Eukaryotes and protists belong to Kingdom Protista. In the 1860s, John Hogg observed that this kingdom was composed of primitive unicellular forms of plants and animals. Back then, it was called Kingdom Protoctista (Sengupta, 2011). In 1886, Ernst Haeckel proposed that a third kingdom, Kingdom Protista, must be established. By the 1960s, R.Y. Stanier, C.B. Van Niel and their colleagues proposed that living things be divided into prokaryotes and eukaryotes. This division is based on whether or not the organisms have a true nucleus, the complexity of their genetic material and the presence or absence of membranes that enclose the cell’s organelles. Today the Kingdom Protista includes eukaryotic microorganisms as well as bacteria (Encyclopedia Britannica, 2011).
Kingdom Protista can be broken down into several subgroups: plant-like, animal-like and fungus-like. Plant-like protists are further classified into euglenoids, diatoms and dinoflagellates. Euglenoids are unicellular organisms with a nucleus, chloroplasts, flagellum and cell membrane. Diatoms are golden brown in color and can store their food in the form of oil. Dinoflagellates, the last classification of plant-like protists, have two flagella and are phosphorescent. Animal-like protists are sub classified into sarcodina, flagellates, ciliates and sporozoans. Sporozoans are distinct from the other groups with their capability to produce spores. These protists are heterotrophs and are saprophytic. The fungus-like protists are multicellular and tend to live in sites where moisture is high. Some of them are capable of forming two forms during their life cycle (Sengupta, 2011).
Members of the Kingdom Protista have very distinct locomotory organelles. Some move with their flagella and others use hair like structures called cilia that are embedded around the cell. These locomatory organelles also serve as basis for classifying protists. For instance, those with flagella are called flagellates, while those that have cilia are called ciliates. However, some groups of protists are not named based on their organ of locomotion. For example, those that belong to the Phylum Sarcodina move with the help of their pseudopods (Encyclopedia Britannica, 2011).
Cilia and flagella should not be confused with the prokaryotic flagellum found in bacteria. Locomotory organelles in protists contain tubulin, while those in bacteria have flagellin. Also, the prokaryotic flagellum is entirely extracellular and does not have a common ancestral origin with the flagella found in protists (Encyclopedia Britannica, 2011).
Respiration and Nutrition
Most protists are obligate aerobes. However, there are also some which are anaerobic. These anaerobes include parasitic organisms that inhabit sites that are depleted of free oxygen and ciliates that are found in the sulphide zone of some marine and freshwater sediment. These organisms do not have mitochondria but rather they have hydrogenosomes or symbiotic bacteria that act as respiratory organelles (Encyclopedia Britannica, 2011).
Some members of the Kingdom Protista, the algal protists, are autotrophic. This means that they require a small amount of inorganic material and some light energy to make their own food for their survival and growth. Other protists such as ciliates engulf particulate food through phagotrophy. They ingest prey that serve as their source of energy, nitrogen, carbon, vitamins and growth factors. Protists feed in different ways. Some use their pseudopods or rhizopods to capture their prey, others trap particles of food through their buccal organelles and some use simple diffusion to absorb nutrients through their cell membrane (Encyclopedia Britannica, 2011).
Typically, protists reproduce through binary fission. The body is pinched into two halves and a pair of daughter nuclei appears in place of the “parental” body. For unicellular plant-like protists, fragmentation is their mode of reproduction. Multiple fission, budding, sporogony and schizogony are other ways by which protists reproduce (Encyclopedia Britannica, 2011).
The complexity of the protest cells sets them apart from those in plants and animals. They are not just cells, but are also whole, independent organisms that are capable of adapting to different habitats, much like higher forms of life.
- Protist. (n.d.). Retrieved November 4, 2011, from Encyclopedia Britannica: http://www.britannica.com/EBchecked/topic/480085/protist/41620/Respiration-and-nutrition
- Sengupta, S. (2011, September 20). Protists-Characteristics. Retrieved November 4, 2011, from Buzzle.com: http://www.buzzle.com/articles/protista-characteristics.html