- The Kingdom Protista comprises of eukaryotic protists.
- Members of this kingdom are typically unicellular and less complex in structure than other eukaryotes.
- All the members of this kingdom do not share many similarities, but are grouped together because they do not fit into any of the other kingdoms.
- Some protists are capable of photosynthesis while some live in mutualistic relationships with other protists
- They mostly live in aquatic environments like moist land habitats, and even inside other eukaryotes.
- Up till now 60,000- 200,000 protist species have been discovered, and many have yet to be identified.
SALIENT FEATURES OF PROTISTS
- Protists are eukaryotes that cannot be classified separately as a plant, animal, or fungus.
- Since they possess an eukaryotic cellular organization; along with a well-defined nucleus, protists also have membrane-bound organelles in their cytoplasm.
- The endoplasmic reticulum and Golgi complexes are significant for the synthesis of proteins and exocytosis of cellular molecules.
- Many protists also contain lysosomes, which help in the digestion of ingested organic materials.
- Certain organelles may only be found in some protist cells and not in others.
- Protists that resemble animal cells also have mitochondria, which provide energy for the cell.
- Protists that resemble plants have a cell wall and chloroplasts.
- Chloroplasts make photosynthesis possible in these cells.
- Some of them are single celled while some are multicellular or form colonies.
- They vary in size, some are microscopic while some are enormous (giant kelp)
- Protists also have bioluminescent characteristics.
- Moreover, they are responsible for a number of diseases that occur in plants and animals.
- The members of kingdom Protista reproduce asexually and sexually by the processes of cell fusion and zygote formation
CLASSIFICATION OF PROTISTS
The members of kingdom Protista are divided into three major groups on the basis of their similarities with other organisms:
- Plant-like protists
- Fungi-like protists
- Animal-like protists
- Plant-like protists are known as algae
- They form a large and diverse group of kingdom Protista.
- Some algae are single-celled organisms e.g. diatoms; while others, such as seaweed, are multicellular organisms
- Algae are considered plant-like because they contain chloroplasts and produce their own food through photosynthesis.
- However, they lack many other structures and features of true plants like roots, stems, or leaves.
- Some algae are also differentiated from plants in being motile as their movement occurs due to pseudopods or flagella.
- Although not plants themselves but algae were probably the ancestors of plants.
- Algae play vital role as producers in aquatic ecosystems.
- Microscopic forms of algae live suspended in the water columns.
- They are the main component of phytoplankton as they contribute to the nutrition of most marine ecosystems
- Multicellular forms of algae may grow as large as trees e.g. kelps
- They are the source of nutrition of ecosystems called kelp forests.
Plant like protists have three main types:
- Dinoflagellates are usually found in marine habitat and are photosynthetic in nature.
- Apparently they are of various colors i.e. yellow, green, brown, blue or red.
- Their color is influenced and imparted by the main pigments present in their cells.
- The cell walls of dinoflagellates have stiff cellulose plates on its outer surface.
- These organisms usually comprise of two flagella; one lies longitudinally while the other one is present transversely in a furrow between the wall plates.
- Euglenoids are predominantly freshwater organisms mostly found in stagnant water.
- They lack cell wall, rather they have a protein-rich layer-pellicle, that provides flexibility to their body.
- They have two flagella. One is short and the other is comparatively long. The two flagella join with each other and form a swelling called para-flagellar body.
- Euglenoids are a connecting link between animals and plants due to its resemblances with both.
- Nutrition in Euglena is mixotrophic depending on the absence or presence of light. When light is available it acts as photosynthetic organisms while in darkness, it is saprophytic absorbing food from surrounding water.
- Chrysophytes comprise of the diatoms and golden algae (desmids).
- They are present in fresh water as well as in marine environments.
- Chrysophytes are microscopic organisms and float passively on water currents (plankton).
- Animal-like protists are commonly known as protozoa.
- Protozoa are mostly single-celled eukaryotes.
- Their cells contain membrane-bound organelles and commonly show animal-like characteristics, such as mobility and heterotrophy.
- According to biologists, protozoa may resemble the first eukaryotes that evolved from prokaryotes.
- Most protozoa are too small to be seen with the naked eye (10-50 µm) and are best seen under a microscope.
- They are found in moist habitats, such as in damp soil, leaf litter and also inside and on the bodies of multicellular animals.
- Protozoa have the ability to act as consumers in the food chain.
- Some protozoa are predators that feed upon unicellular algae, bacteria and fungi while some are herbivores and decomposers.
- They also play a vital role in controlling bacterial population.
- Some of the protozoa such as the malarial parasite (plasmodium), trypanosomes, and leishmania act as parasites and symbionts of multicellular animal.
