• 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

  • 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

Amoeboid 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.

Flagellated protozoans

  • 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


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.
  • 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.