Cell organelles are small structures within cells that perform specific functions. The main organelles include the nucleus, which contains DNA and controls the cell; mitochondria, which generate energy; and the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, chloroplasts, and cytoskeleton, each of which has a specialized function in the cell. Organelles are surrounded by membranes that allow transport of materials in and out and provide structure and protection to the organelle.