- Describe the protist kingdom.
- Identify protist characteristics.
- cilia (singular, cilium)
- short, hairlike projections, similar to flagella, that allow some cells to move
- the ability to move
- kingdom in the domain Eukarya that includes all eukaryotes except plants, animals, and fungi
- temporary, foot-like extension of the cytoplasm that some cells use for movement or feeding
Protists are the simplest eukaryotes. They are easiest to define by what they are not. Protists are not animals, plants, or fungi.
The protist kingdom is sometimes called the “trash can” kingdom. It includes all eukaryotes that don’t fit in one of the other three eukaryote kingdoms: Animalia, Plantae, or Fungi. There are thought to be between 60,000 and 200,000 protist species. Many have yet to be identified. The protist kingdom is very diverse, as shown in Figure below.
Characteristics of Protists
Like all other eukaryotes, protists have a nucleus containing their DNA. They also have other membrane-bound organelles, such as mitochondria (which carries out cellular respiration). Most protists are single-celled. Some are multicellular. Because the protist kingdom is so diverse, their ways of getting food and reproducing vary widely.
Most protists are aquatic organisms. They need a moist environment to survive. They are found mainly in damp soil, marshes, puddles, lakes, and the ocean. Some protists are free-living organisms. Others are involved in symbiotic relationships. They live in or on other organisms, including humans.
Motility of Protists
Most protists have motility. This is the ability to move. Protists have three types of appendages for movement. As shown in Figure below, they may have flagella,cilia, or pseudopods (“false feet”). There may be one or more whip-like flagella. Cilia are similar to flagella, except they are shorter and there are more of them. They may completely cover the surface of the protist cell. Pseudopods are temporary, foot-like extensions of the cytoplasm.
Protists have complex life cycles. Many have both asexual and sexual reproduction. An example is a protist called Spirogyra, a type of algae, shown Figure below. It usually exists as haploid cells that reproduce by binary fission. In a stressful environment, such as one that is very dry, Spirogyra may produce tough spores that can withstand harsh conditions. Spores are reproductive cells produced by protists (and other organisms). If two protist spores are close together, they can fuse to form a diploid zygote. This is a type of sexual reproduction. The zygote then undergoes meiosis, producing haploid cells that repeat the cycle.
Protists get food in one of three ways. They may ingest, absorb, or make their own organic molecules.
- Ingestive protists ingest, or engulf, bacteria and other small particles. They extend their cell wall and cell membrane around the food item, forming a food vacuole. Then enzymes digest the food in the vacuole.
- Absorptive protists absorb food molecules across their cell membranes. This occurs by diffusion. These protists are important decomposers.
- Photosynthetic protists use light energy to make food. Protists that are photosynthetic have chloroplasts. Photosynthesis takes place inside the chloroplasts. They are major producers in aquatic ecosystems.
- Kingdom Protista includes all eukaryotes that are not animals, plants, or fungi. It is a very diverse kingdom. It consists of both single-celled and multicellular organisms.
- Protists have nuclear membranes around their DNA. They also have other membrane-bound organelles. Many live in aquatic habitats, and most are motile, or able to move. Protists have complex life cycles that may include both sexual and asexual reproduction. They get food through ingestion, absorption, or photosynthesis.
Lesson Review Questions
1. What are protists?
2. Identify three structures that protists use to move.
3. Describe three ways that protists get food.
4. A mystery organism consists of one cell. It could be a protist or a prokaryote. What single fact about the mystery cell would allow you to determine which type of organism it is? Explain your answer.
5. Compare and contrast asexual and sexual reproduction in protists.
Points to Consider
Protists are traditionally classified as animal-like, plant-like, or fungi-like. You will read more about each of these types of protists in the next lesson.
- Based on what you already know about animals, plants, and fungi (such as mushrooms), how might the three types of protists differ?
- Why do you think these protists are not classified with the organisms they resemble? For example, why aren’t animal-like protists classified as animals? What sets protists apart from other eukaryotes?
Next: Types of protists
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