Introduction to Protists
Protists are organisms in the highly diverse kingdom Protista. Essentially, Protista is made up of organisms that don't fit well into any other kingdom. Examples include paramecium, amoebas, euglenas, stentors, diatoms, dinoflagellates, and algae. In this article, I'll cover:
1. The characteristics of the kingdom Protista
2. Individual types of protists, including:
- Giant Kelp
- Cellular and plasmodial slime molds
3. Observation in the lab
Characteristics of Protists
Although most protists are microscopic unicellular organisms, protists are a very diverse group. Many of them are in the Protista kingdom primarily because scientists don't know where else to put them. They are not plants, animals, or fungi, each of which belongs to its own kingdom.
- Unicellular or Multicellular? Most are unicellular (they have only one cell), but not all. Kelp, for example, is a multicellular protist.
- Prokaryotes or Eukaryotes? All protists are eukaryotes, which means they have a nucleus. In fact, protists were the first eukaryotes!
- Reproduction: Many reproduce by binary fission, which means that they split in two. However, others reproduce by gametes (sexual reproduction.) Some protists have complex life cycles.
- Autotrophic or Heterotrophic? Some protists engage in photosynthesis (like plants), others eat food they find (like animals), and some just absorb their food (like fungi).
- Locomotion: How do protists get around? Some use flagella (a whip-like tail) or cilia (short hairs) to help them move. Amoebas use pseudopodia, which are extensions of their cytoplasm.
- Location: Most live in water, damp soil, sand, moist leaf litter, and other damp or wet locations.
- Parasites? Some protists are parasites, but not all.
- Awareness of Environment: Many do have mechanisms that help them to be aware of their surroundings. Eyespots that can pick up the intensity of light are one of these mechanisms.
- Amoebas do not have cell walls, so they are very flexible. They move by using extensions of their cytoplasm, which are called pseudopodia. These pseudopodia bulge out and anchor to another surface. The cytoplasm then flows into the extension.
- To eat, amoeba surround bits of food and absorb them.
- They live in soil, and in both fresh and salt water.
- They reproduce through binary fission.
- Amoebas vary greatly in size. Giant amoebas in both the Chaos genus and the Pelomyxa genus have many nuclei, and can grow up to five millimeters long, unlike smaller amoebas that have only one nucleus.
An Amoeba in Action - Moving and Eating
- Forams are snail-like animals with long, thin "legs" of cytoplasm that stick out of their shells. These "legs" help them swim as well as catch food.
- Forams have porous shells, which are called tests. The shells are usually arranged in a spiral shape.
- The shells contain calcium carbonate and have accumulated on the ocean floors for millions of years, forming limestone.
Forams in action
Phyla Chlorophyta, Rhodophyta, and Phaeophyta
- There are three types of algae: Phyla chlorophyta (green algae), rhodophyta (red), or phaeophyta (brown).
- Algae engage in photosynthesis, like plants. Green, red, and brown algae each have different photosynthetic pigments.
- One type of brown algae, kelp, is one of the largest organisms on earth, reaching house-like proportions at times! You can read more about giant kelp in the next section.
- Giant kelp grow in forests in the ocean. Some types of kelp can grow as fast as half of a meter every day, eventually reaching 30 to 80 meters in height!
- Kelp produces methane as it decays. In addition, kelp's sugars can be converted to ethanol. For these reasons, the organism may one day be used as a source of renewable energy for humans.
- Kelp are not actually plants, although they resemble them. Instead of roots, kelp has holdfasts, which grip onto rocky substrates and hold the kelp in place. Unlike roots, holdfasts do not extend down into the substrate, but only anchor the kelp to it. Holdfasts also do not take in nutrients.
- The "leaves" of many types of kelp are called blades.
- Kelp have bladders, called pneumatocysts, which are filled with a gas. These gas bladders help the upper portions of the kelp float near the surface.
- Giant kelp have a pneumatocyst at the base of every blade, but bull kelp has only one for several blades.
- Diatoms are protists with double shells made out of silica. The shells of diatoms form diatomaceous earth, which is used by people to control pests, add sparkle to road paint, and abrade surfaces.
- Diatoms engage in photosynthesis.
