The Kingdom : Protista
All single-celled eukaryotes are placed under the kingdom Protista, but the boundaries are not well defined. What could be ' a photosynthetic protistan' to one biologist, may be 'a plant' to another. Members of Protista are mainly aquatic. This kingdom forms a link with others dealing with plants, animals and fungi. Being eukaryotes, protistan cell bodies contain a well-defined nucleus and other membrane-bound organelles. Some have flagella or cilia. Protists reproduce asexually and sexually by the process of cell fusion and zygote formation.
Structure:
In protists, the cells are surrounded by a membrane. Photosynthetic protista cells contain chlorophyll. Every cell contains organelles surrounded by phases like mitochondria, golgi bodies, endoplasmic reticulum, nucleus, chromosomes, etc.
Reproduction:
Reproduction in protista mainly takes place in two ways:
1. Asexual reproduction and
2. Sexual reproduction
Asexual reproduction occurs by bifurcation and cyst formation, whereas in sexual reproduction the male and female gametes combine to form a zygote. Meiosis takes place in the zygote and eventually haploid organisms develop.
Locomotion :
1. by flagellum
2. by romance
3. by cephalopods or footpads
Mode of nutrition :
Autotrophic (Photosynthetic) and Heterotrophic
1. Chrysophytes
The chrysophytes group includes diatoms and golden algae and they are found in fresh water as well as in marine environment. Mostly they are microscopic, chrysophytes swim passively in water currents (plankton) and most of them are photosynthetic. The cell wall in diatoms forms two thin overlapping spheres, which fit together like a soap box. The walls are inlaid with silica and thus the walls are indestructible.
2. Dinoflajillates
These organisms are mostly marine and photosynthetic and the color is yellow, green, brown, blue or red depending on their main pigment present in their cells. On the outer surface of the cell wall are hard cellulose plates. Most of them have two flagella:
3. Euglenoids
Most of them are freshwater organisms found in stagnant water and have a protein-rich layer called pellicle which makes their body flexible. They have two flagella, one short and one long.
They are photosynthetic in the presence of sunlight, when they are deprived of sunlight they behave like heterotrophs by preying on other small organisms. Interestingly, the pigments of euglenoides are similar to those present in higher plants.
4. Slime Moulds
Slime moulds are saprophytic protists. The body continues to absorb decaying twigs and leaves as well as organic matter. Under suitable conditions, they form an aggregation called Plasmodium which can grow and spread for several feet and during unfavorable conditions, the Plasmodium detaches and forms fruiting bodies with spores at their ends. Spores have true walls. They are extremely resistant and live for many years even under adverse conditions.
The spores are spread by air currents.
5 Protozoans
All protozoans are heterotrophs and live as predators or parasites. they are believed to be primitive relatives of animals and categorized in four major groups of protozoans as follows.
a. Amoeboid protozoans:
These organisms live in the fresh water, sea water or moist soil. They can move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. The marine forms have silica shells on their surface. Its example is Entamoeba.
b. Flagellated protozoans:
They protozoans of this group are either free-living or parasitic. They have flagella. The parasitic forms cause diseases such as sleeping sickness. Its Example is Trypanosoma.
c. Ciliated protozoans:
These protozoans 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. Its example is Paramoecium .
d. Sporozoans:
The sporozoans group 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 effects the human population.