All living things are made from one or more cells. The nucleus is the control center of the cell. It houses the nucleolus and genetic material (chromatin) used for directing cell functions. Nuclear pores allow materials to pass in and out of the nucleus. The nuclear envelope is a membrane which surrounds and protects the nucleus. The nucleolus produces ribosomes. Ribosomes are factories that produce proteins needed by the cell. Lysosomes contain chemicals (enzymes) that break down and recycle harmful materials.