Protists are simple microscopic unicellular microorganisms. They are made up of only one cell but the simple cell structure is highly organized and consists of a nucleus and organelles or specialized cellular machinery.
Whatever scientists can’t classify under animal, plant, fungi, and bacteria are classified under the category of kingdom Protista. As a result, you’ll find that most of the protists share very little similarities.
Table of Contents
In Archaeplastida, eukaryotic life forms containing chloroplasts are enclosed by two membranes. It is generally thought that this is a type of structure which hints at the endosymbiosis of cyanobacteria. Most Archaeplastida have cells with walls, and, more often than not, those walls are made of cellulose.
Their cell organization, however, varies widely and can range from isolated cells to colonies to filaments to multi-celled organisms. The earliest cells were all unicellular, and today many groups are still unicellular. Eventually, however, multi-cellularity evolved across several groups, and these groups include ulvo-phyte green algae and red algae, and the green algae have given rise to stoneworts and land plants.
Archaeplastida, or primoplantae, comprise green and red algae, the land plants, and a small group known as the glaucophytes. They have two major evolutionary lines. The red algae have chlorophyll which is pigmented and phycobiliproteins, just like most cyanobacteria. The green algae and the land plants have chlorophyll that is pigmented but lack phycobiliproteins. Some studies have suggested that other groups – including haptophytes, katablepharids, and cryptophytes – might be related more closely to red and green algae than the glaucophytes are.
Chromalveolata are types of eukaryotes and are also single-celled organisms. Most of their members have morphologies that resemble those of terrestrial plants, and they have cell walls. These kinds of protists also have the plant-like ability to perform photosynthesis. They are usually not considered a kingdom and are not given any formal taxonomical classification.
However, they do include groups such as heterokontophyte, alveolate, dinoflagellate, apicomplexa, ciliophoran, haptophyte, and cryptophyta. Most groups are autotrophic, although some no longer have the ability to photosynthesize, including the water molds and ciliates. All chromalveolates that are photosynthetic use chlorophylls a and c; many also use accessory pigments.
Currently, it is believed that species classified as chromalveolates are derived from a common ancestor that engulfed a photosynthetic red algal cell, and this had since evolved chloroplasts from an endosymbiotic relationship with a photosynthetic prokaryote.
Hence, the chromalveolates’ ancestor is thought to have resulted from a secondary endosymbiotic event, even though some chromalveolates appear to have lost plastid organelles that have derived from red alga and also lack plastid genes altogether. Because of this, the supergroup is best considered a hypothesis-based working group subject to change.
Chromalveolates include photosynthetic organisms that are considered very important, including brown algae, diatoms, and significant disease agents in both plants and animals. Chromalveolates can be further divided into stramenopiles and alveolates.
Excavata are made up of single-celled eukaryotic organisms that might be symbiotic as well as free-living. Most of these life forms lack typical mitochondria, and most of them have two or more flagella. The excavates that do have mitochondria usually have it in the form of tubular, discoidal, or laminar cristae.
Most in this supergroup are single-celled and asymmetrical organisms that have a feeding groove that is “excavated” from one side. The group includes parasites, heterotrophic predators, and photosynthetic species. There are three main subgroups of Excavata – parabasalids, diplomonads, and euglenozoans.
These consist only of single-celled eukaryotic organisms with mitochondria and tubular cristae. Most of them are also like amoeba, because they have pseudopodia, or false feet. The Rhizaria are a supergroup consisting mostly of unicellular eukaryotes, even though recently a multicellular form has been described.
This subgroup has only been around since 2002, and for the most part it consists of amoeba-like structures with pseudopods. Many of them produce skeletons or shells that have structures which are complex. The vast majority of protozoan fossils consist of rhizaria, and nearly all have mitochondria with folds.
In the past, many rhizarians were considered animals, in part because of their movement and their heterotrophy. Later, when the five-kingdom system started being used, rhizarians were put into the Protista kingdom. After Carl Woese’s three-domain system was published, however, rhizarians were put into a monophyletic group.
With a genetic structure of three genes that are fused with one another, the unikont has a cellular structure that is eukaryotic, and most of them are either organisms that have just one protruding flagellum or amoeboid without flagella. This group includes Amoebozoa and Obazoa, the latter of which contains the Opisthokonta, which encompasses animals, fungi, and choanoflagellates.
With a triple-gene fusion missing in the bikonts, the genes are fused together, and the three genes encode enzymes for synthesis of the pyrimidine nucleotides, which include dihydroorotase, carbamoyl phosphate synthase, and aspartate carbamoyltransferase. This likely involved a double fusion, which are rare, that support the shared ancestry of Amoebozoa and Apisthokonta.
Subcategories of Protists
As an example, microscopic freshwater alga characterized by the inclusion of a murein layer and which is thought to be a sign of endosymbiosis of cyanobacterial plasmids. The glaucophyte is often used as a synonym for plantae, even though the most accurate use of this term includes only the land plants and green algae, and even though the relationship among green algae, red algae, and the glaucophytes is unclear. An important feature of glaucophytes is a chloroplast with a peptidoglycan layer, which is a signal of a possible remnant of the endosymbiotic origin of plastids from cyanobacteria. The glaucophytes consist of four major genera.
