In 1969, R.H Whittaker classified living organisms into five kingdoms. He classified organisms based on various criteria like cell structure, mode of nutrition, lifestyle, etc. There were few organisms which are eukaryotic but mostly made of a single cell. He classified and called them Protists. They belong to the kingdom Protista, one of the five kingdoms.Let’s see different groups of protists and their characteristic features.
Kingdom Protista Classification
Protists exhibit characteristic features of all eukaryotic organisms like animals, fungi, and plants. Hence, they categorized to a separate kingdom called kingdom Protista. Kingdom Protista consists of the unicellular eukaryotic organisms. These organisms have a well-defined cell structure with membrane-bound organelles, as is the characteristic of eukaryotes. Protists are majorly aquatic and can reproduce by both means of reproduction i.e. sexual and asexual. The main groups of the kingdom Protista are as follows:
These are microscopic and are found floating in water currents. They include the diatoms and desmids and are located in both marine and freshwater bodies. Diatoms are the primary producers of the water bodies and have two layers of shells made up of silica. As a result of this shell, long time accumulation of diatoms leads to the formation of the diatomaceous earth.
These organisms possess coloring pigments which impart various colors to the bodies like red, green, blue, yellow and brown. The famous red tides occur due to these organisms as the red dinoflagellates start multiplying rapidly. Meanwhile, they also release toxins which kill many marine organisms.
These organisms are mostly found in fresh water. One of the major features of euglenoids is that they have a protein-rich layer – pellicle instead of the cell wall. This layer gives flexibility to the organism. They can be both autotrophic and heterotrophic depending on the availability of sunlight. They possess two flagella, one short and one long.
These are saprophytic in nature. They feed on decaying twigs and leaves. When they find the condition suitable, they form plasmodium – an aggregation which may grow and spread over several feet. The plasmodium forms fruiting bodies which bear spores. This happens only when the conditions are unfavorable. The spores have strong protective covering and are dispersed by air. These spores can remain in this state together for years till they find the suitable condition.
These members of the Protista kingdom are necessarily heterotrophs. They live as parasites or predators. The protozoans can be divided into four major groups:
- Amoeboid protozoans – Mostly found in water bodies, either fresh or saline. They have pseudopodia (false feet)which help to change their shape and in capturing and engulfing food.
- Flagellated protozoans – As the name suggests, the members of this group have flagella. They can be free-living as well as parasitic.
- Ciliated protozoans – They have cilia all over their body which help in locomotion as well as nutrition. They are always aquatic.
- Sporozoans – These organisms are so called because their life cycle has a spore-like stage. For example, the malarial parasite, Plasmodium.
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