Protists are placed in the Protist Kingdom. This kingdom is one of four kingdoms in the Eukarya domain. The other three Eukarya kingdoms are the Fungi, Plant, and Animal Kingdoms. The Protist Kingdom is hard to define. It includes many different types of organisms. You can see some examples of protists in Figure 9.1. The Protist Kingdom includes all eukaryotes that dont fit into one of the other three eukaryote kingdoms. For that reason, its sometimes called the trash can kingdom. The number of species in the Protist Kingdom is unknown. It could range from as few as 60,000 to as many as 200,000 species. For a beautiful introduction to the amazing world of protists, watch this video: MEDIA Click image to the left or use the URL below. URL: Scientists think that protists are the oldest eukaryotes. If so, they must have evolved from prokaryotes. How did this happen? How did cells without organelles acquire them? What was the origin of mitochondria, chloroplasts, and other organelles? The most likely way organelles evolved is shown in Figure 9.2. First, smaller prokaryotic cells invaded, or were engulfed by, larger prokaryotic cells. The smaller cells benefited by getting nutrients and a safe place to live. The larger cells benefited by getting some of the organic molecules or energy released by the smaller cells. Eventually, the smaller cells evolved into organelles in the larger cells. After that, neither could live without the other. Despite the diversity of protists, they do share some traits. The cells of all protists have a nucleus. They also have other membrane-bound organelles. For example, all of them have mitochondria, and some of them have chloroplasts. Most protists consist of a single cell. Some are multicellular but they lack specialized cells. Most protists live in wet places. They are found in oceans, lakes, swamps, or damp soils. Many protists can move. Most protists also have a complex life cycle. The life cycle of an organism is the cycle of phases it goes through until it returns to the starting phase. The protist life cycle includes both sexual and asexual reproduction. Why reproduce both ways? Each way has benefits. Asexual reproduction is fast. It allows rapid population growth when conditions are stable. Sexual reproduction increases genetic variation. This helps ensure that some organisms will survive if conditions change. Protists are classified based on traits they share with other eukaryotes. There are animal-like, plant-like, and fungus- like protists. The three groups differ mainly in how they get carbon and energy. Animal-like protists are called protozoa (protozoan, singular). Most protozoa consist of a single cell. Protozoa are probably ancestors of animals. Protozoa are like animals in two ways: 1. Protozoa are heterotrophs. Heterotrophs get food by eating other organisms. Some protozoa prey on bacteria. Some are parasites of animals. Others graze on algae. Still others are decomposers that break down dead organic matter. 2. Almost all protozoa can move. They have special appendages for this purpose. You can see different types in Figure 9.3. Cilia (cilium, singular) are short, hair-like projections. Pseudopods are temporary extensions of the cytoplasm. Flagella are long, whip-like structures. Flagella are also found in most prokaryotes. Plant-like protists are commonly called algae (alga, singular). Some algae consist of single cells. They are called diatoms. Other algae are multicellular. An example is seaweed. Seaweed called kelp can grow as large as trees. You can see both a diatom and kelp in Figure 9.4. Algae are probably ancestors of plants. Algae are like plants mainly because they contain chloroplasts. This allows them to make food by photosynthesis. Algae are important producers in water-based ecosystems such as the ocean. On the other hand, algae lack other plant structures. For example, they dont have roots, stems, or leaves. Also unlike plants, some algae can move. They may move with pseudopods or flagella. Fungus-like protists include slime molds and water molds, both shown in Figure 9.5. They exist
Output: Most protist diseases in humans are caused by