These are Eukaryotes which lack flagella, and they are a great example of that which is red alga; in fact, this is often a synonym for red algae. In the classification with five kingdoms, this is a kingdom that is comprised of a variety of unicellular and simple multinuclear and multicellular eukaryotic organisms. Red algae are photosynthetic organisms, and they were previously classified as thallophytes, which is a primitive subdivision in the plant kingdom and lack sterns, leaves, true roots, and flowers. There are approximately 4,000 species of red algae, and nearly all of them are marine; however, a few of the species occur in fresh water.
Red algae are found in all oceans but are most common in those with warm temperatures and tropical climates, and they can also be found at greater depths than other photosynthetic organisms. Red algae are multicellular and characterized by an enormous amount of branching, although it takes place without differentiation into complex tissues. The cell wall of the red algae has an inner layer that’s firm and contains cellulose, as well as a mucilaginous or gelatinous later on the outside. The cells can have one or more nuclei, and this depends on the species itself. In addition, cell division is by mitosis.
The red algae’s life cycle is very complex and involves one haploid phase and two diploid phases. Although most marine red algae have delicate, soft bodies (thalli), the coralline algae have thalli that get stronger due to calcification and contribute to the growth of coral reefs located in tropical seas. Because they produce structures that are permanent, coralline algae have a rich fossil record dating back as far as 700 million years.
Alveolate has cortical alveoli, mitochondria, flagella that are distinctly structured, and flattened vesicles.
These include most chloroplast that contain algae.
These are eukaryotes with pigmented chloroplasts, and an example is the coccolithophore alga.
These are diatomic organisms – mostly kelp and other algae – and they are characterized by motile cells and chloroplast content.
These are life forms with protozoa and flagella, and they are unicellular, with some being parasitic and others being free-living.
These are anaerobic flagellate protozoa that are mostly either parasitic or in a symbiotic relationship.
These are protozoan organisms, and some have the ability to transform into encysted forms, amoeboid, or flagellate. They also lack color pigmentation.
This group includes the Giardia lamblia, which is an intestinal parasite. Until recently, they were believed to lack mitochondria, and mitosomes, which are mitochondrial remnant organelles, are now identified in diplomonads, although they are essentially nonfunctional. Diplomonads are found in anaerobic environments and utilize alternative pathways to generate energy, including a pathway called glycolysis. In addition, every diplomonad cell has two nuclei that are identical and also uses several flagella for locomotion.
The parabasalids exhibit semi-functional mitochondria. The structures function anaerobically and produce hydrogen gas as a byproduct, which is why they are called hydrogenosomes. Parabasalids move with the help of flagella and membrane rippling. There are numerous types of parabasalid, including a sexually transmitted disease in humans called Trichomonas vaginalis. With T. vaginalis, women can be at risk of serious complications if they are pregnant, and both men and women are more susceptible to infections such as HIV and certain types of cancer.
This subgroup includes heterotrophs, autotrophs, parasites, and mixotrophs, which range in size from 10 to 500 um. Euglenoids move through their aquatic habitats with the use of two long flagella that help guide them to light sources that are sensed by an eyespot, which is a primitive ocular organ. The genus Euglena includes some mixotrophic species displaying a photosynthetic capability only when there is light, because when it is dark, the euglena’s chloroplasts shrink up and cease functioning temporarily. In addition, the cells will instead take up organic nutrients from the environment that they are in.
These are flagellates and amoeboids that can form pseudopods if there is no mouth well-defined enough for feeding.
Amoeboids which possess cytoplasmic strands that are very fine and which branch out and merge in order to give the protist the appearance of a microscopic net that has a nucleus.
Amoeboid protists that possess complex skeletons that are made of various minerals, some of which can be seen as marine zooplankton.
Generic amoeboids that have movements which are dependent on their internal cytoplasmic flow.
Animal-like protists that are mostly parasitic.
Glossary of Terms
Algae (singular: alga): Algae is an informal term for a very diverse and large group of photosynthetic organisms that may not always be related, which is why they are considered polyphyletic. The organisms included in this group are unicellular microalgae genera, including the diatoms and Chlorella; and multicellular forms, such as the giant kelp and a large brown alga that can grow up to over 160 feet in length.
Most are autotrophic and aquatic, and they lack a lot of the distinct tissue and cell types, including xylem, stomata, and phloem – all of which are ingredients found in land plants. Seaweeds are the most complex and the largest type of algae, and the most complex type of freshwater algae is a division of green algae called Charophyta.
Amoeboid: This term is a version of the word amoeba, which refers to an organism that can change its shape, mainly by retracting and extending pseudopods. Amoebae are not a single taxonomic group but instead, they are found in every main lineage of eukaryotic organisms. Microbiologists often use the terms “amoeboid” and “amoebae” interchangeably, and they include many well-known species, including a type of intestinal parasite.
Ciliate: Ciliates are protozoans that have hair-like organelles called cilia, which are structurally identical to eukaryotic flagella, yet they are generally shorter and are in much larger numbers. They also have an undulating pattern that is a little different than flagella. Cilia occur in all members of this group and can be utilized for feeding, crawling, attachment, and even sensation.
