Question

Describe kingdom Protista

Answer 1

Kingdom Protista contains the protists or the organisms that do not fit into any of the other categories. These organisms were traditionally considered the first eukaryotic forms of life, predecessors to the organisms in the plant, animal, and fungus kingdoms. Eukaryotes are organisms whose cells have membrane-bound organelles.

General characteristics of Kingdom Protista are as follows:

• They are simple eukaryotic organisms.
• Most of the organisms are unicellular, some are colonial and some are multicellular like algae.
• Most of the protists live in water, some in moist soil or even the body of humans and plants.
• These organisms are eukaryotic since they have a membrane-bound nucleus and endomembrane systems.
• They have mitochondria for cellular respiration and some have chloroplasts for photosynthesis.
• Nuclei of protists contain multiple DNA strands, the number of nucleotides is significantly less than complex eukaryotes.
• Movement is often by flagella or cilia.
• Protists are multicellular organisms, they are not planted, animals or fungus.
• Respiration - cellular respiration is a primarily aerobic process, but some living in the mud below ponds or in the digestive tracts of animals are strict facultative anaerobes.
• Nutrition - they can be both heterotrophic or autotrophic.

Major Groups of Protists

Chrysophytes

This group comprises the diatoms and golden algae (desmids). These organisms float passively in water currents (plankton).

Dinoflagellates

These organisms are usually marine and photosynthetic. They have an appearance of various colours like yellow, green, brown, blue or red. Their colour is influenced and decided by the main pigments present in their cells. The cell wall has stiff cellulose plates on its outer surface. These organisms usually have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.

Euglenoids

These are mostly freshwater organisms. We can find them in stagnant water. They do not have a cell wall. Rather, they are built with a protein-rich layer, a pellicle that makes their body flexible. They have two flagella. One is short and the other is a long one. The two flagella join with each other at a swelling called paraflagellar body. Euglena is a connecting link between animals and plants. Nutrition in Euglena is mixotrophic, when the light is available it is photosynthetic, in darkness, it is saprophytic absorbing food from the surrounding water.

Slime Moulds

Slime moulds are saprophytic protists. Their body is capable of moving through 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.

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 protozoan:

1. Flagellated Protozoans: They possess flagella for locomotion. They may be free-living aquatics, parasites, commensals or symbionts. Zooflagellates are generally uninucleate, occasionally multinucleate.

2. Amoebid Protozoans: They develop pseudopodia which are temporary protoplasmic outgrowths. These are used for locomotion and engulfing food particles. Sarcodines are mostly free-living, found in freshwater, seawater and damp soil.

3. Sporozoans: All sporozoans are endoparasites. Some sporozoans such as Eimeria cause severe diseases like coccidiosis in the birds. Nutrition is parasitic (absorptive). Phagotrophy is rare.

4.Ciliated Protozoans: Ciliates are protozoan protists. These develop a number of cilia during a part or whole of the life cycle. They use cilia for locomotion and driving food. There is a high degree of morphological and physiological specialisation. There are definite regions for ingestion and egestion. The region of ingestion consists of an oral groove, cytostome (mouth) and gullet.