They feed on other microbes or organic particles and are heterotrophic. Protozoa consume food in two ways. The first is phagocytosis, which occurs when a flexible section of the cell membrane wraps and engulfs a food particle, bringing it into the cell in a vacuole. The second method is called autophagy. During this process, the cell breaks down its own components to provide nutrients for itself. Protozoa are single-celled organisms that lack a true nucleus and contain no organelles except for some granules inside their cytoplasm. However, they do contain many ribosomes, which are the building blocks of proteins. Therefore, they are able to build large structures such as membranes and fibers to help them survive in an environment that is not favorable for normal cells. They also have the ability to move around using flagella and cilia. Some types of protozoa can reproduce asexually by dividing into two new organisms with the same type. These new individuals will have the same characteristics as the original protozoan. Some species of protozoa can cause disease in humans. For example, Giardia parasites are found in the feces of infected people and animals. People become infected with Giardia when they drink water containing contaminated feces. The parasites use several methods to avoid being killed by heat, light, and chemicals.