An animal cell contains several organelles that each have an important function. The cell membrane forms the boundary of the cell and is selectively permeable. Inside the cell are the nucleus, which controls cell functions, mitochondria which produce energy, and ribosomes and endoplasmic reticulum which produce proteins. Other organelles include the Golgi apparatus for packaging materials, lysosomes for breaking down waste, and the cytoskeleton which maintains the cell's shape. Each organelle works together for the cell to carry out its required functions.