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Protista Characteristics and Classification

Protista is defined as a group of eukaryotic organisms that are neither plant, fungi nor animals. All protists may share a common ancestral history and are studied under the special branch of science, which is called protistology.

A separate kingdom was assigned to the species, which was decided by the famous biologist John Hogg in 1860, followed by Ernst Haekel, who assigned the name Protista to the respective kingdom. The meaning of the term comprises the definition that the primitive ones were called the Protists. Later the prokaryotic organisms were assigned into a separate kingdom called the Monera. Robert Whittaker, in 1869 proposed the Five Kingdom classification where the protists were classified as unicellular prokaryotic species.

The kind of nutrition observed in the Protists-

  • Most of the eukaryotic algae are phototrophic and can produce their food. In certain protists, the ability to produce their food is absent because of the lack of the pigments. As a result, these animals exhibit heterotrophic modes of nutrition.

  • Some of the protists also exhibit the saprophytic mode of nutrition, while many also show phagotrophy and osmotrophy, and parasitic mode of nutrition.

  • Some of the protists show the property of the mixotrophic mode of nutrition which is observed in certain species of the same. In certain cases, due to loss of pigment or mitochondria, these organisms enter into a symbiotic relationship with the algae or other bacteria. They exhibit the properties of ectosymbiont and endosymbiont based on the preference and criteria of the species in the environment.

  • They possess the cytoplasmic extension, which is called the flagella. Once the protists can locate the prey, they conduct filter-feeding to suffice their nutritional needs. They also possess the ability to engulf the bacteria or the entire food particle for replenishing their nutritional needs. They do so by extending their cytoplasm around the food particle and forming a complete vacuole around it. This helps them to digest the food by the process of phagocytosis.

Protist metabolism

There are, however, two main metabolism techniques in protists. The photoautotrophs require sunlight, and the main source of carbon is provided by organic compounds or fixation of carbon in the environment.

The chemoheterotrophs, on the other hand, use the chemical substance present in nature to bring about metabolism in the body.

Kind of reproduction shown in the protists

Both sexual and asexual mode of reproduction is practiced by the protists. Sexual reproduction occurs through gamete formation, while the process of asexual reproduction requires binary fission to carry out the task.

The eukaryotes came into being around 1.5 billion years ago, and it is believed that the protists were the first of their kind. Initially, the protists were observed not to exhibit the sexual means of reproduction, but after much studies and experiments conducted recently, the biologists have concluded that the sexual mode of reproduction that is meiosis is seen in these species.

Facultative sexual reproduction was also observed in certain species of the protists where the organisms could switch between sexual and asexual reproduction depending on the environment and needs of survival.

Amoeba is referred to as the protists, which are asexual but exceptional cases have shown that ameboid cells could switch to a sexual mode of reproduction.

The studies have suggested that generally, the protists reproduce asexually, but when present in a relatively stressful situation, the organisms switch to sexual means of reproduction as well. Such a stressful situation could be a heat shock or starvation situation. Oxidative stress is another factor that induces sexual reproduction in the species. In this case, the oxygen reacts to form reactive species of the compound, which damages the DNA present in the organisms. Under such circumstances, the animal may switch to a sexual mode of reproduction.

The activity of protist pathogens

It is observed in the protist pathogens that they are capable of infecting the host and undergo an asexual mode of reproduction for multiplication. In the secondary hosts, the pathogen can undergo an asexual mode of reproduction. These are also called intermediate hosts. In the case of the primary hosts or the definitive hosts, the protist pathogens are compelled to undergo a sexual mode of reproduction.


The free-living protists are found in the swamps, and anywhere it finds water to survive. They also exist in the ocean in the form of planktons. The protists are set to contribute to a large amount of biomass in the marine as well as the terrestrial environment for survival.





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