Animal cell structure. Artwork of a section through an animal cell. At lower centre is the nucleus, which contains the cell's genetic information in the form of DNA (deoxyribonucleic acid). Surrounding the nucleus is endoplasmic reticulum (ER), a membrane bound organelle that is the site of lipid synthesis and the production of membrane-bound proteins. ER may be either rough (RER), that is with ribosomes (purple dots), or smooth (SER), without ribosomes. Also present are mitochondria, which produce the cell's energy, and golgi apparatus, a membrane-bound organelle that modifies and packages proteins.