Lesson Explainer: Kingdom Protista Biology • First Year of Secondary School

In this explainer, we will learn how to recall the characteristics of organisms belonging to the Kingdom Protista.

Taxonomy is the branch of biology that aims to classify organisms into distinct groups based on their visible characteristics or their evolutionary history. All living organisms that have been discovered so far can be classified into kingdoms, which are groups containing a large number of organisms that share some characteristics.

In 1969, a scientist called Robert Whittaker proposed the 5-Kingdom classification system, outlined in Figure 1.

An example of a kingdom that organisms may be classified into is kingdom Protista. Kingdom Protista has a wide range of organisms belonging to a variety of different species.

All organisms belonging to kingdom Protista are classified as eukaryotes. The cells of eukaryotes contain membrane-bound organelles, including a nucleus, which is the location of the cell’s genetic material.

Other common characteristics belonging to all of the organisms in this kingdom are harder to find; over time, kingdom Protista has become a large mix of organisms that cannot, for whatever reason, be classified as an animal, plant, or fungus. This means that this kingdom is incredibly diverse but hard to define!

There are some other characteristics that apply to the majority of organisms belonging to kingdom Protista. The majority of protists are unicellular, so they are only made up of one cell, for instance, the organism Amoeba proteus, which is shown in the micrograph below.

Not only do protists show a huge variety in their cellular structure, but they also show this variation in their nutrition. Protists can be autotrophs and produce their own food via photosynthesis, or they can be heterotrophs and consume organic material for nutrition.

Example 1: Recalling the Key Characteristics of Protists

The picture provided shows Amoeba proteus, an organism belonging to the kingdom Protista.

Which of the following statements describes the characteristics of organisms belonging to the kingdom Protista?

1. Organisms are eukaryotic, they contain membrane-bound organelles like a nucleus, and the majority are unicellular.
2. Organisms are prokaryotic, autotrophic, and multicellular.
3. Organisms are prokaryotic, so they do not contain a membrane-bound nucleus, and they are all unicellular.
4. Organisms are eukaryotic, they all contain photosynthetic pigments, and the majority are multicellular.

Answer

Kingdom Protista is a large taxonomic group that contains a huge variety of organisms. However, all these organisms have some characteristics in common. All organisms belonging to kingdom Protista are eukaryotic, which means their cells contain a membrane-bound nucleus. Kingdom Protista is largely composed of small organisms that are generally unicellular, meaning that they are only one cell big. Some characteristics that vary between organisms belonging to this kingdom are their forms of nutrition, so while some protists are able to synthesize their own nutrition (autotrophic), some need to consume other organisms or organic material to obtain their nutrition (heterotrophic). Some autotrophic protists will carry out photosynthesis to form glucose and so they contain photosynthetic pigments, but this is a small portion of the kingdom.

Therefore, the statement that best describes kingdom Protista is that the organisms are eukaryotic, they contain membrane-bound organelles like a nucleus, and the majority are unicellular.

As kingdom Protista is such a large group, it is useful for scientists to further divide them into smaller groups. The diagram provided in Figure 3 outlines how kingdom Protista can be split into four phyla: Euglenophyta, Protozoa, Pyrrophyta (also called Dinoflagellata), and Chrysophyta. It also provides some examples of classes belonging to each of these phyla.

Protozoa is a large phylum of the kingdom Protista. Organisms of this phylum all share some characteristics: they are unicellular, they can reproduce sexually and asexually, they may live individually or in groups called colonies, and they can be free-living or act as parasites of other organisms. Organisms belonging to this phylum are heterotrophic and consume other organisms for nutrition.

The phylum Protozoa is further divided into four classes, primarily due to the forms of locomotion they use. Table 1 below describes the key characteristics of each of these classes.

Some of these protozoan species can be harmful to humans.

Some species of Plasmodium, such as Plasmodium falciparum, cause the disease malaria. The Plasmodium parasites are carried by Anopheles mosquitoes and are transmitted to humans via a bite. Malaria is a flu-like illness and can be fatal; the parasitic Plasmodium species are common in sub-Saharan Africa, and the World Health Organization estimated that in 2018 around 405‎ ‎000 people died from malaria, most of them children in Africa.

Trypanosoma brucei is a species of the class Flagellata that causes the tropical disease “sleeping sickness.” This disease causes fevers, swollen lymph nodes, and headaches and can be fatal if left untreated. This parasitic protist is carried by the tsetse fly, which spreads the infection by biting humans. A micrograph of a blood smear containing Trypanosoma protists (shown in darker purple) is given below.

Example 2: Comparing the Characteristics of Protista Phyla

Which of the following statements would correctly replace X in the Venn diagram comparing the classes Sarcodina and Sporozoa?

