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Protists are eukaryotes, which means their cells have a nucleus and other membrane-bound organelles. Most, but not all, protists are single-celled. The simplest definition is that protists are all the eukaryotic organisms that are not animals, plants or fungi.

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For classification, the protists are divided into three groups:

1. Animal-like protists: also known as Protozoa, are unicellular and heterotrophic, meaning they seek out food in their surrounding environments. Some animal-like protists prey on other, smaller microorganisms, which they engulf and digest in a process known as phagocytosis. Others may feed on non-living, organic matter. Many Protozoa have a mouth like structure through which they can ingest food particles, while some absorb nutrients through their cell membrane.

2. Plant-like protists: also know as algae, are autotrophs that photosynthesize. Algal cells usually have a cell wall which, like the cell walls of true plants, contain cellulose. However, unlike true plants, algae lack leaves, stems, and roots. Plant-like protists may reproduce asexually or sexually.

3. Fungi-like protists: also known as molds, are heterotrophic feeders and absorb nutrients from decaying organic matter in their environment. They also reproduce using spores. However, they differ from true fungi in that their cell walls contain cellulose, rather than chitin.

But remember, protists are not animals, nor plants, nor fungi.

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Protists serve as the foundation of the food chain.

Protists are symbionts – having a close relationship between two species in which, one is benefited.

Some protists also produce oxygen and may be used to produce biofuel.

Protists are the primary sources of food for many animals.

In some rare cases, Protists are harvested by humans for food and other industrial applications.

Phytoplankton is one of the sole food sources for whales

Seaweed is an alga, which is considered a plant-like protist.

Zooplankton is fed on by various sea creatures including shrimp and larval crabs.

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There are approximately 60,000 - 200,000 species of protists and many of them have not been identified.

Amoeba Proteus (Animal-like)

Is unicellular.

Uses extensions called pseudopodia to move and to eat smaller unicellular organism.

Food is enveloped inside the cell's cytoplasm in a food vacuole, where ingested matter is slowly broken down by enzymes. Typically feeds on protozoans, algae, rotifers, and even other smaller amoebae.

It inhabits freshwater environments because they need a moist environment to survive. Damp soil, marshes, puddles, lakes, and the ocean are the usual places they live in.

Is colourless, but may have coloured inclusions derived from its food.

Red Algae, Rhodophyta Florideophyceae (Plant-like)

Red algae are typically found in tropical marine environments where they often grow on flat surfaces, such as reefs. Though red algae may be unicellular, they are typically multicellular organisms and form a variety of seaweeds.

The red algae form a distinct group characterized by having eukaryotic cells without flagella and centrioles, chloroplasts that lack external endoplasmic reticulum and contain unstacked (stroma) thylakoids, and use phycobiliproteins as accessory pigments, which give them their red color.

Lack of flagella and centrioles

Presence of photosynthetic pigments

Stored food is in the form of starch and polymers of galactan sulphate

Their cell walls consist of cellulose and many different types of carbohydrates.

Grow on solid surfaces independently or sometimes found attached to other algae.

Presence of pit in the cell walls, through which cytoplasmic connections are maintained.

Mode of Reproduction: It takes place by all the three means: vegetative, asexual and sexual.

Slime molds, Myxogastria Mycetozoa (Fungi-like)

Slime molds are often found on rotting logs, where they feed on decaying organic matter.

They are often unicellular but, when food is scarce, can swarm together to form a slimy mass. These brightly colored blobs can move very slowly in their search for food and, in some cases, can fuse to form one enormous, multinucleated cell.

They feed on microorganisms that live in any type of dead plant material. They contribute to the decomposition of dead vegetation, and feed on bacteria, yeasts, and fungi.

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BYJUS (2021, January 14). Kingdom protista - characteristics and classification of Protists. BYJUS. Retrieved December 1, 2021, from https://byjus.com/biology/protista/.

Encyclopædia Britannica, inc. (2021). Protist. Encyclopædia Britannica. Retrieved December 1, 2021, from https://www.britannica.com/science/protist.

Foundation, C. K.-12. (2021). 12 foundation. CK. Retrieved December 2, 2021, from https://flexbooks.ck12.org/cbook/ck-12-middle-school-life-science-2.0/section/6.1/primary/lesson/protist-characteristics-ms-ls/.

Latham, B. K., By: & Latham, K. (2021, March 1). Animal-like, fungus-like, and plant-like protists. Biology Dictionary. Retrieved December 2, 2021, from https://biologydictionary.net/animal-like-fungus-like-and-plant-like-protists/.

Libretexts. (2021, March 6). 8.1: Protist kingdom. Biology LibreTexts. Retrieved December 1, 2021, from https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.01%3A_Protist_Kingdom.

Libretexts. (2021, March 6). 8.5: Algae. Biology LibreTexts. Retrieved December 2, 2021, from https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.05%3A_Algae.

Protist. Protist - an overview | ScienceDirect Topics. (2021). Retrieved December 2, 2021, from https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/protist.

