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Learning goals

Finding protista on the tree of life

The different types of protista

What are some uses of protista?

What are some protist diseases?

Where on the tree of life

Like animals and plants, protists are in the eukaryote domain.

Look closely at this cladogram (history of evolution shown in a tree form)

It shows most kingdoms of life on Earth.

Question: We say there are forms of life called protista, yet there’s no kingdom named protista. What does this mean?

Phylogenetic Tree of Life by Ciccarelli in March 2006 Science

Image by Madeleine Price Ball. Simplified universal phylogenetic tree, made using information from the Interactive Tree of Life. Ciccarelli, et al., Mar 3 2006, Science Vol. 311

To help answer this question, let’s look at how another scientist diagrams much of the same data.

Notice this next diagram only show eukarya.

In this diagram:

pink: all forms of fungi
blue: all forms of animals
green: all forms of plants
yellow: dozens of different kingdoms of life.

Q: What domains of life are not shown on this second cladogram?

Q: In what ways are the two different cladograms similar? In what way are they different?

Q: Which yellow-colored kingdoms are the closest to plant life?

Putting it all together

We know about kingdoms of life such as animals, plants and fungus. We’ve heard about kingdoms of single celled organisms – bacteria and archaea.

We can see them grouped together in the next diagram.

Now look at the organisms in the upper right corner:

Q: Based on the previous two cladograms, and this image, what can we now say about the protista kingdom?

Six Kingdom System Britannica

Protists aren’t a single kingdom. They are a super-kingdom, covering every kingdom of life on Earth that doesn’t fit into the more commonly known ones.

Characteristics of protists

All are eukaryotic (their cells have nuclei and other organelles)

They can be either unicellular or multicellular.

They can be single-celled (microscopic), or their cells can connect to make colonies hundreds of feet long.

Some are heterotrophs (can’t make their own food; so they need to eat)

but others are autotrophs (they can make their own food through photosynthesis)

One way to categorize protists

1. Animal-like protists

protozoans – single celled organisms that can move/swim, to find food

heterotrophs (need to ingest another organism for food)

2. Plant-like protists

Single-celled, and get power from photosynthesis

Autotrophs (aka producers) – Capture solar energy to produce their own food

Some species connect to create colonial organisms, tens of feet long.

Can live in soil, on the bark of trees, in fresh water, and in salt water.  Important because they produce a lot of oxygen.

They form the base of aquatic food chains.

3. Fungus-like protists

These are not related to true fungi.


Like plants, these have:

   cell membranes (plasma membranes), soft and flexible.

   cell walls (rigid)

Plant cell has a wall adapaproject

Plant cell has a wall adapaproject

Reproduce by forming spores.

They are able to move at some point in their lives, but once they connect to form a colony they stop moving.

Can create water molds, downy mildews, and slime molds.

Uses of protists

Protists a a major part of the food chain in the oceans.

Some protists are decomposers: These break down organic matter in the soil.

Plant-like protists produce large amounts of oxygen gas.

Most types of seaweed are protists, and many are edible. Good sources of nutrition.

Some medicines have been made from molecules found in protists.

Carrageenan is a common food ingredient, made from red algae.

Protist diseases

Plasmodium protozoa cause malaria.

Parasites enter a host’s blood through the bite of an infected mosquito.

The parasites infect the host’s red blood cells, causing fever, joint pain, anemia, and fatigue.

Malaria is common in tropical and subtropical climates.

It is one of the most common infectious diseases on the planet.

It kills several million people each year, most of them children.

A tremendous amount of research has gone in to developing a malaria vaccine. Currently, scientists have developed one weak vaccine, that offer 25% to 50% protection. Further improvements are expected.

(from CK-12-Biology-Concepts/section/8.7)

Worldwide map of malaria

Trypanosoma Protozoa

Members of the genus Trypanosoma are flagellate protozoa that cause sleeping sickness, which is common in Africa.

They also cause Chagas disease, which is common in South America.

The parasites are spread by insect vectors. The vector for Chagas disease is shown in Figure below.

Trypanosoma parasites enter a person’s blood when the vector bites. Then they spread to other tissues and organs. The diseases may be fatal without medical treatment.

(from CK-12-Biology-Concepts/section/8.7)

Triatomine bugs and Chagas disease

Giardia Protozoa cause giardiasis

The parasites enter the body through food or water that has been contaminated by feces of infected people or animals.

The protozoa attach to the lining of the host’s small intestine, where they prevent the host from fully absorbing nutrients.

They may also cause diarrhea, abdominal pain, and fever.

(from CK-12-Biology-Concepts/section/8.7)

giardia protozoa life cycle

Image from Transmission and Epidemiology of Zoonotic Protozoal Diseases of Companion Animals Article, Jan 2013 Kevin J Esch and Christine Anne Petersen


Learning Standards

Massachusetts Science and Technology/Engineering Curriculum FrameworkLife

Science (Biology), Grades 6–8.
Classify organisms into the currently recognized kingdoms according to characteristics that they share. Be familiar with organisms from each kingdom.

Biology, High School
5.2 Describe species as reproductively distinct groups of organisms. Recognize that species are further classified into a hierarchical taxonomic system (kingdom, phylum, class, order, family, genus, species) based on morphological, behavioral, and molecular similarities.

Benchmarks for Science Literacy, American Association for the Advancement of Science

Students should begin to extend their attention from external anatomy to internal structures and functions. Patterns of development may be brought in to further illustrate similarities and differences among organisms. Also, they should move from their invented classification systems to those used in modern biology… A classification system is a framework created by scientists for describing the vast diversity of organisms, indicating the degree of relatedness between organisms, and framing research questions.

SAT Biology Subject Area Test

Evolution and diversity: Origin of life, evidence of evolution, patterns of evolution, natural selection, speciation, classification and diversity of organisms.

Teaching About Evolution and the Nature of Science, National Academy Press (1998)

Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification.

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