1. Move using extensions called pseudopodia
2. Move by beating cilia surrounding the cell
3. Move using flagella that extend from the cell
4. Have multiple, varied appendages for locomotion

Answer

Classes Sarcodina and Sporozoa both belong to the phylum Protozoa of kingdom Protista. As we can see from the diagram, class Sarcodina and class Sporozoa have some characteristics in common. Organisms belonging to both classes are unicellular, eukaryotic, and able to reproduce sexually or asexually.

Protozoa classes are classified based on their method of locomotion. These classes include Sarcodina, Sporozoa, Ciliophora, and Flagellata. The diagram, however, only refers to Sarcodina and Sporozoa, and the X indicates a missing description that applies to Sarcodina only. We can see that organisms belonging to class Sporozoa do not have any specialized mechanism for locomotion. As the names suggest, organisms of the Ciliophora class move by use of cilia, and organisms of the Flagellata class move by using flagella that extend from the cell. Examples of organisms classified into the Sarcodina class include Amoeba proteus; this organism forms temporary extensions from its cell body called pseudopodia. Pseudopodia are sometimes referred to as “false feet,” and they change their form in order to move the cell.

Therefore, the statement that should replace X in the diagram is that the organisms in group Sarcodina move by cell extensions called pseudopodia.

Euglenophyta is another phylum of kingdom Protista. Organisms belonging to this phylum are unicellular and are able to move by the use of flagella. Euglena organisms possess chloroplasts, which allows them to carry out photosynthesis to make their own food. When this is not possible, they consume other organic material. This makes Euglena mixotrophs; they can be autotrophic or heterotrophic depending on the environment. They inhabit a wide range of aquatic environments, including ponds, rivers, seawater, and even puddles!

Organisms belonging to the Chrysophyta phylum include diatoms. Diatoms have a unique and fascinating cell wall. Their cell walls are made up of silicon dioxide (silica) surrounded by an organic matrix, which gives the appearance of a glass-like structure. This structure allows diatoms to withstand huge amounts of pressure, up to kg/m²! Diatoms are hugely diverse and numerous and are particularly important as a food source for marine organisms. An example of a diatom is shown in the micrograph below.

The Pyrrophyta phylum, also referred to as the dinoflagellates, are important species of phytoplankton (microscopic algae) in marine ecosystems. Dinoflagellates are motile organisms and possess two flagella, which they use for locomotion. A micrograph of a dinoflagellate is shown below.

Dinoflagellates often contain photosynthetic pigments, such as carotenoids, which give them a reddish-brown color. When coastal areas of oceans or seas are particularly nutrient-rich, dinoflagellates build up around the coast leading to a phenomenon called a “red tide.” A photograph of a red tide is given below. Although it looks interesting, the rapid population expansion of dinoflagellates can release a large amount of toxins into these areas and kill other aquatic organisms like fish.

Example 3: Identifying the Characteristics of the Euglenophyta Phylum

The kingdom Protista is divided into multiple phyla.

Which of the following statements best describes organisms belonging to the Euglenophyta phylum?

1. Organisms are unicellular, they inhabit fresh water or salt water, and most contain chloroplasts to carry out photosynthesis.
2. Organisms move by the use of cilia, and they often predate on smaller protists.
3. Organisms are mostly unicellular algae, with their cells often containing chloroplasts and the cell walls containing silica.
4. Organisms are unicellular, and they are often found living as parasites on the skin of large animals.

Answer

Kingdom Protista is a large taxonomic group that contains a huge variety of organisms with a range of characteristics. In fact, the only thing that all organisms belonging to this kingdom have in common is that they are eukaryotic!

A phylum is a large taxonomic group, although smaller than a kingdom in the taxonomic hierarchy; multiple phyla make up a single kingdom. Protists can be divided into the following phyla: Protozoa, Euglenophyta, Chrysophyta, and Pyrrophyta.

This question is asking us to recall the key characteristics of the Euglenophyta phylum. Organisms belonging to this phylum include those classified into the genus Euglena, and some examples of these organisms are shown in the photo below.

We can see from the photo that these organisms are only made up of one cell, so they are referred to as unicellular. The substance that gives the Euglena its characteristic green color is chloroplast, which is a photosynthetic pigment commonly found in plant cells. This allows the Euglena to produce its own nutrition through the process of photosynthesis. Species of Euglena commonly inhabit fresh water, like ponds or lakes, but can also be found in salt water.

Looking through our options, we can see statement A best describes these organisms: they are unicellular, they inhabit fresh water or salt water, and most contain chloroplasts to carry out photosynthesis.

Let’s summarize the information we have covered regarding the wide variety of organisms of kingdom Protista.