Cells are comprised of a cell membrane, organelles and nucleus. The cell membrane holds the cell's internal contents from its environment. The cell membrane is comprised of two layers of fat. The cytoplasm is a liquid in which the various organelles of the cell are suspended. The cytoplasm keeps organelles in place. It also allows different materials to flow from one organelle to another. The nucleus of a cell contains genetic material. DNA, RNA and mitochondria replicate in the nucleus. The nucleus also produces and replicates proteins, and sends the proteins to other parts of the cell.