What is Kingdom Protista and its Characteristics | Class 11 Biology
A protist is a group of eukaryotic organisms which are not classified as either an animal, plant or fungus. They possibly 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, plant, or fungus the cells of protists have highly organized nucleus and specialized cellular organelles. Some protists have flagella or cilia for locomotion. They inhabit an environment that contains water.
CHARACTERISTICS OF KINGDOM PROTISTS
- Protists are simple eukaryotic organisms. Most of them are unicellular, some are colonial and some are multicellular like algae but they do not have specialized tissue organization.
- Most of the protists live in water. Some live-in moist soil and even in the body of humans and plants.
- Like all eukaryotic cells, the protists also possess a characteristic central compartment called the nucleus, where the genetic material is present. They also have specialized cellular organelles that perform definite functions within the cell.
- Their nucleus is membrane bound. Nuclei of protists contain multiple DNA strands.
- Photosynthetic protists like various types of algae contain These organelles are the site of photosynthesis (the process of harvesting sunlight to produce nutrients in the form of carbohydrates). The plastids of some protists are similar to those of plants.
- They show movement by flagella or cilia.
- Most protists have mitochondria (organelle which generates energy for cells to use). Some protists that live in anoxic conditions (environments that lack oxygen) use an organelle called the hydrogenosome (a modified version of mitochondria) for some of their energy production; for example, in Trichomonas vaginalis.
- In protists that have mitochondria, aerobic respiration takes place, the protists, that live in the mud below ponds or in digestive tracts of animals are strict facultative anaerobes, the respiration occurs with the help of hydrogenosome.
- The mode of nutrition can be either heterotrophic or autotrophic in protists.
- Some protists feed by the process of endocytosis (formation of food vacuole by engulfing a bacterium and extending their cell membrane) for example in case of Amoeba.
- Mode of reproduction in protists is both asexual and sexual. They can reproduce by mitosis and some are capable of meiosis for sexual reproduction.
- They form cysts in adverse conditions. When the protozoan parasites, are exposed to very harsh conditions during various stages in their life cycle, they form cysts around them. An encysted form allows them to survive in extreme conditions.
- Some protists are pathogens for both animals and plants. Example: Plasmodium falciparum causes malaria in humans.
The kingdom Protista is divided into three groups:
- Animal-like protists: They are heterotrophs and have the ability to move.
- Plant-like protists: They are autotrophs and have the ability photosynthesize.
- Fungi-like protists: They are heterotrophs. Their cells have cell walls and have the ability to reproduce by forming spores.
ANIMAL-LIKE PROTISTS (PROTOZOANS)
Protists that are close to animals are known as protozoans. They are different from animals because they are unicellular while animals are multicellular. They live in moist watery environments. The characteristics that resemble that of animals are –
- They have the ability to move and
- They cannot produce their own food (heterotrophs).
Protozoans are classified into four categories (phyla) on the basis of their mode of movement:
- Sacordinians – They use pseudopodia for the body movement.
- Zooflagellates – They move using flagella.
- Ciliaphorans – They move by the cilia present all over the body.
- Sporozoans – forms spores.
Phylum Sarcodina – The sarcodinians show body movements by extending cytoplasm in the form of lobes known as pseudopodia. This pseudopoda is used for movement and feeding. Example: Amoeba, Foraminiferans (they are amoeba-like, single-celled protists, characterized by an external shell, which has holes or chambers).
Phylum Mastigophora (Zooflagellata) – The member protozoans of this group move with the help of flagella. They are mostly parasitic. Many flagellates are found in the intestine of humans, in termites and other animals. Some flagellates are harmful. Example: Trypanosoma gambiense cause sleeping sickness in cattle and human.
Phylum Ciliophora (Ciliates) – These protozoans move with the hair-like structures called cilia, which helps the organism to move their body as well as taking in the food particles. The movement of cilia is paddle-like, moving back and forth for movement. Example: Paramecium, Lacrymaria, Coleps, Stentor, Dileptus,
Phylum Sporozoa – All members of this phylum are non-motile and parasitic. They form spores, hence termed as sporozoa. They lack locomotory structures, therefore, they are carried in their hosts by their body fluids. Many sporozoans cause serious diseases in humans. Example: Plasmodium it causes malaria in humans.
Check out another article for class 11 biology – Theories of a Cell
PLANT-LIKE PROTISTS – ALGAE
These protists resemble the plants in having the chlorophyll. The chlorophyll enables them to make food by photosynthesis. They produce and release oxygen like the plants. Most of the supply of oxygen on Earth is believed to be from plant-like protists. They are the major food source as well as primary producers for water organisms.
Phylum Chlorophyta (Green Algae) – The green algae comprise of unicellular and multicellular algae. They are found in fresh water. Their cell wall is made of cellulose and pectin. Example: Spirogyra; it is a unicellular green alga, and grows as a green thread or filament.
Phylum Rhodophyta (Red Algae) – Red algae are named so, because of their bright red color. Red algae are mostly large and multicellular. They are found in oceans and are generally referred to as sea vegetables as they are used as food. Coralline algae are red algae which secrete calcium carbonate onto the surface of their cells and helps in forming coral reefs. Agar, which is a gelatin-like substance is prepared primarily from Gracilaria and Gelidium species and is important as a culture medium for bacteria and fungi.
Phylum Phaeophyta (Brown Algae) – They are multicellular organisms. They grow on rocks of the sea. Large brown algae are called kelps. Kelps may grow densely in the sea and form kelp forests. They form important food sources for fish and invertebrates. The brown algae growing on rocks are known as rockweed. Examples of Brown Algae include Sargassum, Laminaria, Macrocystis, Nerocystis. Brown algae are an important source of algin, (a colloidal gel) which is used as a stabilizer in the baking and ice-cream industries. Some Brown Algae are used as fertilizer, and several are eaten as a vegetable (e.g., Laminaria) in East Asia and elsewhere
Phylum Chrysophyta (Golden algae) – ‘Chryso’ means ‘color of gold’. There are three types of golden-algae: yellow-green algae, golden-brown algae, diatoms. They are found in both marine and fresh waters. Diatoms are the most abundant and live both in seawater and freshwater habitats. They are a major source of food for many aquatic organisms.
Phylum Pyrrophyta (Fire Algae) – Dinoflagellates are called fire algae because some forms of these protists are bioluminescent. They store food in the form of starch and oils. Some species cause the ‘red tide phenomenon’. The Red tide is a phenomenon commonly termed as algae bloom, caused by species of dinoflagellates and other algae. During this phenomenon, algae become so numerous that they discolor coastal waters (hence the name “red tide”). The algal bloom may cause depletion in oxygen in the waters and/or release toxins that may cause illness in humans and other animals.
FUNGUS LIKE PROTISTS-
Slime Moulds – Slime moulds are saprophytic protists. They are very bright in appearance. They live in moist soil, decaying plants, and trees. Slime mould is a name given to several kinds of unrelated eukaryotic organisms that can live freely as single cells but can aggregate together to form multicellular reproductive structures. During favorable condition, they form multicellular aggregations called plasmodium. During unfavorable conditions, plasmodia differentiate to form fruiting bodies bearing spores at the tip. These spores possess resistant true walls, which help in survival for a long time during adverse conditions. These spores disperse by air currents.
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