Color enhanced TEM of a normal cell. The cell membrane, nucleus and the nucleolus are all distinguishable. The cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". The study of cells is called cell biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Cells emerged on Earth at least 3.5 billion years ago.