cell
The cell is the smallest unit of structure and function of all living organisms. Cell Biology focuses on significant aspects of the cell from its structure to its division. Some organisms contain just one cell, and others contain trillions. Some have a nucleus with DNA, others do not. Some have many organelles, others do not. But all cells are surrounded by a cell membrane. And it is this semipermeable membrane that determines what can enter and leave the cell. All cells need energy, and for many organisms, this energy comes from photosynthesis and cellular respiration. All cells come from preexisting cells through the process of cell division, which can produce a new prokaryotic organism. The cell cycle, which includes mitosis, defines the life of an eukaryotic cell.