Kingdom ProtistaTerm "Protista" by C. Cuvier and Kingdom established by E.Hackel. All the organisms included in Protista are unicellular (acellular) eukaryotes.
Living organisms included in Protista are as follows:
2. Absorptive and Photosynthetic Organisms (Eg: Dinoflagellates, Diatoms, Euglenoids)
3. Protozoans or Consumer - Decomposer Organisms (Eg: Slime Mould)
4. Eg: Zooflagellates, Sacrodines, Sporozoans, CiliatesMode of NutritionMode of nutrition in Protista is of different types.
1. Holophytic or Photosynthetic
- They synthesize their own food through photosynthesis.
Autotrophs: Synthesize their own food
- Some protists have a holozoic mode of nutrition, which is similar to animals, i.e. food is first ingested and then digested.
- Some protists obtain their food from dead organic substances. These protists secrete some extracellular enzymes.
- These enzymes convert complex organic substances into simpler substances. Now, these simple substances can be easily absorbed through the body surface.
- Some protists have both holophytic and saprophytic type of nutrition.
Protists reproduce both asexually and sexually.
1. Asexual Reproduction
This is the most common method of reproduction in protists. Asexual reproduction takes place in favourable conditions.
It is of following types:
(a) Binary Fission
Two daughter cells are formed by the division of one mother cell. After this, each daughter cell grows to form a normal organism.
(b) Spore Formation
- Some protists have a special structure known as sporangia.
- Spores are formed in these sporangia.
- The sporangia bursts after some time and all the spores become free.
- These spores form a new cell after germination.
2. Sexual Reproduction
- In sexual reproduction, two haploid gametes fuse to form a diploid zygote. This process is known as syngamy.
Syngamy is of three types:
Syngamy Types(a) Isogamy
- It is the easiest way of sexual reproduction.
- In isogamy the fusing gametes are morphologically (i.e. shape, size, structure) similar but physiologically(i.e. functionally or genetically) they may be similar or dissimilar.
- When fusing gametes are physiologically dissimilar, the process is called physiological anisogamy.
- The fusing gametes are morphologically dissimilar (smaller - larger, motile - immotile) but physiologically they may be similar or dissimilar.
- It is the developed form of anisogamy.
- The male gamete is small and motile, while female gamete is large and non-motile. This female gamete is known as an egg.
- The formation of male & female gametes takes place in sex organs.
Life Cycle of Protists
Protists Life Cycle
1. Life Cycle Showing Zygotic Meiosis
- When a protist is haploid, and meiosis occurs in the zygote, i.e. after fertilization, then it is known as zygotic meiosis.
- In this type of life cycle, the zygotic phase is only diploid and remaining all the phases are haploid, so this type of life cycle is known as a haplontic life cycle.
- Example: Ceratium, Gymnodinium, Cellular slime moulds.
Note: In this type of life cycle the zygotic phase is only diploid and remaining all the phases are haploid so this type of life cycle is known as Haplontic life cycle.
2. Life Cycle Showing Gametic Meiosis
- When a protist is diploid, and meiosis takes place during gamete formation, then it is called gametic meiosis.
- In this type of life cycle during sexual reproduction, meiosis takes place in a diploid cell, due to which haploid gametes are formed.
- Now haploid gametes fuse to form a diploid zygote. And after that mitosis takes place in the zygote, due to which diploid cells are formed again.
- Example: Noctiluca, Diatoms, Acellular slime moulds.
Note: In this type of life cycle only gametic phase is haploid and remaining all phases are diploid so this type of life cycle is known as Diplontic life cycle.
Economic Importance of Protists
- Protists serve as the foundation of the food chain.
- Protists are symbionts – having a close relationship between two species in which, one is benefited.
- Some protists also produce oxygen and may be used to produce biofuel.
- Protists are the primary sources of food for many animals.
- In some rare cases, Protists are harvested by humans for food and other industrial applications.
- Phytoplankton is one of the sole food sources for whales.
- Seaweed is an alga, which is considered a plant-like protist.
