Overview of kingdom Protista, Practice Problems, Frequently Asked Questions

Do you know under which kingdom unicellular or acellular eukaryotic organisms are placed? It was quite difficult to classify these organism. Do you know why? This is because these organisms do not have well defined characteristic features. Some of them show characterististics like plants, some like animals and others like fungi. Still, not able to figure out the kingdom? It is Protista.

Have you heard of Amoeba? Did you know that Amoeba is a protist? Well, it is. Now think of Euglena. Do you think an Amoeba and an Euglena have anything in common? Euglena can photosynthesise but Amoeba cannot. Amoeba shows locomotion with the help of pseudopodia but Euglena uses its flagella for locomotion. So there are many differences right? But you will be amazed to know that both of these have been put under the same kingdom, that is, Protista. This shows that although this kingdom includes all unicellular eukaryotes, the members still show a lot of variety in their mode of nutrition, locomotion and many other facets. Let us learn more about this.

Table of contents

• Habitat of Protists
• Mode of Nutrition in Protists 
• Locomotion in Protists
• Reproduction in Protists
• Practice problems
• Frequently Asked Questions

Structure

These organisms are unicellular and microscopic. Being eukaryotic they have a well-defined nucleus covered by a nuclear membrane. The cells may be with or without a protective covering such as cell wall, cuticle, pellicle, etc. Genetic material is composed of DNA and proteins and lies within the nucleus. The cytoplasm shows continuous streaming or cyclosis. Ribosomes in the cytoplasm are of the 80S type whereas the ones in the mitochondria and chloroplasts are 70S type. Centrosome occurs in the protists which possess cilia or flagella at some phase of their life. 

Habitat

Members of Protista are primarily aquatic (both marine and freshwater) while some grow in moist and humid places like moist soil. In aquatic habitats, around 95% of the planktons is constituted of protists. Slime moulds are present in dead and decaying plant matter and parasitic protists are found within living hosts which can be humans, other animals and plants.

Fig: Common habitat of protists

Mode of Nutrition

Protists show different types of nutrition as follows:

Fig: Modes of nutrition in protists

• Photosynthetic protists can prepare their own food, e.g, diatoms.
• Saprophytic protists feed on dead decaying organic matter, e.g., slime moulds.
• Holozoic protists engulf their food, e.g., Paramoecium.
• Parasitic protists depend on a host for nutrition, e.g., Plasmodium.
• Some organisms like Euglena can be mixotrophic and show both autotrophic (photosynthetic) and heterotrophic modes of nutrition.

Locomotion

Most protists have different locomotory organs such as cilia, flagella, and pseudopodia. Very few protists like Vorticella are sedentary while some like the diatoms remain afloat in water and do not show active locomotion.

Fig: Types of locomotion in protists

• Flagellar locomotion is seen in euglenoids, e.g., Euglena, dinoflagellates, e.g., Gonyaulax, zooflagellates, e.g., Giardia, Leishmania, etc.
• Ciliary locomotion occurs in protozoan protists belonging to group Ciliata, e.g, Paramoecium.
• Movement with the help of temporary protoplasmic extensions called pseudopodia is seen in slime moulds, sarcodines and amoeboid protists, e.g, Amoeba.
• Diatoms exhibit mucilage propulsion by secreting mucilage in one direction while the cells are propelled in the other direction.
• In Monocystis, a wriggling locomotion is exhibited with the help of a wave of contraction and expansion through the body.

Respiration

Most free-living protists and the parasitic protozoan protists, which live in the body fluids, respire aerobically. Free-living protists living at the bottom of water bodies or parasitic protists residing in the alimentary canal are obligate anaerobes.

Reproduction

There are two types of reproduction observed in protists as follows:

• Asexual reproduction
• Sexual reproduction

Asexual reproduction

This is the most common method of reproduction in protists. Asexual reprodiction takes place in favurable conditions. Asexual reproduction can occur by:

• Binary fission
• Multiple fission
• Cyst formation
• Spore formation

Fig: Asexual reproduction in protists

Sexual reproduction

In sexual reproduction, two haploid gametes fuse to form a diploid zygote. This process is called syngamy. In protists, cells that produce gametes are called gamonts.

