In this video, we will learn about
common characteristics of organisms within the protist kingdom. We will contrast the main phyla
within this kingdom, and then we’ll practice classifying example organisms using our
knowledge of kingdom Protista.
The protist kingdom, or kingdom
Protista, was first proposed in the 19th century to include microscopic organisms
that were neither plants nor animals. The protist kingdom is not a
phylogenetic classification. Many of these organisms are more
closely genetically related to multicellular organisms in other kingdoms than they
are to each other. The protist kingdom is probably the
most controversial kingdom in modern taxonomy because it relies completely on
artificial classification as opposed to classification by genetic relationships.
Since microscopic life makes up the
majority of life on Earth, the protist kingdom contains a huge number of different
species. There are over 200,000 known
species of protists. Scientists estimate that there may
be almost two million species of protists in total.
Protists are eukaryotic. This means that their cells possess
a true nucleus as well as other membrane-bound organelles. The majority of protists are
unicellular, although some are considered multicellular. Multicellular protists generally
join together to form colonies. They do not form tissues in
organisms like other complex, multicellular life forms.
All protists live in water or on
other moist surfaces like soil. In the five-kingdom system,
unicellular and multicellular algae are sometimes considered protists and sometimes
considered plants. For the purposes of this lesson,
the phyla that can contain what we call higher algae, which are commonly known as
red algae, green algae, and brown algae, will be considered plants, while the phylum
that contains the more simple golden algae will be classified as protist.
You’ll recall that in the Linnean
system, kingdom is the most general taxonomic rank. There are seven taxonomic ranks in
total, each more specific than the last, and each one tells us something about the
traits and relationships of certain organisms. One kingdom contains several
phyla. A phylum is more specific than a
kingdom, so it contains fewer organisms, but it tells us additional information
about their characteristics. Within the protist kingdom, we will
learn to identify four different phyla. We will also learn to identify
certain classes within these phyla.
And we will start with the phylum
Euglenophyta. The Euglenophyta phylum is
characterized by a class of protists called euglena. Euglena are interesting because
they demonstrate the mixed nature of the protist kingdom. Euglena are unicellular, eukaryotic
organisms. They typically live in fresh water
and swim by using one flagellum. These organisms possess
chloroplasts and are able to carry out photosynthesis, which is a characteristic of
autotrophs. Euglena are able to absorb
nutrients through diffusion or even capture and consume other organisms as food,
which are characteristics of heterotrophs.
Since euglena possess traits of
both autotrophs and heterotrophs, we call them mixotrophs. Euglena also possess an organelle
called an eyespot or stigma. This structure is able to detect
light intensity and direction so that euglena can either swim towards or away from
light, depending on their metabolic needs. Euglena are single-celled organisms
with traits of both plants and animals, which makes them typical of the protist
Next, we’ll look at the Chrysophyta
phylum. The phylum of Chrysophyta contains
a unique group of organisms called diatoms that belong to the class
Bacillariophyta. Diatoms are protists. They can be unicellular or
colonial. They can be autotrophs,
heterotrophs, or mixotrophs. One of their most unique traits is
that part of their cell wall is made of a chemical called silicon dioxide, which
gives the diatoms shimmering, glass-like shells, which remain long after the
organism inside has perished.
Not only are diatoms varied, but
they’re also extremely numerous. Diatoms produce 20 to 50 percent of
our planet’s oxygen each year. And they make up up to 50 percent
of the total biomass found in our oceans.
Next, we’ll look at the phylum
protozoa. The protozoa phylum is a large and
varied phylum within the protist kingdom. Protozoans were first established
as a group for what scientists then called the simplest animals. Protozoans are largely motile or
able to move around on their own and heterotrophic, which means they consume other
organisms from nutrition or absorb it from their surroundings. The cells of protozoa do not
possess a cell wall, and they can reproduce sexually or asexually. So you could see how scientists
might consider these protists to be animal-like.
The protozoa phylum is further
divided into four classes based on the method of movement or locomotion. A paramecium is an example of a
protist from the class Ciliophora. Ciliophora use cilia or hair-like
projections on the outside of the cell to move around from place to place. They can also use their cilia to
help them to capture prey and to take in food.
