Cells are called the building blocks of life because they are the basic unit for all living organisms, and must be present for life to exist. Plants and animals have trillions of cells, while protists have only one cell, yet protists are still considered to be living organisms.
Cells are the smallest unit of life that can reproduce independently. Cells must come from preexisting cells, as scientists are not able to create or produce a live cell in a laboratory.
All cells are made with similar parts. For instance, there is a plasma membrane around each cell that allows nutrients and fluids to pass in and out of the cell. This membrane has channels and pores. The plasma membrane is like a wall that holds all parts of the cell in place. The tiny parts of each cell are called organelles, which means "little organs" in Latin.
Each cell in the human body is a small and simple form of life. Each type of cell is programmed to do different things, such as carry oxygen, transmit signals to the brain, or defend the human body against viruses and bacterial infections. The origin of cells is linked directly to the origin of life.