Kingdom : Protista

All unicellular eukaryotic organism, irrespective of their mode of nutrition, are included in the Kingdom Protista. This kingdom forms a connecting link between the prokaryotic Monera on one hand and other three eukaryotic kingdoms. Fungi, Plantae and Animalia on the other hand. Protists were the first eukaryotes to evolve on this earth some 1600 million years ago. Protists may be considered as the ancestors of all multicellular eukaryotes.

The term Protista was proposed by Ernst Haeckel and also created the kingdom Protista in 1886.

General characteristics

They are solitary unicellular or colonical unicellular eukaryotic organisms.

Mostly they are aquatic organisms. Many protists are planktons. Some forms are parasites.

Protists have different body shapes. The cell wall, when present, contains cellulose. Pellicle, shell, etc. may be present.

Cytoplasm contains membrane bound cell organelles; many have centrioles also.

Cytoplasm shows streaming movement.

Photosynthetic forms contain chloroplasts with internal thylakoids and act as chief producers of food in the oceans and fresh waters.

Nucleus has typical structure viz. porous nuclear envelope, chromatin material, nucleoplasm and nucleolus. A mesokaryon occurs in dinoflagellates.

They commonly move with the help of pseudopodia, flagella or cilia. Flagella have 9 + 2 organization of microtubules that are composed of a protein named tubulin.

Nutrition may be photosynthetic, holozoic, saprobic or parasitic. Some protists have mixotrophic nutrition (photosynthetic and saprobic) as in euglenoids.

Food reserve can be starch, paramylum, chrysolaminarin, glycogen and fat.

Reproduction occurs by both asexual and sexual methods. An embryo stage is absent.

In protists asexual reproduction is a rapid method of multiplication and occurs by binary fission, multiple fission, spore formation, budding, etc.

Sexual reproduction is believed to have originated in primitive protists. It involves meiosis and syngamy.

Parasitic protists may cause diseases such as dysentery, malaria, sleeping sickness, etc.