An amoeba is an example of a
protist in the class Sarcodina. Members of this class move from
place to place using extensions of the cell structure called pseudopods. Pseduopod is a word that means
false or fake and foot. Not only do these pseudopods allow
these organisms to move from place to place, but it’s also how they engulf and take
in their food. Some protozoa are parasites that
cause diseases in other organisms. A certain type of amoeba is known
to cause dysentery in humans when transmitted through contaminated water.
The organism Trypanosoma is an
example of a protist in the Flagellata class. Flagellata move by using long,
whip-like extensions called flagella. This particular flagellate is a
parasite passed from person to person by biting flies, and it can cause an illness
called sleeping sickness.
Plasmodium is an example of a
protist in the class Sporozoa. Sporozoans do not have any special
appendage that they use for movement. Plasmodium is a parasite
transmitted by mosquitoes that causes malaria.
Before we try our practice
question, let’s look at one more phylum of protists. Protists in the phylum Pyrrophyta
are also called dinoflagellata. These organisms are often
considered to be a type of algae or phytoplankton. Dinoflagellata possess two
different flagella. These organisms possess
chloroplasts. Their chloroplasts, instead of
being filled with green chlorophyll, are filled with a pigment that has a gold to
reddish-brown color, which is why they’re commonly called golden algae. When large numbers of these
organisms build up near the coasts, they can cause a phenomenon known as red
tide. Red tide can infect shellfish and
poison the humans that eat them.
Now that we’ve learned about the
classification of protists, let’s try a practice question.
The following is a list of the
different classes of protozoa. (1) Sarcodina, (2) Ciliophora, (3)
Flagellata, (4) Sporozoa. A single-celled eukaryotic organism
is found that possesses a foot-like extension to aid its movement. Which of the classes listed is this
organism most likely to belong to? (A) Sporozoa, (B) Sarcodina, (C)
Ciliophora, or (D) Flagellata.
This question is asking us to use
the information provided and our prior knowledge to classify an organism into one of
the four classes of protists that’s listed. In order to answer this question,
we’ll first recall the traits that distinguish these classes of protozoa, and then
we’ll use the information provided about our mystery organism to choose the correct
The four classes of protozoa are
distinguished by their method of locomotion. Some organisms move by cilia or
hair-like extensions found on the outside of the cell. Some move using flagellum or
whip-like structures that allow them to swim. Some protozoa move using pseudopods
or moving extensions of the cell body. And finally, our last class of
protozoa have no special appendages that help them to move.
Now that we know what distinguishes
each class, we need to figure out which is which. Well, we can easily tell that
Ciliophora likely move using cilia, since that’s indicated in the name of the
class. Similarly, we can tell that
Flagellata use flagella. The last two are a little more
tricky. Sarco- is a word part that means
flesh or muscle. The Sarcodina use pseudopods or
extensions of their cell bodies to move from place to place. That leaves us with Sporozoa, which
are the class that possess no special appendages which help them to move.
Now that we’ve identified each of
the different classes of protozoa, we need to figure out which one our mystery
organism belongs to. Let’s start by recalling the fact
that all protozoa are single-celled or colonial, eukaryotic organisms. So those two traits do not give us
a clue as to which class our organism belongs to.
Our classes of protozoa are
distinguished by their modes of movement. And the organism in our question
has a foot-like extension which allows it to move. So, we can tell that it’s not a
Sporozoa, because it does possess a special appendage. And that appendage is described as
foot-like, so it doesn’t sound like it’s hair-like or whip-like.
Another clue is that the word
pseudopod actually means false foot. So, pseudopod is another name for
the foot-like extension of our mystery organism. Now, we have the answer to our
question. The class that this organism is
most likely to belong to is Sarcodina.
Next, we’ll wrap up our lesson by
reviewing what we’ve learned. In this video, we learned about the
protist kingdom. Protists are eukaryotic organisms
that live in water or on most surfaces like soil. Otherwise, protists vary
widely. Protists can be unicellular or
multicellular. They might be autotrophs,
mixotrophs, or heterotrophs. Some members of the protist kingdom
reproduce sexually, some reproduce asexually, and some are able to use both. We also learned about four phyla of
the protist kingdom, and we learned about various classes of organisms within these