- All single-celled eukaryotes are placed under Protista.
- Chrysophytes, Dianoflagellates, Euglenoids, Slime moulds and Protozoans are under Protista.
- The protistan cell body contains a well-defined nucleus and other membrane-bound organelles.
- Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.
- This group includes diatoms and golden algae (desmids), found in fresh water as well as in marine environments.
- In diatoms the cell walls form two thin overlapping shells embedded with silica.
- diatoms have left behind large amount of cell wall deposits in their habitat, which accumulated over billions of years is referred to as ‘diatomaceous earth’ .
- Most of them are photosynthetic and are the chief ‘producers’ in the oceans.
- Mostly marine and photosynthetic.
- The cell wall has stiff cellulose plates on the outer surface.
- Yellow, green, brown, blue or red pigments are present in their cell.
- Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
- Red dianoflagellates undergo such rapid multiplication that they make the sea appear red (red tides). Example: Gonyaulax.
- Majority of euglenoids are fresh water organisms.
- They have a protein rich layer called pellicle which makes their body flexible.
- They have two flagella, a short and a long one.
- They are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs.
- Slime moulds are saprophytic protists.
- Under suitable conditions, they form an aggregation called plasmodium, and during unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.
- The spores possess true walls, and they are extremely resistant and survive for many years.
- All protozoans are heterotrophs and live as predators or parasites.
- There are four major groups of protozoans
- Amoeboid protozoans:
- These organisms live in fresh water, sea water or moist soil.
- They show Amoeba like movements and move and capture their pray with the help of pseudopodia.
Example- Entamoeba, a parasite.
- Flagellated protozoans:
- The members of this group are either free-living or parasitic, and have flagella.
- The parasitic forms cause diseases such as sleeping sickness.
- Example: Trypanosoma.
- Ciliated protozoans:
- These are aquatic, actively moving organisms because of the presence of thousands of cilia.
- They have a cavity (gullet) that opens to the outside of the cell surface.
- Sporozoans includes diverse organisms that have an infectious spore-like stage in their life cycle.
- The most notorious is Plasmodium (malarial parasite) which causes malaria.