Most of the simple organisms known as protozoans are too small to be seen without the help of a microscope. Despite their size, however, they are able to do many of the same things as other living creatures—eat, get rid of wastes, grow, and reproduce. Many can even move. Although protozoans are often called one-celled animals, some scientists like to think of them as living organisms that are separate from plants and animals. Thousands of protozoans can be found throughout the world. Two of the most familiar are the amoeba and the paramecium. Where protozoans liveMany protozoans live in freshwater and oceans, where they are important members of the food chain because many other animals eat them. Others live in moist soil, where their waste products help to make the soil richer. Many protozoans are parasites, meaning that they live inside the bodies of larger animals. Some are found within human beings. A few types cause malaria and other diseases in humans and other animals. Physical featuresProtozoans are made of cell material called cytoplasm. A membrane, or thin skin, surrounds the cytoplasm to form a barrier between the creature and the outside world. Small pouches called vacuoles within the cytoplasm help with digestion and the removal of wastes. All protozoans have at least one nucleus. The nucleus is the central portion of a cell that directs the functions of the cell. It also contains the genetic material DNA. BehaviorDifferent protozoans move in different ways. Some have a covering of short, hairlike structures called cilia that are used like oars to propel the organism. Others have whiplike structures called flagella, which they lash about in order to move. The amoeba and related protozoans create fingerlike projections out of their bodies. The rest of the body then flows to catch up. Many protozoans that are parasites do not move. Instead they just attach themselves firmly inside their host. Protozoans have different ways of eating as well. Some contain plantlike structures that let them make their own food with the help of sunshine and water. Others feed on algae, bacteria, and other protozoans. Parasites feed by absorbing the blood and other fluids of the animal in which they live. Most protozoans are asexual, meaning that they reproduce without a partner. In many, the nucleus pinches in two in the middle, and the two halves pull apart with equal amounts of cytoplasm to form two separate organisms. |