- Describe the protist kingdom.
- Outline the evolution of protists.
- Identify protist characteristics.
- cilia (singular, cilium)
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.
Protists range from single-celled amoebas to multicellular seaweed. Protists may be similar to animals, plants, or fungi.
Evolution of Protists
Scientists think that protists are the oldest eukaryotes. If so, they must have evolved from prokaryotic cells. How did this happen? The endosymbiotic theory provides the most widely-accepted explanation. That’s because it is well supported by evidence.
The First Eukaryotic Cells
According to the endosymbiotic theory, the first eukaryotic cells evolved from a symbiotic relationship between two or more prokaryotic cells. Smaller prokaryotic cells were engulfed by (or invaded) larger prokaryotic cells. The small cells (now called endosymbionts) benefited from the relationship by getting a safe home and nutrients. The large cells (now called hosts) benefited by getting some of the organic molecules or energy released by the endosymbionts. Eventually, the endosymbionts evolved into organelles of the host cells. After that, neither could live without the other.
As shown in Figure below, some of the endosymbionts were aerobic bacteria. They were specialized to break down chemicals and release energy. They evolved into the mitochondria of eukaryotic cells. Some of the small cells were cyanobacteria. They were specialized for photosynthesis. They evolved into the chloroplasts of eukaryotic cells.
Endosymbiotic theory explains how eukaryotic cells arose.
Evidence for the Endosymbiotic Theory
Many pieces of evidence support the endosymbiotic theory. For example:
- Mitochondria and chloroplasts contain DNA that is different from the DNA found in the cell nucleus. Instead, it is similar to the circular DNA of bacteria.
- Mitochondria and chloroplasts are surrounded by their own plasma membranes, which are similar to bacterial membranes.
- New mitochondria and chloroplasts are produced through a process similar to binary fission. Bacteria also reproduce through binary fission.
- The internal structure and biochemistry of chloroplasts is very similar to that of cyanobacteria.
Characteristics of Protists
Like all other eukaryotes, protists have a nucleus containing their DNA. They also have other membrane-bound organelles, such as mitochondria. 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 use cilia, pseudopods, or flagella to move.
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.
Spirogyra is a genus of algae with a complex life cycle. Each organism consists of rectangular cells connected end-to-end in long filaments.
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. 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.
- Scientists think that protists are the oldest eukaryotes. They most likely evolved from prokaryotic cells, as explained by the endosymbiotic theory. This theory is well-supported by evidence.
- 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. How did the first eukaryotic cells evolve, according to endosymbiotic theory?
3. Identify three structures that protists use to move.
4. Describe three ways that protists get food.
5. 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.
6. Identify one piece of evidence for endosymbiotic theory. Explain how this evidence supports the theory.
7. 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?