The three main components of a cell are the cell membrane, cytoplasm, and nucleus. The cell membrane serves as a protective barrier, controlling the movement of substances in and out of the cell. The cytoplasm is a jelly-like substance that contains organelles, which perform various functions necessary for cell survival. The nucleus houses the cell's genetic material (DNA) and is responsible for regulating gene expression and cell division. Together, these components enable cells to function, grow, and replicate, forming the basis for all living organisms.