In the last article, we have discussed the Kingdom Monera. In this article, we shall study the Kingdom Protista.

All single-celled eukaryotes are placed under Kingdom Protista (Greek – protistos – First of all), but the boundaries of this kingdom are not well defined. We include Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans under Protista. Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals, and fungi.

Characteristics of Kingdom Protista:

• They are with or without a cell wall. If the cell wall is present it is composed of cellulose.
• Being eukaryotes, the protistan cell body contains a well-defined nucleus with the nuclear envelope, nucleoplasm, nucleolus and helical DNA, chromosomes and other membrane-bound organelles such as mitochondria, chloroplast, ER, Golgi Complex etc.
• Some have flagella or cilia.
• They show the photosynthetic, holozoic, saprophytic or parasitic mode of nutrition.
• Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.

Classification of Protists:

Plant Like Protist:

Chrysophytes:

• This group includes diatoms and golden algae (desmids).
• They are found in freshwater as well as in marine environments.
• They are microscopic and float passively in water currents (plankton).  Most of them are photosynthetic. Diatoms are the chief ‘producers’ in the oceans.
• In diatoms, the cell walls form two thin overlapping shells, which fit together as in a soapbox. The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind a 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.

Dinoflagellates:

• These organisms are 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 (Example: 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.

Fungi Like Protist:

• 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.

Euglenoids (Plant or Animal):

• Majority of them are freshwater 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.
• The pigments of euglenoids are identical to those present in higher plants.
• Example: Euglena

Animal-Like Protist 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.

Amoeboid protozoans:

 These organisms live in freshwater, seawater 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:

The members of this group are either free-living or parasitic. They 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. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. Example: Paramoecium

Sporozoans:

 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 the human population.

Science > Biology > Classification of Microbes, Protists, and Fungi > Kingdom Protista

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