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The Protists

The Protists

Introduction: The kingdom Protista is probably the most diverse of all the eukaryotic kingdoms. There are over 200,000 species of organisms ranging from unicellular protozoans to multicellular algae. They live in almost every habitat and are known for their ability to reproduce sexually or asexually. They may move using very specialized methods, or be sessile, not moving at all. They are all eukaryotic, which means they have a membrane-bound nucleus in their cell(s). They also have organelles that carry out many metabolic functions for those cells.

Protists are usually divided into two main categories, the protozoans and the algae. The protozoans are the animal-like protists. Members of this group include the amoeba, the euglena, and the paramecium.

The amoeba, shown above, is a voracious predator. It is unicellular and actively hunts bacteria and other protists for food. It can often be found in pond water and moves using structures called pseudopods. As you can see, the amoeba does not have a definite shape. It pushes the cytoplasm inside of itself towards its cell membrane. This causes the membrane to be forced outward, creating the "false feet" and moving the amoeba in that direction.

When it approaches potential prey, it will wrap the pseudopods around it until it is complete enclosed. This chamber will then form a structure called a vessicle, which the amoeba will then bring into its body through endocytosis.


Euglena (shown above) are often considered the missing link between the protozoans and the algae. This is because it is able to photosynthesize to make its own food, but when there is no light, it is able to actively hunt for prey. Euglenas move using a long whiplike tail called a flagellum. This tail sways back and forth, propelling the euglena through the water.

Since they photosynthesize, euglena have chloroplasts containing chlorophyll inside of themselves. They also have a large eyespot that is used to detect light.


The paramecium (above) is another protozoan that is an active predator. It eats bacteria and other protists, including other paramecia. It moves using tiny hair-like projections coming off of its body. These hairs are called cilia and they move the paramecium through the water by sweeping back and forth. This motion is also beneficial in feeding, as it creates a current of water and food particles to the mouth of the organisms.

Unlike other eukaryotic organisms, a paramecium has two nuclei. The micronucleus is adapted specifically for the process of reproduction, while the macronucleus handles all other cell activities.

There are many other species of protists, but these are some of the most common.

The plant-like protists are called algae. They range in size from the single-celled diatoms to the multicellular giant kelp. As plants do, these organisms photosynthesize, but may use pigments other than chlorophyll. Algae are often classified by the pigments they contain, with green, brown, and red being the most common.

Diatoms (see image) are an aquatic group of algae that form the basis of most food chains. They are active photosynthesizers, producing a large proportion of the Earth's oxygen. Without them, all live on land would perish.

Diatoms are special because they produce a shell, or test, that is made of a substance called silica (a lot like glass). When diatoms die, they sink to the sea floor where their shells are harvested for use by people in certain products. Toothpaste and jewelry cleaner are just a couple of places where diatom shells are found.

Kelp is one of the largest species of algae. It is marine, living in cold waters. Since it grows so large, it can form very dense forests (see image). These forests are home to many animals, including sea otters, seals, and a variety of invertebrates. Kelp is also used by humans. It is very high in iron. It is a prime component of sushi and is harvested by the ton by specially designed boats. These boats have special blades attached to their undersides that move through the forest and chop off the blades of kelp. Since kelp can grow upwards of 6 feet per day, there is no shortage.

Directions for this Lesson: Answer the practice questions and then watch the videos to learn more about this diverse group of organisms.

Required Videos: