What are Protists? Protists are organisms belonging to a kingdom which exclusively contains unicellular eukaryotic organisms. Did you know that before the five kingdom classification given by Whittaker, most protists were incorrectly placed in the plant or animal kingdom? Well, yes they were and this is because of their similarities to both the kingdoms. So, while some protists like Euglena, contain chlorophyll and are photosynthetic like plants, there are also some which show characteristics similar to fungi and animals. For example, some of them show a saprophytic mode of nutrition similar to fungi. Hence, are called Fungi-like protists. On the other hand, protists who show a holozoic, parasitic mode of nutrition are classified as animal-like protists.
Let’s study about these protists.
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. Slime moulds are saprophytic in nature. Reproduction is both sexual and asexual.
On the basis of structure, they are of two types:
Acellular slime moulds
Under favourable conditions, the body of acellular slime moulds is made up of a wall-less multinucleated protoplasmic mass covered in slime known as Plasmodium. It does not have a definite shape and moves from one place to another with the help of pseudopodia. During unfavourable conditions, the plasmodium forms spore-bearing fruiting bodies called sclerotia.
Vegetative reproduction occurs by fragmentation, sclerotia or by the formation of sporangia. Examples: Physarum, Fulgio
Cellular Slime Moulds
Their body consists of many wall-less amoeba-like cells known as Myxamoebae that are found in groups or a slimy mass but are not fused. This type of body is called Pseudoplasmodium. The nuclei of the pseudoplasmodium are haploid in nature. These extend pseudopodia to capture food. Examples: Dictyostelium, Protostelium.
They are unicellular, animal-like protists. They are mostly heterotrophs and show a holozoic or parasitic mode of nutrition. Locomotory organs such as flagella, cilia or pseudopodia are present for movement. Protozoans can be classified into 4 types - amoeboid, flagellated, ciliated and sporozoans.
Amoeboid protozoans are mostly aquatic and live in freshwater, sea water or moist soil. They show amoeboid movement with the help of pseudopodia. Silica shells are found on the surface of marine forms. Some amoeboid protozoans are parasitic in nature. These protozoans possess contractile vacuoles for osmoregulation. Asexual reproduction is by binary fission and sexual reproduction, if present, occurs by syngamy. Example : Amoeba, Entamoeba histolytica.
These protozoans possess flagella for locomotion and food capture. Nutrition can be saprotrophic or parasitic. They are found in both marine and freshwater. Some flagellated protozoans such as Trypanosoma brucei cause diseases such as sleeping sickness.
These are aquatic protozoans. They possess cilia which help both in movement and feeding. Nutrition is holozoic. They have food vacuoles where digestion of food takes place. They excrete out waste material through anal pore. Contractile vacuole is present for osmoregulation. Trichocysts are organs of defence and are used to stun the prey. Ciliates have two nuclei, a macronucleus which controls the vegetative functions and a micronucleus which takes part in sexual reproduction. Example: Paramoecium, Vorticella.
All sporozoans are endoparasites and many such parasites cause diseases in humans. They show gliding movements due to lack of locomotory structures. Sporozoans reproduce asexually as well as sexually. Sexual reproduction involves syngamy follwoed by formation of haploid spores called sporozoites. Some sporozoans such as Plasmodium have more than one host. Example: Plasmodium (malarial parasite)
Question 1. Protozoans include diverse types of organisms. Which class of protozoa have all parasitic organisms?
Answer: All sporozoans are endoparasites, most of which are also pathogenic. Thus, Sporozoa is the class of protozoa that has all parasites. Hence, option d is correct.
Question 2. Which option correctly matches the mode of reproduction of each protozoan.
a. Plasmodium - Multiple fission
b. Amoeba - Multiple fission
c. Euglena - Transverse binary fission
d. Paramecium - Longitudinal binary fission
Answer: Plasmodium reproduces by multiple fission in which it produces multiple daughter cells from a single parent cell. Thus, Plasmodium correctly matches its mode of reproduction. Hence, option a is correct.
Question 3. Select the feature which is common in all protozoans.
a. Being eukaryotic
c. Contractile vacuole
d. All of them
Answer: All protozoans are unicellular eukaryotic organisms. Only amoeboid protozoans use pseudopodia for locomotion. Contractile vacuole is also found only in amoeboid protozoans. Thus, being eukaryotic is common to all eukaryotes. Hence, option a is correct.
Question 4. Identify the wrong statement about protozoans?
a. Trypanosoma is a flagellated protozoan.
b. Amoeba or Entamoeba uses pseudopodia for locomotion.
c. Protozoans include Plasmodium
d. Paramecium is an amoeboid protozoan.
Answer: Paramecium has tiny hair-like structures called cilia on its body for locomotion. Thus, it is a ciliated protozoan. Hence, option d is correct.
Question 1. What is the mode of nutrition in flagellated protozoans?
Answer: All protozoans are heterotrophic in nature. The nutrition in flagellated protozoans can be saprotrophic or parasitic.
Question 2. Name the classes in which protozoans are classified on the basis of locomotory structures.
Answer: Protozoans are classified into 4 classes based on the locomotory organs as follows:
Question 3. What is plasmodium in slime moulds?
Answer: Plasmodium is the characteristic feature of slime mould. It is the body of acellular slime moulds, under favourable conditions, which is made up of a wall-less multinucleated protoplasmic mass covered in slime.
Question 4. Name the protist which causes sleeping sickness.
Answer: Trypanosoma belongs to the kingdom Protista and is a flagellated protozoan responsible for causing sleeping sickness.