Protozoans are single-celled organisms that carry out various life processes. Like other living things, they produce waste products, which must be excreted from their bodies to maintain the proper internal environment. The excretory product in protozoans is usually ammonia, a compound that is produced when proteins are broken down during cellular metabolism. Ammonia is highly toxic to cells and needs to be excreted quickly to avoid damaging the organism. Therefore, protozoans have specialized structures that help them get rid of excess ammonia, such as contractile vacuoles or diffusion through the cell membrane. Other excretory products such as urea, uric acid or amino acids are commonly found in more complex organisms with more advanced excretory systems.