- All single-celled eukaryotes are placed under Protista.
- Being eukaryotes, the protistan cell body contains a well defined nucleus and other membrane-bound organelles.
- Some have flagella or cilia.
- Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.
- Chrysophytes, Dianoflagellates, Euglen oids, Slime moulds and Protozoans are under Protista
- Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals and fungi.
- They are found in fresh water as well as in marine environments.
- This group includes diatoms and golden algae (desmids).
- They are microscopic and float passively in water currents (plankton).
- Most of them are photo synthetic.
- In diatoms the cell walls form two thin overlapping shells, fit together as in a soap box.
- The walls are embedded with silica and thus the walls are indestructible.
- Diatoms have left behind large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as diatomaceous earth’.
- Being gritty this soil is used in polishing, filtration of oils and syrups.
- Diatoms are the chief ‘producers’ in the Oceans.
- 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.
- These organisms are mostly marine and photosynthetic.
- Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
- Red dianoflagellates (Example: Gonyaulax) undergo such rapid multiplication that they make the sea appeared (red tides).
- Toxins released by large numbers kill other marine animals such as fishes.
- 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 by predating on other smaller organisms.
- Majority of them 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.
- The pigments of euglenoids are identical to those present in higher plants.
- Example: 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 unfavorable 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.
- All protozoans are heterotrophs and live as predators or parasites.
- They are believed to be primitive relatives of animals.
- There are four major groups of protozoans
- These organisms live in fresh water, sea water or moist soil.
- They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba.
- Marine forms have silica shells on their surface.
- Some of them such as Entamoeba are parasites.
- They have flagella.
- The members of this group are either free living or parasitic.
- The parasitic forms cause diseases such as sleeping sickness.
- Example: Trypanosoma
- 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.
- The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet.
- Example: Paramoecium
This includes diverse organisms that have an infectious spore-like stage in their life cycle.
The most notorious is Plasmodium (malarial parasite) which causes malaria which has as taggering effect on human population.