Cells are considered the smallest unit of life because they are the smallest components in living things, and some organisms, such as bacteria, are made up of single cells. In addition, each part of the animal body, including blood vessels, organs and skin, is made up of a congregation of cells.
The cell was discovered by scientist Robert Hooke in the mid 1660s and is often referred to as "life’s building block." All organisms are made up of one or more cells. While bacteria have one cell, the body of an average human being is comprised of approximately 100 trillion cells.
Another reason why cells are considered the smallest unit of life is that all crucial operations of an organism take place within a cell. All cells are born from previously existing cells, with the older ones degenerating. The cells carry hereditary attributes, such as DNA, and pass them from one generation to the next. When an organism is unable to produce new cells, it dies off, because the body’s vital functions no longer take place. Cells are responsible for basic functions, such as metabolism, photosynthesis and protein synthesis.
All cells have a selectively permeable membrane. The covering controls what is allowed to pass through it. An individual cell comprises a nucleus, organelles and a cytoskeleton.