With cilia, the organism can grab food, move around, and much more. Today there are more than 5,500 species, and they can be found in both salt-water and freshwater oceans and lakes. Ciliates are also the most specialized of the protozoans and have many different organelles that perform certain processes.
Flagellate: This term relates to organisms that have flagellum, which is a mobile, very long, whip-like appendage that appears from a basal body at the surface of a cell. The appendages serve as a locomotor organelle, and in eukaryotic cells, the flagella contain nine separate pairs of microtubules that are arranged around a central pair. In bacteria, their strands are tightly wound and called flagellin.
The word comes from the Latin word flagellum, which means whip. Flagella are organelles that are defined by their function rather than their structure, and the main role of the flagellum is movement; however, it is often used as a sensory organelle and is even sensitive to temperatures and chemicals outside of the cell.
Kelp: Kelp is large brown algae seaweeds that are part of the order Laminariales. There are approximately 30 different types, and they all grow in shallow oceans in areas known as underwater forests. It is thought by some that kelp has been around five to twenty three million years. Kelp needs water that is rich in nutrients if the temperature of the water is between 42 and 57 degrees Fahrenheit. Growing up to 1.5 feet per day, they are known for their high growth rate, and they can even reach up to more than 260 feet in length.
Protozoa (singular: protozoan): Protozoa are single-celled eukaryotes and can be either parasitic or free-living, which means it feeds on organic matter that includes organic tissues and debris, as well as other microorganisms. Protozoa historically have been known as one-celled animals thanks to their animal-like behaviors, which include predation and motility.
They also lack a cell wall, such as the ones found in many algae and in plants. The traditional practice of grouping protozoa with animals no longer is in existence, but the term is still sometimes used as a way to loosely identify single-celled organisms that feed by heterotrophy and move independently.
Slime Mold: Slime mold is an informal name used to identify numerous types of unrelated eukaryotic organisms that live freely as single cells but which aggregate together in order to form multicellular reproductive structures. Formerly classified as fungi, slime mold is no longer considered part of that kingdom. There are approximately 500 species of primitive organisms that contain true nuclei and resemble both fungi and protozoan protists.
Sporozoa (singular: sporozoan): Sporozoa are a large class of non-motile, strictly parasitic protozoans with a complex life cycle that usually involves both sexual and asexual generations, often in various hosts. The class also includes important pathogens that include babesias and parasites. Sporozoa are parasitic, spore-forming protozoan and include many different species.
One of these species is known as plasmodia, which is the organism that causes malaria. The mature forms do not have external organs that give it some locomotive capabilities, and some of the most common, well-known forms include Toxoplasma, Microsporidia, Plasmodium, Isospora, and Cryptosporidium.
Water Mold: Belonging to a group known as oomycetes, water molds look like other fungi thanks to their branched filaments and form spores. The water molds, however, have cellulose in their walls, even though other fungi have chitin. Oomycetes have a complicated reproductive cycle that includes zoospores, which bear flagella.
Some water molds are actually parasites of fish, while others can cause disease in plants such as potatoes, grapes, and even tobacco. Water molds are microscopic and reproduce both sexually and asexually. They thrive under high-humidity conditions and continuous running water, and they are tiny and absorptive in nature. They also have a thallus, or body, that is composed of mycelia, which is a tube-like vegetative body.
Examples of Protists
Discovered in the year 1757, the Amoeba proteus is found commonly in species of this microbe. It can be from 220 to 740 micrometers in size, and it has a body structure that is characterized by the presence of one or more nuclei. They participate in asexual reproduction in the form of cytokinesis.
A unicellular microbe, the Euglena, has more than 1,000 species. They exhibit both heterotrophy and autotrophy, and the latter produces sugars by means of photosynthesis. Raw materials are used in this process and include the carotenoid pigments, and chlorophyll a and c.
There is some confusion regarding how to classify them, and reproduction takes place asexually through binary fission. Locomotion is accomplished through flagella in the organs, and their eyespot is the body part that is photosensitive. They detect light through the eyespot, and they make all adjustments that are necessary for photosynthesis.
The diatom is a phytoplankton that forms one of several groups of algae. Most are unicellular in nature, and their cell wall is called the frustule and is made up of hydrated silicon dioxide. There is a lot of variety when it comes to these frustules, and diatoms are found in freshwater bodies like lakes and rivers, and also in oceans. Diatoms boast over 100,000 species and 200 genera. They are useful when the need for studying water quality is necessary, and most of them number the species that are found in the tropical regions. Reproduction is through binary fission.
Paramecium are unicellular microorganisms and possess a locomotory organ called cilia. Their body ranges in size from 50 to 350 micrometers, and contractile vacuoles are used for osmoregulation. This organism has an oral groove that is present on the side of the body, and it takes in food with a sweeping motion through that oral groove. Its diet consists of bacteria, yeasts, and algae, and it is commonly found in freshwater regions, although a few of the species are found in oceans. There is a symbiotic relationship between Paramecium aurelia and bacterial endosymbionts.