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Defining the protists

From the time of Aristotle, near the end of the 4th century bce, until well after the middle of the 20th century, the entire biotic world was generally considered divisible into just two great kingdoms, the plants and the animals. The separation was based on the assumption that plants are pigmented (basically green), nonmotile (most commonly from being rooted in the soil), photosynthetic and therefore capable solely of self-contained (autotrophic) nutrition, and unique in possessing cellulosic walls around their cells. By contrast, animals are without photosynthetic pigments (colourless), actively motile, nutritionally phagotrophic (and therefore required to capture or absorb important nutrients), and without walls around their cells.

When microscopy arose as a science in its own right, botanists and zoologists discovered evidence of the vast diversity of life mostly invisible to the unaided eye. With rare exception, authorities of the time classified such microscopic forms as minute plants (called algae) and minute animals (called "first animals," or protozoa). Such taxonomic assignments went essentially unchallenged for many years, despite the fact that the great majority of those minute forms of life-not to mention certain macroscopic ones, various parasitic forms, and the entire group known as the fungi-did not possess the cardinal characteristics on which the "plants" and "animals" had been differentiated and thus had to be forced to fit into those kingdom categories.

In 1860, however, British naturalist John Hogg took exception to the imposition of the plant and animal categories on the protists and proposed a fourth kingdom, named Protoctista (the other three kingdoms encompassed the animals, the plants, and the minerals). Six years later German zoologist Ernst Haeckel (having dropped the mineral kingdom) proposed a third kingdom, the Protista, to embrace microorganisms. In the late 1930s American botanist Herbert F. Copeland proposed a separate kingdom for the bacteria (kingdom Monera), based on their unique absence of a clearly defined nucleus. Under Copeland's arrangement, the kingdom Protista thus consisted of nucleated life that was neither plant nor animal. The following decade he revived the name Protoctista, using it in favour of Protista.

The next major change in the systematics of lower forms came through an advancement in the concept of the composition of the biotic world. About 1960, resurrecting and embellishing an idea originally conceived two decades earlier by French marine biologist Edouard Chatton but universally overlooked, Roger Yate Stanier, Cornelius B. van Niel, and their colleagues formally proposed the division of all living things into two great groups, the prokaryotes and the eukaryotes. This organization was based on characteristics-such as the presence or absence of a true nucleus, the simplicity or complexity of the DNA (deoxyribonucleic acid) molecules constituting the chromosomes, and the presence or absence of intracellular membranes (and of specialized organelles apart from ribosomes) in the cytoplasm -that revealed a long phylogenetic separation of the two assemblages. The concept of "protists" originally embraced all the microorganisms in the biotic world. The entire assemblage thus included the protists plus the bacteria, the latter considered at that time to be lower protists. The great evolutionary boundary between the prokaryotes and the eukaryotes, however, has meant a major taxonomic boundary restricting the protists to eukaryotic microorganisms (but occasionally including relatively macroscopic organisms) and the bacteria to prokaryotic microorganisms.

During the 1970s and '80s, attention was redirected to the problem of possible high-level systematic subdivisions within the eukaryotes. American biologists Robert H. Whittaker and Lynn Margulis, as well as others, became involved in such challenging questions. A major outcome was widespread support among botanists and zoologists for considering living organisms as constituting five separate kingdoms, four of which were placed in what was conceived of as the superkingdom Eukaryota (Protista, Plantae, Animalia, and Fungi); the fifth kingdom, Monera, constituted the superkingdom Prokaryota.

In the late 1970s, realizing distinctions between certain prokaryotes, American microbiologist Carl R. Woese proposed a system whereby life was divided into three domains: Eukarya for all eukaryotes, Bacteria for the true bacteria, and Archaea for primitive prokaryotes that are distinct from true bacteria. Woese's scheme was unique for its focus on molecular characteristics, particularly certain RNA sequences. Although imperfect, RNA analyses have provided great insight into the evolutionary relatedness of organisms, which in turn has led to extensive reassessment of protist taxonomy such that many scientists no longer consider kingdom Protista to be a valid grouping.

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Protist | Definition, Characteristics, Reproduction, Examples, & Facts

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Protist, any member of a group of diverse eukaryotic, predominantly unicellular microscopic organisms. They may share certain morphological and physiological characteristics with animals or plants or both. The term protist typically is used in reference to a eukaryote that is not a true animal,

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Protists may share certain morphological and physiological characteristics with animals or plants or both.

Most have mitochondria.

They can be parasites.

They all prefer aquatic or moist environments.

Protista reproduces by asexual means. The sexual method of reproduction is extremely rare and occurs only during times of stress.

Protists can be autotrophic or heterotrophic: Autotrophic means the organism produces its own food either via light as in photosynthesis or via chemical interactions called chemosynthesis. Heterotrophic means the organism can not produce its own food and instead gains energy by consuming other organisms, such as plants or animals.

Protists exhibit locomotion through cilia and flagella. A few organisms belonging to kingdom Protista have pseudopodia that help them to move.