All living beings are made up of microscopic structures known as cells. Cells are considered the basic structural and functional unit of life because each cell is a discrete and distinct unit that is capable of independent existence and carrying out all functions vital for living. Cells replicate and develop, resulting in growth. They carry out functions such as metabolism, nutrition, respiration, reproduction, transportation, and excretion, in the same way that an individual organism does. 

Unicellular organisms, which are capable of existing independently, are an example that supports the theory that the cell is the basic unit of life.