The membrane is composed predominately of a double layer of phospholipids. The membrane also contains proteins most of which span its entire width. In the process of exocytosis, invaginations of the cell membrane allow the cells to take up molecules from the external environment. Lysosomes contain digestive enzymes and are responsible for the elimination of structures and molecules within the cell and for digestion of the contents of the phagocytic food vacuoles.
Mitochondria serve as the major sites for energy production within the cell.
They have an outer membrane with a smooth contour and an inner membrane with infoldings called cristae. The endoplasmic reticulum is a system of membranous tubules in the cell. The rough endoplasmic reticulum is covered with ribosomes and is involved in protein synthesis. The cell nucleus is surrounded by a double-layered nuclear membrane. At some points, the two layers are fused by nuclear pore complexes that allow for the passage of molecules.