They are eukaryotes and unicellular. Some of them have flagella or cilia for locomotion. Reproduction is asexual and sexual. Sexual reproduction is by a process of cell fusion and zygote formation. Kingdom Protista is divided into following groups:
The diatoms and golden algae (desmids) are included in this group. They are found in freshwater and marine habitats. They are microscopic and float passively in water currents and hence are considered as planktons. Organisms which cannot swim against the current are called planktons. Most of the chrysophytes are photosynthetic.
The cell walls in diatoms form two thin overlapping shells, which fit together as the two parts of a soapbox. The cell walls are embedded with silica and hence are indestructible. Due to this, the diatoms leave behind a large amount of cell wall deposits in their habitat.
The cell wall accumulation of diatoms, over billions of years is called diatomaceous earth. This soil is gritty and hence is used in polishing, and for filtration of oils and syrups.
Diatoms are the main producers in the oceans, and thus are integral part of the marine foodchain.
They are usually marine and photosynthetic. They appear yellow, green, brown, blue or red, according to the pigment present in them. Stiff cellulose plates are present on the outer surface of cell wall in dianoflagellates.
Most of the dianoflagellates have two flagella. One flagellum lies longitudinally and another transversely in a furrow between the wall plates.
- Most of them live in freshwater habitat in stagnant water.
- Cell wall is absent. There is a protein rich layer (called pellicle) in place of cell wall. The pellicle makes their body flexible.
- Two flagella (one short and another long) are present.
- They are photosynthetic, but behave as heterotrophs in the absence of sunlight. Example: Euglena.
- These are saprophytic. The body moves along decaying twigs and leaves and feeds on organic material. Under favourable conditions, they form an aggregation called plasmodium.
- The plasmodium may grow and spread over several feet.
- Under unfavourable conditions, the plasmodium differentiates and forms fruiting bodies. The fruiting bodies bear spores at their tips.
- True walls are present on the spores. The spores are extremely resistant and survive for many years.
- The spores are dispersed by air currents.
They are heterotrophs and live as predators or parasites. The protozoans are classified into following four major groups:
- Amoeboid Protozoans
- Flagellated Protozoans
- Ciliated Protozoans
Amoeboid protozoans: The amoeboid protozoans live in freshwater, sea water or in moist soil. They produce pseudopodia for locomotion and for capturing food. The marine forms have silica shells on their surface. Some of them are parasites, e.g. Entamoeba histolytica.
Flagellated protozoans: They are either free-living or parasitic. Flagella are present for locomotion. Many of them are parasites, e.g. Trypanosoma.
Ciliated protozoans: They are aquatic. Cilia are present for locomotion. A cavity (gullet) is present which opens to the outside of the cell surface. The coordinated movement of cilia facilitates the entry of food-laden water into the gullet. Example: Paramoecium.
Sporozoans: The sporozoans have an infectious spore-like stage in their life cycle. Example: Plasmodium (malaria parasite)
Note: Organisms in which nucleus is not in the form of organized structure and is devoid of nuclear membrane are called prokaryotes. Organisms in which nucleus is in the form of organized structure and is enclosed in nuclear membrane are called eukaryotes. All organisms; other than bacteria; are eukaryotes.