- Diatoms have either radial symmetry (like a wheel) or bilateral symmetry (like a person's body where the right side matches the left).
- Diatoms move by gliding, aided by chemicals that they secrete through their shells.
- Diatoms have an unusual method of reproducing: They separate the two halves of their shells, then each half regrows another half. One of the results of this method of reproduction is that the diatoms become smaller. When one becomes too small, it leaves its shell, grows in size, and then grows a new shell.
- Most dinoflagellates have two flagella. One encircles the body and the other runs perpendicular to the body. These flagella allow dinoflagellates to spin through the water.
- Dinoflagellates have a cellulose coat which is often covered in silica. This gives them unique shapes!
- Dinoflagellates can be autotrophic, heterotrophic, or both.
- They are often found in marine water, and are part of the plankton.
- Like dinoflagellates, euglena have two flagella.
- They have an eye spot, which helps them see light.
- Some Euglena are autotrophic, while other types are heterotrophic.
- Kinetoplastids have a DNA-containing particle within their mitochondrian. This particle is called a kinetoplast.
- They also have at least one flagellum.
- Kinetoplastids are heterotrophic.
- Some kinetoplastids cause disease in humans and other animals.
- Some taxonomists classify kinetoplastids in the Euglenophyta phylum instead.
- Cilates get their names from their cilia: short hairs that help them move around.
- They are heterotrophs and do not engage in photosynthesis.
- They have flexible bodies that can squeeze around or through other things.
- Most Cilates have two nuclei: the micronucleus and the macronucleous.
- Paramecia are one type of cilate.
Cell structure of a ciliophora (as seen in the diagram below):
- Contractile vacuole
- Digestive vacuole
Cilates in action
Cellular Slime Molds
- Cellular slime molds are a group of amoebas that have come together during times of stress. They form colonies called slugs.
- Each slug forms a base, a stalk and a spore-forming tip.
- When the spores are released, each one becomes an amoeba.
Cellular Slime Molds
Plasmodial Slime Molds
- Plasmodial Slime Molds are a collection of organisms that stream along as a plasmodium.
- As they stream, they engulf bacteria and other things they encounter.
- Plasmodial Slime Molds have many nuclei, which are not separated by cell walls.
- If a plasmodial slime mold begins to get too dry or hungry, it divides into smaller mounds, which produce stalks topped with spore-containing capsules.
- These spores eventually germinate into haploid cells. (Haploid cells contain half of the number of chromosomes as the adult of that species.) When two haploid cells get together, they create a diploid zygote. These zygotes undergo mitosis and become a new plasmodial slime mold.
Plasmodial Slime Molds
Oomycetes - Water Molds, White Rusts, Downy Mildews
- Oomycetes have two flagella. One of these points forward and the other points backward.
- Some types of oomycetes cause disease in plants. The potato blight in Ireland was called by the oomycete Phytophthora infestans.
Diversity and Characteristics of Protists - Paul Anderson (Bozemanbiology) talks about protists and their diversity.
Observation in Labs
Observing Live Protists
To observe live protists, you'll need a microscope and some slides. Having a few clean pipets (very inexpensive to buy) is also helpful. You can obtain samples of paramecium, amoebas, euglenas, etc, from Carolina Biological Supply, and/or you can collect pond water. Our homeschool co-op did both. We had the best results with seeing the euglenas from Carolina Biological Supply. They were everywhere, swimming all around! There was no way to miss seeing them in the microscope.
It was also fascinating to see what organisms could be found in water from a nearby pond!
Here are some activities (and tips) you may want to try.
Although it's lots of fun (as well as educational!) to watch live protists swim around, it can be difficult to find some types in the microscope, and others won't hold still long enough for you to study them. You won't be able to watch live protists swim around via these slides, but you will be able to examine the physical structure of their bodies.
Online Protists Flashcards and Games
- Protists Flash Cards and Games
Are you preparing for a test on protists? On this website, you'll find some flash cards to help you study the different types. There are also some free games available on this site!
For more info about Protists
- Kingdom Protista
For more information about protists, visit Kingdom Protista. In addition to information, you'll find great pictures as well as coloring sheets.