Kingdom Protista and Types of Protista
· All single-celled eukaryotic organisms are placed under Kingdom Protista.
· The cell body contains a well-defined nucleus and other membrane-bound organelles. Presence of flagella or cilia.
· Protists reproduce asexually and sexually by cell fusion and results in formation of a zygote.
Protists can be further classified into:
4. Slime Moulds
· Chrysophytes in fresh water as well as in marine environments. Float passively in water currents (plankton). Most of them are photosynthetic in nature.
· In diatoms the cell walls are referred to as Frustule which forms two thin overlapping shells, which fit together as in a soap box.
· The walls are embedded with silica and are indestructible. Thus, diatoms have left behind large amount of cell walls deposits in their habitat over billions of years which is referred to as ‘diatomaceous earth’.
· Diatomaceous earth is used in polishing, filtration of oils and syrups.
· Diatoms are the chief ‘producers’ in the ocean.
· Dinoflagellates Gellates are mostly marine and photoautotrophs. They appear yellow, green, brown, blue or red depending on the main pigments present in their cells.
· The cell wall has stiff cellulose plates on the outer surface. Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
· Red dinoflagellates (Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides). They release a potent neurotoxin called Saxitoxin or paralytic shellfish toxin (PST) which may even kill other marine animals such as fishes.
· Most of Euglenoids are fresh water organisms found in stagnant water. Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible.
· They have two flagella, a short and a long one.
· They have Mixotrophic mode of nutrition, they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms. Therefore, they are known as connecting link between plants and animals.
· The pigments of euglenoids are identical to those present in higher plants. e.g., Euglena.
· 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, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls. They are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed by air currents.
Kingdom Protista – Protozoans
All protozoans are heterotrophs and live as predators or parasites.
There are four major groups of protozoans.
1. Amoeboid protozoans: They move and capture their prey by pseudopodia (false feet). Marine forms have silica shells on their surface. E.g., Amoeba, Entamoeba (Parasite)
2. Flagellated protozoans: They are either free-living or parasitic. They have flagella. The parasitic forms cause diseases such as sleeping sickness e.g, Trypanosoma.
3. Ciliated protozoans: Aquatic, actively moving organisms. Have thousands of cilia. They have a cavity (gullet) that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. E.g., Paramoecium.
4. Sporozoans: These organisms have an infectious spore-like stage in their life cycle. E.g., Plasmodium (malaria parasite).