- Zooplankton is fed on by various sea creatures including shrimp and larval crabs.
Division - Euglenophyta - Euglenoids
- Previously euglenoids were placed in plant kingdom due to their photosynthetic ability. But due to the absence of cell wall and animals like nutrition, some scientists placed them in the animal kingdom. But now according to five kingdom classification, they are included in Protista.
- They are found as free-living organisms in freshwater lakes, ponds, etc. But sometimes they are also found in damp soil and brackish water.
- On the basis of their mode of nutrition, they are called as mixotrophic because they have the holophytic, holozoic and saprophytic mode of nutrition.
Example: Euglenoids - Euglena, Paranema.
- They are unicellular and the cell wall is absent around them. They are surrounded by a cell membrane which is made up of lipoprotein and this cell membrane is covered with a pellicle. The pellicle is made up of lipoprotein and it is elastic in nature.
Structure of Euglenoid
- At the anterior end of Euglenoids, a cavity is present, which is known as the reservoir. The flagellum is originated from the base of the reservoir. Euglenoids have only one functional flagellum and one non-functional flagellum. One eyespot is present at an anterior position.
- They have a contractile vacuole. These contractile vacuoles help in osmoregulation.
- Euglenoids have a haploid nucleus and chloroplast.
- Chloroplast has the following pigments: Chl. 'a', Chl. 'b' and Xanthophyll (Zeaxanthin).
Note: Stored food - Paramylum and fat- Paramylum is a carbohydrate, which is formed by the modification of starch.
- Wriggling movement: Euglenoids are motile. They are of two type - flagellated and Non-flagellated.
- Flagellated Euglenoids locomote with the help of flagella. But non-flagellated Euglenoids are also motile.
- These non-flagellated euglenoids locomote by wriggling movement, which is also called a Euglenoid movement.
- Wriggling movement is due to the wave motion of the pellicle.
- Asexual reproduction by longitudinal binary fission.
Longitudinal Binary Fission in Euglenoid
- Also, reproduce by cyst formation during unfavourable conditions.
- Sexual reproduction has not been seen yet.
- These organisms develop a slimy mass at the time of their vegetative phase. Therefore they are called slime moulds. They are also called false fungi.
- They are found on decaying stem, leaves etc, so these are saprophyte.
- Slime moulds have characters of both animals & fungus, therefore, they are also called Fungus animals.
- Scientist Anton De Bary placed them in Mycetozoa by relating them with animals. While mycologist 'Ainsworth' placed them in Myxomycota by relating them with fungi.
- But nowadays modern scientists place them in Class - Gymnomycota (Gymnomycota-naked fungi) of Kingdom Protista and now these are known as Protistan fungi.
➢ On the basis of structure, Slime Moulds are of two types:
(i) Acellular Slime Moulds
- Their body is made up of wall-less multinucleated protoplasmic mass. This type of body is known as plasmodium. (Plasmodium = wall-less coenocyte).
- During unfavourable conditions, plasmodium forms fruiting bodies.
- They are diploid, i.e. every nucleus found in the cell is diploid.
Example: Physarum, Stemonitis, Physarella, Fulgio.
(ii) Cellular Slime Moulds
- Their body consists of many wall-less amoeba-like cells (the group of amoeba-like cells is known as cellular slime mould).
- These cells are found in groups but they are not fused. Every cell has a haploid nucleus. This type of body is called as myxamoeba or pseudoplasmodium.
- They are haploid because the nucleus found in each cell is haploid.
Example: Dictyostelium, Protostelium, Acytostelium
➢ Slime moulds (asexual & sexual type of reproduction)
- Asexual reproduction: It is mainly with the help of spore formation (sporangia). The mucilaginous sporangium of slime moulds is known as capillitium.
- Sexual reproduction: The cells of acellular slime moulds are diploid. So they reproduce by gametic meiosis. Therefore, their life cycle is diplontic. The cells of cellular slime moulds are haploid, so they reproduce by zygotic meiosis. Therefore, their life cycle is haplontic.
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 freshwater, seawater 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.