You must have seen ponds covered with green plants. You always thought that those are mosses. Didn’t you? Well no! We will see what those are. It is time to peek deeper into another kingdom in this chapter: Protista. We will look at some of the most interesting groupings of this kingdom. We will also take a look at their characteristics and examples.
Characteristics of Kingdom Protista
We place all single-celled eukaryotes under Protista. However, the boundaries of this kingdom are not well defined. Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals and fungi. Being eukaryotes, the protistan cell body contains a well-defined nucleus and other membrane-bound organelles.
Some have flagella or cilia. Protists reproduce asexually and sexually by, the process involving cell fusion and zygote formation. It may be photosynthetic or holotrophic. These could also be saprotrophic, parasitic and symbionts. On the other hand, some could have mixotrophic nutrition (holotrophic + saprobic). Phytoplanktons are photosynthetic, floating protists. Zooplanktons are free-floating, holozoic protozoans.
Grouping of Unicellular Protists
We can classify unicellular protists into three major groups:
- Photosynthetic Protists. Example: Dinoflagellates, Diatoms, Euglenoids
- Consumer Protists. Example: Slime moulds or Myxomycetes
- Protozoan Protists.Example: Zooflagellate, Sarcodina, Sporozoa, Ciliata
Life Cycles in Protists Showing Zygotic Meiosis
By life cycle, what we mean is nothing but a sequence of events between any given phase in one generation and that similar phase in the succeeding generation. It occurs in some dinoflagellates (Example: ceratium, gymnodinium; von stosch, 1973) and cellular slime moulds.
The zygote is in the form of 2n. It usually divides by meiosis (also called zygotic meiosis). These produce vegetative cells with the chromosome number of 1n. These cells divide repeatedly by mitosis. The resultant daughter cells maintain the 1n number of chromosomes. Some of the vegetative cells produce gametes. When these gametes combine in fertilization, a zygote forms and the life cycle gets complete.
Major Groups of Protists
This group comprises of the diatoms and golden algae (desmids). We find them in fresh water as well as in marine environments. They are microscopic. These organisms float passively in water currents (plankton).
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.
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, 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 surrounding water.
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.
The spores possess true walls. The spores are dispersed by air currents. They are extremely resistant to changes in the atmosphere. They are capable of surviving for many years, even under adverse conditions.
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:
- Flagellated Protozoans: They possess flagella for locomotion. They may be free-living aquatics, parasites, commensals or symbionts. Zooflagellates are generally uninucleate, occasionally multinucleate.
- Amoebid Protozoans: They develop pseudopodia which are temporary protoplasmic outgrowths. These are used for locomotion and engulfing food articles. Sarcodines are mostly free-living, found in fresh water, sea water and on damp soil.
- 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.
- 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.
Solved Examples for You
Question: Give some examples of disease-causing protozoans.
Answer: The various types of disease-causing protozoa include:
- Trypanosome gambiense: The parasite of sleeping sickness. It is transmitted by tsetse fly. It causes Gambian sleeping sickness.
- Trypansoma rhodesiense: It causes Rhodesian sickness. The parasite is transmitted by the bites of tsetse fly (Glossina palpalis and glossina morsitans). Initially parasite is present in the blood of man but later on, it enters the cerebrospinal fluid.