Protists are a group of organisms that don’t really fit into any of the other kingdoms. Since the protists are so diverse, some members of the kingdom have features not seen in other members. This lesson will describe the function of the cell wall in protists.
What Are Protists?
Protists are a mixed-up kingdom of organisms. An organism is classified in the Protist Kingdom if it does not fit in any of the other eukaryotic kingdoms. Protists are not plants, animals, or fungi.
Most protists are microscopic and unicellular, but some can be multicellular, such as colony-forming protists, or even macroscopic, like kelp or seaweed.
Do Protists Have a Cell Wall?
A cell wall is a covering that surrounds the plasma membrane of a cell and provides protection and support. So, do protists have cell walls? Because so many protists vary from one another, the answer to this question varies as well. Protists that are similar to animals do not have cell walls at all. Plant- and fungi-like protists have cell walls that are similar to plants. Other protists have a unique cell wall that is different from cell walls seen in other eukaryotic kingdoms.
Some protists have cell walls composed of polymers similar to cellulose found in plants. A polymer is a large molecule made up of repeating subunits. In the case of cellulose, the repeating subunit is glucose. Cellulose is the material that gives the wood, stem, and leaves of a plant its strength. The exact composition of the cell wall varies with the species of protist. Some protists have cell walls that are made of cellulose, but others have cell walls made of sugars other than glucose, modified sugars, or proteins.
First, let’s look at unicellular protists.
Plant-like protists can be unicellular, filamentous, or colonial. Most unicellular protists are microscopic, but some, like Caulerpa, are very large. Caulerpa form structures that resemble leaves, roots, and other tissues found in plants. Each plant-like structure formed by Caulerpa is actually a single cell.
The cell wall is composed of cellulose. The cell wall extends into the cytoplasm, forming structures called trabeculae to give shape to the leaf-, root-, and stem-like structures formed by the organism. The cell wall also functions to give the cell shape and protection in microscopic unicellular algae.
Colonial Plant-Like Protists
Now, let’s look at colonial plant-like protists. Colonial plant-like protists have a cell wall surrounding each cell, while a gelatinous ooze, or extracellular matrix (ECM) , surrounds all cells in the colony.
The ECM allows the individual cells of the colony to act as a single unit. Colonial algae can be microscopic, like Volvox, or large, like Ulva. Ulva is a seaweed that forms structures that resemble plants, but they are actually a colony of single-celled organisms embedded in an ECM. Volvox colonies operate as a single unit because the cell walls form intercellular bridges between cells that are embedded in an ECM, as indicated by the arrows in the image below:
|Filamentous Plant-Like Algae
Next, let’s look at filamentous plant-like algae. Filamentous plant-like algae are also groups of single-celled organisms that work cooperatively. As the name implies, the cells form long filaments instead of colonies. Filamentous algae can also be microscopic, such as Spirogyra or Klebsormidium, or macroscopic, such as members of the Cladophoras. Each cell will have its own cell wall and a second cell wall allowing it to interact with other cells in the filament. For example, closer examination of the leaves of Cladosporas will show that the organism is made up of many cells connected by a cell wall, which you can see in the image below:
You can also see that each Klebsormidium cell has its own cell wall and a cell wall that connects it to other cells.
Fungi-like protists resemble fungi both macroscopically and microscopically. One key difference between protists and fungi is the composition their cell walls. The fungal cell wall is composed of chitin, while fungi-like protists have cell walls made of cellulose or similar polymers. Water molds or oomycetes can be unicellular or filamentous, but they don’t have chitin in their cell walls.
Dictyostelium, a kind of slime mold, forms a single cell wall to surround and protect the colony of cells from the environment.
In fact, individual amoeba will gather to form the structures seen in slime molds, like you can see in the image above. Some amoeba in the colony secrete cellulose to form a protective cell wall.
Other Types of Cell Covering
Euglenoids are motile photosynthetic algal protists that are covered by a stiff pellicle made of protein strips and microtubules. This structure gives support and shape to members of the Euglena species.
Diatoms are beautiful photosynthetic algal protists that come many shapes and sizes. These organisms have a unique cell wall made of silica glass called a frustule. Each frustule has two halves called theca, which are joined together by girdle bands.
Amoebas do not usually have a cell wall, but some amoebas have shells called tests. These ornate shells contain calcium carbonate or silica. Some amoebas build their own shells, but others build them from particles found in the environment, including other protists. Having a hard shell offers the protists protection from the environment and from predators.
Check out the image below.
Some amoebas steal materials to build shells from the environment or other protists, as indicated by the arrows in Panel B.
Some protists, which are eukaryotic organisms that aren’t plants, animals, or fungi, have a cell wall (a covering that surrounds the plasma membrane of a cell and provides protection and support), and some don’t. Protists that resemble plants and fungi have a variety of cell walls. The structure and composition of these cell walls depend on the species protists. But, some have cell walls made of polymers (large molecules made up of repeating subunits) which are similar to plants’ cellulose (the material that gives the wood, stem, and leaves of a plant its strength).
Unicellular algae, like Caulerpa, have cell walls that surround the cell just like plants, and it extends into the cytoplasm, forming structures called trabeculae, to give shape to the leaf-, root-, and stem-like structures form by the organism. Colonial and filamentous algae have a cell wall that surrounds the individual cells, and then the group of cells are surrounded by an extra-cellular matrix (ECM), which is a gelatinous substance that allows the individual cells of the colony to act as a single unit. Finally, some protists, like motile photosynthetic algal protists like euglena, are covered by a stiff pellicle which is made of protein strips and microtubules. Others have unique cell walls such as the frustules that cover the diatoms, have a unique cell wall made of silica glass, and are made of two halves called theca that are together by girdle bands. Finally, there are tests, which are ornate shells that contain calcium carbonate or silica possessed by some amoeba.