Biology XI-2. Biological Classification(2 Kingdom Protista)

Kingdom Protista General Characteristics Contains all unicellular eukaryotes Primarily aquatic Cell body contains well-defined nucleus and other membrane-bound organelles Reproduce asexually and sexually, by a process involving cell fusion and zygote formation Classes of Protista  Chrysophytes (Planktons) Includes diatoms and golden algae (desmids) Found in fresh water as well as marine environments Microscopic; float passively in water currents Mostly photosynthetic Diatoms Cell wall forms two thin overlapping shells Walls embedded with silica, and thus, indestructible Fossilised remains of diatoms are referred to as ‘diatomaceous earth’ Chief producers of the ocean Dinoflagellates Mostly marine and photosynthetic Appear yellow, green, brown, blue or red, depending on pigments present in their cells Cell wall has stiff cellulosic plates on the outer surface Most of these have two flagella—one lies longitudinally and the other transversely, in a furrow between the wall plates Red dinoflagellates (Example − Gonyaulax) rapidly multiply and make the sea appear red (red tides) Toxins released by these organisms may kill other marine animals Euglenoids Majority are fresh-water organisms Found in stagnant water Have protein-rich layer called pellicle, instead of cell wall, which makes their body flexible In the presence of sunlight, they behave like autotrophs; in the absence of sunlight, they behave like heterotrophs, predating on smaller organisms Pigments are similar to those present in higher plants Slime Moulds Saprophytic protists Move along decaying twigs and leaves Under suitable conditions, they form an aggregation called plasmodium, which grows and spreads Under unfavourable conditions, plasmodia differentiate to form fruiting bodies bearing spores at their tips Spores possess extremely resistant true walls, which make them capable for surviving for a long time under adverse conditions Spores disperse by air currents Protozoans All are heterotrophs Live as predators or parasites Believed to be the primitive relatives of animals Amoeboid Protozoans Live in freshwater, sea water or in moist soil Move and capture their prey by putting out pseudopodia (false foot); Example: Amoeba Marine forms have silica shells on their surface Some of them, such as Entamoeba, are parasites Flagellated Protozoans Either free-living or parasitic Bear flagella The parasitic forms cause diseases such as sleeping sickness; Example: Trypanosoma Ciliated Protozoans Aquatic and actively moving Show presence of thousands of cilia Have a cavity (gullet) which opens on the outside Movement of cilia causes food-laden water to enter the gullet Example: Paramoecium Sporozoans Include diverse organisms with spore-like infectious stage in their life cycle Example: Plasmodium,which causes malaria