Biological Classification - Notes | Class 11 | Part 3: Kingdom Protista



-    It includes single-celled eukaryotes.

-    The cell contains a well-defined nucleus and other membrane-bound organelles. Some have flagella or cilia.

-    Protists are primarily aquatic.

-    It is a link with plants, animals and fungi.

-    They reproduce asexually and sexually (cell fusion and zygote formation).

-    Protista includes Chrysophytes, Dianoflagellates, Euglenoids, Slime moulds and Protozoans.

I. Chrysophytes

-    Found in fresh water and marine environments.

-    Microscopic and float passively in water currents (plankton).

-    Most of them are photosynthetic.

-    It includes diatoms & golden algae (desmids).

-    Diatoms: They have siliceous cell walls forming two thin overlapping shells, which fit together as in a soap box. The cell wall deposit of diatoms over billions of years in their habitat is known as ‘diatomaceous earth’. This is used in polishing, filtration of oils and syrups.

-    Diatoms are the chief ‘producers’ in the oceans.

II. Dinoflagellates

-    Mostly marine and photosynthetic.

-    They appear yellow, green, brown, blue or red based on the main pigments present in their cells.

-    The cell wall has stiff cellulose plates on the outer surface.

-    Most of them have 2 flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.

-    Red dinoflagellates (E.g. Gonyaulax) undergo rapid multiplication so that the sea appears red (red tides). They release toxins that kill marine animals like fishes.

III. Euglenoids

-    Mainly fresh water organisms found in stagnant water.

-    Instead of a cell wall, they have a protein rich layer called pellicle. It makes their body flexible.

-    They have two flagella, a short and a long one.

-   They are photosynthetic in the presence of sunlight. In the absence of sunlight, they behave like heterotrophs by predating on smaller organisms.

-    The pigments are identical to those in higher plants.

-    E.g. Euglena.

IV. Slime Moulds

-    They are saprophytic protists.

-    The body moves along decaying twigs and leaves engulfing organic material.

-    Under suitable conditions, they form an aggregation called plasmodium. It may spread over several feet.

-  Under unfavourable conditions, plasmodium differentiates and forms fruiting bodies bearing spores at their tips. Spores have true walls. They are highly resistant and survive for many years. Spores are dispersed by air.

V. Protozoans

They are heterotrophs (predators or parasites).

They are the primitive relatives of animals.

There are 4 major groups of protozoans:

o   Amoeboid protozoans: They live in fresh water, sea water or moist soil. They move and capture prey by putting out pseudopodia (false feet). E.g. Amoeba. Marine forms have silica shells on their surface. Some of them are parasites. E.g. Entamoeba.

o   Flagellated protozoans: They are free-living or parasitic. They have flagella. The parasitic forms cause diseases like sleeping sickness. E.g. Trypanosoma.

o   Ciliated protozoans: They are aquatic, actively moving organisms using thousands of cilia. They have a cavity (gullet) that opens to outside. By the movement of cilia, the water with food enters gullet. E.g. Paramoecium.

o    Sporozoans: They have an infectious spore-like stage in their life cycle. E.g. Plasmodium (malarial parasite).


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