Lesson 5 of 12 · 7 min
Kingdom Protista
NCERT §2.2–2.2.4
The green scum from the school pond, seen under the microscope, is full of tiny glassy boxes and whirling cells with grooves. None of them is a bacterium: each has a nucleus.
The lesson in notes
In short
Protista holds all single-celled eukaryotes, but its boundaries are fuzzy: one biologist's photosynthetic protist is another's plant. NCERT includes chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans.
Protists are mostly aquatic, form a link with plants, animals and fungi, have a well-defined nucleus and membrane-bound organelles, may carry flagella or cilia, and reproduce asexually and sexually through cell fusion and zygote formation.
Chrysophytes are diatoms and golden algae (desmids), found in fresh and sea water. They are microscopic, drift with water currents as plankton, and most are photosynthetic.
A diatom's wall is two thin overlapping halves that fit like the lid and base of a soap box. Silica makes the walls practically indestructible, so over billions of years they have piled up as diatomaceous earth, a gritty material used for polishing and for filtering oils and syrups. Diatoms are the chief producers of the oceans.
Dinoflagellates are mostly marine and photosynthetic, coloured yellow, green, brown, blue or red by their main pigments. The outer wall has stiff cellulose plates, and most have two flagella, one running lengthwise and one crosswise in a groove between the plates.
Red dinoflagellates such as Gonyaulax can multiply so fast that the sea turns red (red tides); their toxins can kill fish and other marine animals.
Euglenoids mostly live in stagnant fresh water. They have no cell wall but a protein-rich pellicle that keeps the body flexible, and two flagella, one short and one long.
Euglenoids photosynthesise in sunlight but, kept in the dark, feed like heterotrophs by preying on smaller organisms. Their pigments are the same as those of higher plants. Example: Euglena.
Slime moulds are saprophytic protists that creep over rotting twigs and leaves, engulfing organic matter. In good conditions they gather into a plasmodium that can spread over several feet.
In bad conditions the plasmodium forms fruiting bodies with spores at their tips. The spores have true walls, resist harsh conditions for many years, and are spread by air currents.