Protists

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Printed: July 15, 2015

www.ck12.org Chapter 1. Protists

CHAPTER 1 Protists

FIGURE 1.1An amoeba belongs to the Protist king-dom. It is unicellular, engulfs its food andmust 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. Theother 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 examplesof protists in the figure below. The Protist Kingdom includes all eukaryotes that don’t fit into one of the other threeeukaryote 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 as200,000 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 ofthem have mitochondria, and some of them have chloroplasts.

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FIGURE 1.2These examples of protists show how var-ied 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 untilit returns to the starting phase. The protist life cycle includes both sexual and asexual reproduction. Why reproduceboth ways? Each way has benefits. Asexual reproduction is fast. It allows rapid population growth when conditionsare stable. Sexual reproduction increases genetic variation. This helps ensure that some organisms will survive ifconditions change.

Types of Protists

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

Animal-like protists are called protozoa (protozoan, singular). Most protozoa consist of a single cell. Protozoa areprobably ancestors of animals.

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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 deadorganic matter.

2. Almost all protozoa can move. They have special appendages for this purpose. You can see different types inFigure 1.3. Cilia (cilium, singular) are short, hair-like projections. Pseudopods are temporary extensions ofthe cytoplasm. Flagella are long, whip-like structures. Flagella are also found in most prokaryotes.

FIGURE 1.3Three types of appendages for movementin protozoa

Plant-Like Protists

Plant-like protists are commonly called algae (alga, singular). Some algae consist of single cells. They are calleddiatoms. Other algae are multicellular. An example is seaweed. Seaweed called kelp can grow as large as trees. Youcan see both a diatom and kelp in Figure 1.4. Algae are probably ancestors of plants.

FIGURE 1.4Diatom (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 otherplant 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 cellsor as many cells that form a blob-like colony. They are probably ancestors of fungi. Like fungi, many fungus-likeprotists 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 veryslowly 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 orfish parasites.

FIGURE 1.5The slime mold (top) is called “dog vomit”mold. The water mold (bottom) is a plantparasite 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 invadeand live in the human body. The parasites get a place to live and nutrients from the human host. In return, they makethe host sick. Examples of human diseases caused by protozoa include giardiasis and malaria.

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• Protozoa that cause giardiasis are spread by contaminated food or water. They live inside the intestine. Theymay cause abdominal pain, fever, and diarrhea.

• Protozoa that cause malaria are spread by a vector. They enter the blood through the bite of an infectedmosquito. They live inside red blood cells. They cause overall body pain, fever, and fatigue. Malaria killsseveral 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 theoldest 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. Mostreproduce both asexually and sexually.

• Protists are classified as animal-like, plant-like, or fungus-like. The three groups differ mainly in how theyobtain 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/NaturalHistory 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 3.03. Diatoms: Hannes Grobe; Kelp: Claire Fackler, NOAA. Diatom and kelp . Diatoms: CC BY 3.0; Kelp: CC

BY 2.04. Top: User:Algirdas/Wikimedia Commons; Bottom: United States Department of Agriculture. Molds . Public

Domain

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