Animal like protists or protozoa have three main types:
- Amoeboid protozoans
- Flagellated protozoans
- Ciliated protozoans
- Amoeboid protozoans move by means of pseudopodia. Pseudopodia are temporary extensions of cytoplasm. Amoeba give out pseudopodia by extending their bodies forward and then pulling the rest of their bodies with them.
- Pseudopodia are also used to catch food.
- Amoebae are mainly found in freshwater, mud or in rotten vegetation.
- Amoebae possess a heterotrophic mode of nutrition. They use pseudopodia to catch food.
- Organisms are ingested by extending pseudopodia around them. The pseudopodia fuse and packs the organism into a food vacuole.
- Within the food vacuole, the ingested substances are mixed with digestive enzymes and broken down into constituents which are then absorbed by the cytoplasm.
- This whole process of transporting nutrients is called as phagocytosis.
- Amoeboid protozoans lack a supporting cell wall and instead they have a contractile vacuole that pumps out excess water into the cell by osmosis.
- This process helps the cell maintain homeostasis.
- Flagellated protozoans move by using their flagella.
- They can have one or more than one flagella that allows the cell to move.
- Most of these protists live in the bodies of other organisms.
- They could harm their host by having a parasitic relationship or at other times they are mutualistic with their host.
- They reproduce asexually by mitosis and cytokinesis. During this reproduction, two cells are formed that are genetically identical.
- Some of the zoo-flagellates have a sexual life cycle of reproduction as well.
- During sexual reproduction, gamete cells are produced by meiosis, when gamete from two organisms fuse, an organism with a new combination of genetic information is formed.
- Ciliated protozoans move by using their tiny hair-like structures called cilia.
- The cilia act as propellers that allow the protists to move through watery environments.
- The cilia also help the protists to capture its food. They feed on microorganisms for food like bacteria, algae, and yeasts.
- To gather food, the Ciliated protozoan uses its cilia to sweep the food into the oral groove of the cell.
- The food passes from the cell mouth into the gullet, which is like the stomach.
- When sufficient food accumulates at the gullet base, it forms a food vacuole in the cytoplasm.
- Enzymes from the cytoplasm enter the vacuole and digest the food contents
- Digested nutrients then go into the cytoplasm and the vacuole shrinks.
- Sporozoans are animal like protists which are characterized by being one-celled, non-motile, parasitic and spore-forming.
- Most of them have an alternation of sexual and asexual stages in their life cycle.
- Sporozoan are parasitic; they live on or within the host body and cause harm to the host.
- They lack all the locomotory organs like flagella, pseudopodia and cilia.
- However they depend entirely on host for nourishment and nutrition.
- They pass from host to host or via vectors to host in the form of spores.
- Sporozoans are known as apicomplexan because of the presence of apical complex, a structure which secretes enzymes and enables the protest to wedge itself into the host cell.
- An example of sporozoan is the Plasmodium falciparum, which is known as the causative agent of malaria.
- Fungus-like protists are usually known as molds.
- They act as absorptive feeders on decaying organic matter
- The fungus-like protists look like fungi because they produce sporangia.
- A sporangium is a plant or fungal structure that produces and contains spores.
- The small, slimy-looking fungus-like protists differ from fungi in many ways.
- The cell walls of the fungi-like protists contain cellulose rather than chitin.
- These protists generally do not have divisions between their cells like fungi do.
- Moreover, they have diploid nuclei, whereas fungi have haploid nuclei.
On the basis of characteristics, fungi like protists are divided into two major types:
- Slime molds
- Water molds
- Slime molds resemble amoeba in appearance.
- They are shapeless bags of protoplasm that ooze through damp areas engulfing organic matter and microorganisms.
- When environmental conditions deteriorate, slime molds release spores which then disperse with wind or water.
- Water molds act as filamentous parasites that attack plants and animals in aquatic and moist environments.
- They produce mobile, flagellated spores as a result of reproduction.
- Water molds were the main cause for the Irish potato famine of the 1840s and a number of other agricultural calamities.
ECONOMIC IMPORTANCE OF PROTISTS
- Protists form a huge part of the food chain and supply much of the oxygen we breathe.
- Protists are utilized in medicine and as food additives.
- Algae, a protist, acts as habitat for sea urchins and other marine animals
- Chemicals from some kinds of algae are used to produce various kinds of plastics.
- Some kinds of algae are used in extracts for making cosmetics, ice cream, and many more.
- Protists are important manufacturers of oxygen.
- Some protists such as kelps are edible and are used to overcome shortage of food in world
- Many marine protists are also source of many valuable substances like algin, agar, carrageen and antiseptics.
- Arihant’s handbook of biology. Biological classification. Page no: 7-23.
- NCERT biology; textbook for class 11. Biological classification. Page no: 16-28.