Prtotisa is kingdom of unicellular eukaryotes. It is a kingdom of exclusion. Organisms that are not clearly bacteria, fungi, plants or animals are grouped into this large and diverse kingdom. The organisms in kingdom protista have some common features of other kingdoms. Some are autotrophic and are producers in marine and aquatic ecosystems like plant. Some are saprophyte and break down dead matter, like fungi. Some eat, move and behave very much like animals. This kingdom forms a connecting link between monera (prokaryotes) and plants, animals and fungi (multicellular eukaryotes).
- All members are microscopic, unicellular, eukaryotic organisms.
- Cell with one or more well developed nucleus.
- Presence of well developed membrane bound organelles.
- Mode of nutrition may be parasitic, ingestive, photosynthetic, saprophytic, mixotrophic etc.
- This group includes photosynthetic phytoplanktons, non-photosynthetic zooplanktons and disease causing parasites.
- Reproduction occurs by asexual (binary or multiple fission) and sexual reproduction (by syngamy).
- Two types of life cycles are found in protists: Life cycle showing zygotic meiosis and life cycle showing gametic meiosis.
Classification of Protista
There are four division or phylum in protista:
4. Slime molds
- These organisms are mostly marine and photosynthetic. They appear yellow, green, brown, blue or red depending on the main pigments present in their cells.
- The cell wall is divided in to plates, which is made up of cellulose. Due to this stiff cellulose plates on the outer surface they are called armoured algae.
- Most of them have two flagella, one lies longitudinally and the other transversely in a furrow between the wall plates. Due to this dinoflagellate shows a special type of whorling motion. Therefore they are called as "whorling whips".
- Chromosome of them doesn't contain histone protein, due to this they are known as mesokaryotes.
- Dinoflagellates like Noctilluca, Gonyaulax show "bioluminescence" due to presence of photogenic granules in cytoplasm. These dinoflagellates are known as night light. Due to presence of bioluminescence dinoflagellates are also known as "Fire algae".
- Noctilluca is knwon as Sea ghost, because it glows in night.
- Red tides: Red dinoflagellates (Example: Gonyaulax) undergo such rapid multiplication that they make the sea appear red. This red coloured water bloom is known as Red tides. They secrete toxins, which are known as "saxitoxin". Saxitoxin (STX) is a neurotoxin that can kill other marine animals such as fishes. It is accumulated in these organisms. Human aquire these toxins through food chain. Due to it they cause paralysis in human.
- This group include diatoms and golden algae (desmids).
- They are found in fresh water as well as in marine environment.
- They are microscopic and float passively in water currents (plankton).
- Most of them are photosynthetic and are chief producers in the oceans.
- In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box.
- The cell wall of diatoms is made up of cellulose in which silica particles are embedded in at many places. This silicated cell wall is called "shell" or "frustule". This is hard. This cell wall does not get destroyed after their death. Thus diatoms have left behind large amount of cell wall deposits in their habitat. This accumulation is referred as diatomaceous earth or keiselgura.
- Being gritty diatomaceous earth is used in polishing, filtration of oils and syrups. They are also used as heat insulator and as sound proofing substance.
Euglenophyta or Euglenoids
- This division includes Euglena like flagellates. Majority of them are fresh water organisms found in stagnant water.
- They are unicellular, cell wall absent. Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible.
- They have two flagella, a short and a long one. At the base of the flagellum there is a light-sensitive swelling, and to one side of it a red pigment spot. These structures guide the organism towards light when it is swimming.
- Euglena swim in water with the help of flagellum. If the water dries up, Euglena stops using its flagellum and wriggles around like a little worm. This movement is knwon as metaboly or euglenoid movements.
- Example: Euglena, Peranema etc.
- Euglenais also known as connecting link between plant and animal kingdom. Because it has some features of both kingdoms. They are photosynthetic in presence of sunlight (Plant like feature) but in absence of light or in dim light they behave heterotophs by predating on other smaller organisms (Animal like feature).
Slime moulds are saprophytic protists. Slime moulds consist of spreading slimy mass of protoplasm. The body moves along decaying twigs and leaves engulfling organic material. They are unicellular organism which lack cell walls and have the plasma membrane as their outer covering. They are also knwon as false fungi. Their are two types of slime moulds are present:
- Acellular slime moulds: Their body is made up of wall less multinucleated protoplasmic mass. This type of body is known as plasmodium. (Plamodium = wall less coenocytes). They are diploid.
- Cellular slime moulds: Group of amoeba like cells is known as cellular slime moulds. They are known as myxamoeba or pseudoplasmodium. They are haploid.
Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet. During unfavorable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls. They are extremely resistant and survive for many years, even under adverse conditions. The spore are dispersed by air currents.
- All protozoans are heterotrophs and live as predators or parasites.
- They are believed to be primitive relatives of animals. (Proto = Primitive + Zoan = Animals).
- There are four major groups of protozoans:
- (A) Amoeboid protozoan: They are known as sarcodines. These organisms live in fresh water, sea water or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites. Example: Entamoeba histolytica – Amoebiasis. Entamoeba gingivalis – Pyorrhoes.
- (B) Flagellated protozoans: They are known as zooflagellates. The members of this group are either free living or parasitic. They have one to several flagella for locomotion. Example: Trypanosoma (It produces sleeping sickness disease).
- (C) Ciliated protozoans: They are known as ciliates. These are aquatic, actively moving organisms because of the presence of thousand 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 current with food to be steered into the gullet. Example: Paramoecium.
- (D) Sporozoans: This includes diverse organisms that have an infectious spore like stage in their life cycle. Locomotory organs (cilia, flagella, pseudopodia etc.) are absent. Plasmodium (malarial parasite)