Animal cell structure. Eukaryotic cells are bounded by a cytoplasmic membrane. Chromosomes (not visible as individual structures in a nondividing cell) are located in the nucleus, and the nuclear envelope (nuclear membrane) is a double membrane with large pores through which RNA and proteins move. The nucleolus, a substructure within the nucleus, is the site of rRNA synthesis. All other organelles are in the cytoplasm. The centrosome consists of two centrioles, perpendicular cylinder-shaped protein complexes. Centrosomes organize microtubule spindles that attach to the cytoskeleton. During cell division, the centrosome duplicates and the two centrosomes migrate to opposite poles of the cell. The centrosomes then organize the spindle apparatus, in which microtubules connect chromosomes to the centrosomes for partitioning of the chromosomes into daughter cells. Mitochondria, the energy factories of animal cells, oxidize fuels to produce ATP. Lysosomes, containing degradative enzymes, aid in digestion of intracellular debris and recycle certain components. Peroxisomes help detoxify chemicals and degrade fatty acids. The smooth endoplasmic reticulum (ER) is the site of lipid synthesis and drug detoxification. Ribosomes, composed of both RNA and protein, act as protein-synthesizing factories; many attach to the ER, giving it a rough appearance. The rough ER sorts proteins destined for the cytoplasmic membrane or for other organelles; it is continuous with the outer membrane of the nuclear envelope. The Golgi apparatus, a membranous network, receives proteins from the ER and modifies and directs them to their proper compartments. Cytoskeletal fibers are a network of structural proteins that give shape to the cell and aid in cell movement.