Protists
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C HAPTER
Chapter 1. Protists
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Protists
FIGURE 1.1
An amoeba belongs to the Protist kingdom. It is unicellular, engulfs its food and
must live in a moist environment.
Introduction
Most single-celled eukaryotic organisms are protists, the simplest eukaryotes.
Protist Classification and Evolution
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.
Classification of Protists
The Protist Kingdom is hard to define. It includes many different types of organisms. You can see some examples
of protists in the figure below. The Protist Kingdom includes all eukaryotes that don’t fit into one of the other three
eukaryote kingdoms. For that reason, it’s 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:
http://www.youtube.com/watch?v=0-6dzU4gOJo
General Traits of Protists
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.
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FIGURE 1.2
These examples of protists show how varied they are.
• 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.
Types of Protists
Protists are classified based on traits they share with other eukaryotes. There are animal-like, plant-like, and funguslike protists. The three groups differ mainly in how they get carbon and energy.
Animal-Like Protists
Animal-like protists are called protozoa (protozoan, singular). Most protozoa consist of a single cell. Protozoa are
probably ancestors of animals.
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Chapter 1. Protists
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 1.3. 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.
FIGURE 1.3
Three types of appendages for movement
in protozoa
Plant-Like Protists
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 1.4. Algae are probably ancestors of plants.
FIGURE 1.4
Diatom (left) and kelp (right)
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 don’t have roots, stems, or leaves. Also unlike plants, some algae can move.
They may move with pseudopods or flagella.
Fungus-Like Protists
Fungus-like protists include slime molds and water molds, both shown in Figure 1.5. They exist as individual cells
or as many cells that form a blob-like colony. They are probably ancestors of fungi. Like fungi, many fungus-like
protists are decomposers. They absorb nutrients from dead logs, compost, and other organic remains
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• Slime molds are commonly found on rotting organic matter such as compost. Swarms of cells move very
slowly over the surface. They digest and absorb nutrients as they go.
• Water molds are commonly found in moist soil and surface water. Many water molds are plant pathogens or
fish parasites.
FIGURE 1.5
The slime mold (top) is called “dog vomit”
mold. The water mold (bottom) is a plant
parasite that has infiltrated a potato.
Protists and Human Diseases
Many human diseases are caused by protists. Most of them are caused by protozoa. They are parasites that invade
and live in the human body. The parasites get a place to live and nutrients from the human host. In return, they make
the host sick. Examples of human diseases caused by protozoa include giardiasis and malaria.
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Chapter 1. Protists
• Protozoa that cause giardiasis are spread by contaminated food or water. They live inside the intestine. They
may cause abdominal pain, fever, and diarrhea.
• Protozoa that cause malaria are spread by a vector. They enter the blood through the bite of an infected
mosquito. They live inside red blood cells. They cause overall body pain, fever, and fatigue. Malaria kills
several million people each year. Most of the deaths occur in children.
Lesson Summary
• Protists are eukaryotes in the Protist Kingdom. Most are single-celled organisms. Protists are thought to be the
oldest eukaryotes. They probably evolved when some prokaryotic cells invaded or were engulfed by others.
• Traits of most protists include the ability to move, living in wet places, and complicated life cycles. Most
reproduce both asexually and sexually.
• Protists are classified as animal-like, plant-like, or fungus-like. The three groups differ mainly in how they
obtain carbon and energy.
• Several protozoa are human parasites. They cause diseases such as giardiasis and malaria.
Lesson Vocabulary
•
•
•
•
•
•
•
alga (algae, plural)
cilium (cilia, plural)
life cycle
protist
Protist Kingdom
protozoan (protozoa, plural)
pseudopod
Lesson Review Questions
Recall
1. Define protist.
2. List general traits of most protists.
3. What human diseases are caused by protists?
Apply Concepts
4. Create a table comparing and contrasting the three types of protists.
Think Critically
5. Explain how protists are thought to have evolved.
Points to Consider
Fungus-like protists resemble fungi.
• What are fungi?
• How do fungi differ from plants and animals?
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References
1. Ute Frevert; false color by Margaret Shear; Scott Fay/UC Berkeley; Richard Lampitt and Jeremy Young/Natural
History Museum, London. Protists come in many different shapes . CC BY 2.5
2. Miranda Dudzik. Three types of appendages for movement in protozoa . CC BY 3.0
3. Diatoms: Hannes Grobe; Kelp: Claire Fackler, NOAA. Diatom and kelp . Diatoms: CC BY 3.0; Kelp: CC
BY 2.0
4. Top: User:Algirdas/Wikimedia Commons; Bottom: United States Department of Agriculture. Molds . Public
Domain
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