Fig: Sexual reproduction in Protists

Syngamy among protists can be of different types:

1. Isogamy: Fusion between two morphologically similar gametes

Fig: Isogamy

2. Anisogamy: Fusion between two morphologically different gametes

Fig: Anisogamy

3. Oogamy: Fusion of a smaller motile male gamete and a larger non-motile female gamete.

Fig: Oogamy

Life cycle of protists

Haplontic life cycle: In cellular slime moulds and some dinoflagellates such as Ceratium, Gymnodinium, the zygote is the only diploid phase and undergoes meiosis just after fertilisation to form four haploid offspring. These haploid offspring develop into haploid adults or gametophytes which produce haploid gametes by mitosis. The gametes fuse to form a diploid zygote which again undergoes zygotic meiosis, that is, meiosis just after zygote formation. As the dominant phase of the life cycle is the haploid phase, it is known as the haplontic life cycle.

Fig: Haplontic life cycle

Diplontic life cycle

The adult organism is a diploid which is the dominant phase of life. It undergoes gametic meiosis to form haploid gametes which again fuse to form a diploid zygote that eventually develops into a diploid adult. Thus, this life cycle is called the diplontic life cycle.

Fig: Diplontic life cycle

Classification of kingdom Protista

Kingdom Protista is mainly divided into three groups - Plant-like protists (photosynthetic protists), Animal-like protists and Fungi-like protists.

Fig: Classification of kingdom Protista

• Photosynthetic protists include - Dinoflagellates (Pyrrophyta), Chrysophyta and Euglenophyta.

Fig: Types of photosynthetic protists

• Fungi-like protists include cellular and acellular slime moulds.
• Animal like protists include the protozoans which can be zooflagellates, rhizopods (amoeboid), sporozoans or ciliates.

Fig: Types of photosynthetic protozoans

Practice Problems 

Q1. Which one of the following is not a locomotory organ used by protists?

A. Pseudopodia
B. Cilia
C. Parapodia
D. Flagella

Solution: Parapodia is absent in protists. It is the locomotory organ found in annelids. Protists have flagella, cilia or pseudopodia for locomotion. Some can also show locomotion with the help of mucilage propulsion or wriggling movement of the body. Thus, the correct option is c.

Q2. You found a unicellular eukaryotic organism in the blood sample of a patient. Can you guess the mode of nutrition of this protist?

A. Saprotrophic
B. Photosynthetic
C. Chemosynthetic
D. Parasitic

Solution: Unicellular eukaryotic organisms belong to kingdom Portista. The protists which obtain nutrition from living hosts such as human blood cells, as stated here, show a parasitic mode of nutrition. Thus, the correct option is d.

Q3. Read the following statements and identify the incorrect statement.

A. Protists are found mainly in aquatic environments.
B. Protists reproduce only through asexual mode of reproduction.
C. They show both autotrophic and heterotrophic modes of nutrition.
D. Protists show locomotion due to the presence of locomotory structures.

Solution: Protists can reproduce by both asexual and sexual mode of reproduction. They are mostly found in freshwater or marine, that is, aquatic environments. Some protists can be autotrophic and produce food by photosynthesis while others can be heterotrophic and can show holozoic, parasitic or saprotrophic nutrition. Most Protists show locomotion due to the presence of locomotory structures such as cilia, flagella or pseudopodia.

Hence, option b is correct.

Q4. FInd the incorrect match.

Solution: Diatoms are capable of preparing food by photosynthesis. Amoeba engulfs its food by phagocytosis with the help of extension of the pseudopodia. Thus, it shows holozoic nutrition. Euglena shows mixotrophic nutrition while Plasmodium shows a parasitic mode of nutrition. 

Slime moulds feed on dead and decaying organic matter and are hence saprophytic in nature.

Hence, option c is correct. 

Frequently Asked Questions 

Q1. How do protists harm the body?
Answer: Parasitic protists belonging to group Protozoa can harm the body as they target liver cells, blood cells or cells of the small intestine in the body. These parasites mostly multiply asexually within the body cells and are released by the rupture of the host cells. Thus, they harm the body. The blood parasites are particularly dangerous pathogens.

Q2. Name five diseases caused by parasitic protists.
Answer: Five diseases caused by parasitic protists are - malaria (caused by Plasmodium), amoebic dysentery (caused by Entamoeba histolytica), giardiasis (cause by Giardia), leishmaniasis (caused by Leishmania), trypanosomiasis (caused by Trypanosoma).

Q3. Can protists be infected by viruses?
Answer: Many small RNA viruses and large DNA viruses are capable of infecting protists. For example, Cafeteria roenbergensis is infected by the Cafeteria roenbergensis virus (CroV).

Q4. What is the similarity between cellular slime moulds and Chlamydomonas?
Answer: Both cellular slime moulds and Chlamydomonas undergo zygotic meiosis and have a haplontic life cycle. The dominant phase of their life cycle is the haploid adult which produces gametes by mitosis. The haploid gametes fuse to form a diploid zygotes which immediately undergoes meiosis to form haploid cells that grow into adults.

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