An organ refers to a system of tissues, which functions together on a bigger scale to perform certain kinds of functions within the body of an animal. The examples include the heart, brain, and lungs.
An organ system refers to a group of organs, which functions together to do specific functions of the body. For example, the respiratory system uses the respiratory muscles, airways, and lungs to inhale oxygen and discharge carbon dioxide in animals.
An organism is a self-contained, recognizable individual. It can be unicellular like amoeba or bacteria, or multicellular comprising of organs and organ systems. An example of a multicellular organism is a human being.
