The kingdom protista includes eukaryotic microorganisms, They are uni-cellular and generally aquatic. On evolutionary point of view this kingdom acts as a link between prokaryotic monera and multicellular eukaryotic kingdom plantae, fungi and animalia. Protists show different life styles. Some are photo synthetic like plants or predatory like animals. Some are parasitic.
Kingdom Protista includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans.
i) Chrysophytes:
- This group includes diatoms and golden algae (desmids).
- They are found in fresh water as well as in marine environments.
- They are microscopic and float passively in water currents (plankton).
- Most of them are photosynthetic.
- In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box. The walls are embedded with silica and thus the walls are indestructible. Thus, 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.
- Mostly marine and photosynthetic. 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.
- Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
- Very often, red dinoflagellates (Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides). Toxins released by such large numbers may even kill other marine animals such as fishes.
ii) Euglenoids:
- 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. They have two flagella, a short and a long one. Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.
- Interestingly, the pigments of euglenoids are identical to those present in higher plants.
- Example: Euglena, Pernema, Trachelomonas
iv) 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 unfavourable 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.
- Example of cellular slimemoulds: Dictyostelim
- Example of acellular slimemoulds: Physarella and Physarum
v) Protozoans: 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.
a) 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.
b) Flagellated protozoans: The members of this group are either free-living
or parasitic. They have flagella. The parasitic forms cause diaseases such
as sleeping sickness.
Example: Trypanosoma.
c) 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:
Paramecium, Vorticella, Nyctotherus, Opalina
d)Sporozoans (Parasitic protozoans) : 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, a disease which has a staggering
effect on human population.
Example: Plasmodium
Tags:
Kingdom Protista