KINGDOM PROTISTA
The kingdom protista was proposed by Ernest Haeckal in 1866. Although all single celled eukaryotes are placed in kingdom protista yet it's boundaries are not well defined. Phylogenetically the kingdom protista acts as a connecting link between the prokaryotic kingdom - Monera on one hand and the complex multicellular Kingdom - fungi, Plantae, and Animalia on the other hand Protista are regarded as ancestors of all multicellular eukaryotic organisms.
General characteristics of Protista -
They are simple eukaryotic organisms. Most of the organisms are unicellular, some are colonial and some are multicellular like algae.
Most of the protists live in water, some in moist soil or even, in the body of humans and plants
These organisms are eukaryotic since they have a membrane-bound nucleus and endomembrane system.
They have mitochondria for cellular respiration and some have chloroplasts for photosynthesis.
Nuclei of protista contain multiple DNA strands, the number of nucleotides is significantly less than complex eukaryotes.
Movement is often by flagella or cilia.
Protista are unicellular organisms, They are not a plant, animal or fungus.
(1) Respiration : Cellular respiration is a primarily aerobic process, but some living in the mud below ponds or in the digestive tract of animals are strict facultative anaerobes.
(2) Nutrition : They can be both heterotrophic and autotrophic. Flagellates are filter feeding, some protists feed by the process of endocytosis (formation of food vacuole by engulfing a bacterial and extending their cell membrane)
(3) Reproduction : Some species have a complex life cycle involving multiple organisms. Examples : plasmodium
Some reproduction sexually and other asexually. They can reproduce by mitosis and some are capable of meiosis for sexual reproduction.
They form cyst in adverse conditions.
Some protists are pathogens of both animals and plants. Examples : plasmodium falciparum causes malaria in humans.
Major group of Protista-
Photosynthetic Protists-(protistan algae) : They have plant-like features, such as containing chloroplasts, living in moist, sunny environments. Examples - dinoflagellates,Diatoms, and Euglenoids.
Dinoflagellates: The dinoflagellate belongs to division pyrrophyta and class dinophyceae.
The dinoflagellates are an important component of phytoplankton. Most of them are marine but some occur in freshwater. Some dinoflagellates such as gymnodinium and gonyaulax grow in large numbers in the sea and make the water look red and cause the so-called "red tide". Some marine dinoflagellates show bioluminescence. It means they emit light, e.g.,Noctiluca, gonyaulax, pyrocystis, pyrodinium.
Characteristics
Nutrition is photosynthetic.
Dinoflagellates are basically unicellular motile and biflagellate, golden brown photosynthetic protists. Predominant colour is golden brown but yellow, green, brown and even blue forms also occur due to change in proportion of various pigments. A few are non motile, non flagellate, amoeboid, and filamentous.
Cells are generally covered by a rigid coat the theca or lorica of articulated and sculptured plant of cellulose. Periplast may occur instead of theca. Because of the presence of sculptured plates, these protists are often known as armoured dinoflagellates.
Theca contains two grooves, the longitudinal groove called the sulcus and the circular groove known as the cingulum or annulus or girdle.
The two flagella are different (heterokont), one transverse flagellum and other longitudinal flagellum. The longitudinal flagellum is narrow, smooth, directed posteriorly and lies in the Sulcus.
The transverse flagellum is ribbon -like and lies in the cingulum or annulus.
The two types of flagella beat in different directions.
The nucleus is larger in size and has been named as mesokaryotes by dodge (1966). Chromosomes do not have histone or DNA.
Plastid or chromatophore have chlorophyll a and chlorophyll c, mucilage bodies or vesicles occur below the cell membrane.
A non - contractile vacuole
called pusule is present near the flagella base. Pusule may take part in flotation and osmoregulation. Contractile vacuoles are absent.
Varieties of eye spot occur in dinoflagellates. Some of them are like ocelli.
Reserve food is stored in the form of starch and oils.
Reproduction : Asexual reproduction is commonly through cell division. Cyst occurs in a number of dinoflagellates.
Sexual reproduction has been reported in some dinoflagellates (e.g., ceratium). It is isogamous and anisogamous.
Examples. : Glenodinium, peridinium, Gymnodinium, Gonyaulax, ceratium, Noctiluca etc.
Chrysophytes
Chrysophytes includes diatoms and desmids. They belong to the division chrysophyta/Bacillariophyta.
Diatoms : They occur in various habitats like freshwater, saline water and also in terrestrial condition on or within the soil. Sometimes they occur as epiphytes along with algae, on the leaf of forest trees, mostly in tropical rainforests. Depending on the mode of nutrition they may be photosynthetic autotrophs or photosynthetic symbionts or heterotrophs.
Characteristics
Most of the diatoms occur as phytoplankton both in fresh and marine water.A few forms occur as benthos at the bottom of water reservoirs.
Diatoms constitute a major part of phytoplankton of the oceans.
The cell wall is chiefly composed of cellulose impregnated with glass - like silica. It shows sculpturing and ornamentation. It is composed of two overlapping halves (or Thea) that fit together like two parts of a soap box.
The upper half (lid) is called epitheca and the lower half is called hypotheca
Diatoms do not possess flagella except in the reproductive stage.They show gliding type of movement with the help of mucilage secretions.
They float freely on the water surface due to light weight lipids.
Economic Importance of diatoms-
Diatoms are an important source of food to aquatic animals.
Diatom deposits are often accompanied by petroleum fields.
Diatoms are porous and chemically inert therefore used in filtration of sugar, alcohol and antibiotics.
The walls are embedded with silica and thus the walls are indestructible.Thus, diatoms have left behind large amounts of cell wall deposits in their habitat:, This accumulation over billions of years is referred to as 'diatomaceous earth'.
Being gritty this soil is used in polishing, filtration of oils and syrups.
Diatoms are the chief producer in the oceans.
Euglenoids
General characteristics
These protists are devoid of cellulose cell walls. The body is covered by thin and flexible pellicle.
They have two flagella, a short and a long one.
They are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.
The pigments of euglenoids are identical to those present in higher plants.
Examples : Euglena
Slime Moulds
Slime moulds are saprophytic protists.
The body moves along decaying twigs and leaves engulfing organic material.
Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet.
During unfavourable conditions, bearing spores at their tips.
The spore possess true walls.
They are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed by air current.
Protozoans
All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major group of protozoans
Ameboid protozoans: These
organisms live in freshwater, sea water or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba.
Marine forms have silica shells on their surface. Some of them, such as Entamoeba, are parasites.
Flagellated protozoans :
The members of this group are either free - living or parasitic.
They have flagella.
The parasitic forms cause diseases such as sleeping sickness.
Examples: Trypanosoma, Trichomonas, Leishmania etc.
Ciliated protozoans
These are aquatic, actively moving organisms because of the presence of thousands of cilia.
They have a cavity (gullet) that opens to the outside of the cell surface.
The coordinated movement of rows of cilia cause the water laden with food to be steered into the gullet.
Example : paramecium
Sporozoan :
This includes diverse organisms that have an infectious spore -like stage in their life cycle.
The most notorious is plasmodium (malaria parasite) Which causes malaria, a disease which has a staggering effect on the human population.
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