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Differences between Unicellular and Multicellular Organisms, Practice Problems and FAQs

We know that the cells are the building blocks of life. Our body is made up of different types of cells. Have you ever observed the cells which fall off from our body? Yes, every day some cells in our body die and new cells are formed. Most of the cells have a short life span. Neurons are the type of cells which have the longest life span. Even Though dead cells leave our body, it does not impact our lives, right? This is due to the fact that we have multiple cells in our body and new cells replace the dead cells, that means we are multicellular organisms.

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Fig: Formation of dead cells

Now think about the organisms with only one cell in their body. If the cells have a short life span, then the total lifespan of a single celled organism will be very less right?. By the death of the cell, the organism also will die. We can call them unicellular organisms. Examples include yeast.

We know that when life originated, it started from a single cell and then later developed into multicellular organisms. So there will be a lot of differences between a multicellular and unicellular organism. We are going to discuss more about the differences between these two types of organisms in this article.

Table of contents

Unicellular organisms

Organisms with a body made up of a single cell are called unicellular organisms or single celled organisms. The first protocells were thought to have emerged 3.8 to 4.0 billion years ago and they are considered as the oldest form of life and the first unicellular organisms. Unicellular organisms are very hard to see through naked eyes. Since they can be only visible through a microscope, unicellular organisms are also called microscopic organisms or microbes. Examples include Amoeba.

Fig: Amoeba - Unicellular organism

Types of unicellular organisms

There are two types of cells on the basis of presence or absence of a membrane bound nucleus. If the cell has a membrane bound nucleus then it is called a eukaryotic cell and if the membrane bound nucleus is absent then it is called a prokaryotic cell. Those organisms which are made up of a eukaryotic cell are called eukaryotic organisms. Examples include protozoans. Those organisms with prokaryotic cells are called prokaryotic organisms. Examples include bacteria and cyanobacteria.

Fig: Structure of prokaryotic and eukaryotic cell

Classification of bacteria

All prokaryotic organisms are unicellular organisms and they are mostly bacteria. Bacteria can be classified into two as follows:

Eubacteria

They are found everywhere on the Earth. Examples include Salmonella typhi.

Fig: Salmonella typhi

Archaea

They are primitive bacteria and found in extreme environmental conditions. Examples include methanogens, halophiles and thermoacidophiles.

Fig: Archaebacteria

Unicellular eukaryotes

Most of the eukaryotes are multicellular, but some organisms are unicellular and they are as follows:

  • Kingdom Protista which includes unicellular eukaryotes. It includes Protozoa (Paramoecium), Chrysophytes (Diatoms and desmids), Dinoflagellates (Gonyaulax), Euglenoids (Euglena) and Slime moulds (Physarum).

Fig: Members of Kingdom Protista

  • Subdivision Algae include some unicellular eukaryotes like Chlamydomonas, Chlorella, Spirulina etc.

Fig: Unicellular algae

  • Kingdom Fungi or Mycota include some unicellular eukaryotes like yeasts.

Fig: Yeast

Some unicellular organisms are visible to naked eyes, because of the formation of colonies as in Nostoc. Bacteria which form large interlinked structures like colonies or biofilms can not be considered as a multicellular organism.

Fig: Nostoc

Multicellular organisms

Organisms with a body made up of more than one cell are called multicellular organisms. All the multicellular organisms are eukaryotes. They possess different levels of organisations. In most cases cells combine together to form tissues and tissues combine together to form organs. The combined functioning of organs form the organ systems. Levels of organisation in higher eukaryotes varies as follows:

Tissue level organisation

It is seen in the phylum Coelenterata or Cnidaria (Examples include jellyfishes) and Ctenophora (comb jellies).

Fig: Organisms with tissue level of organisation

Organ level of organisation

It is seen in phylum Platyhelminthes (flatworms). Examples include Planaria.

Fig: Planaria

Organ system level of organisation

Most of the higher multicellular eukaryotic organisms have an organ system level of organisation. It is applicable to phylum Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata.

Fig: Organ system level of organisation in human

Examples of multicellular organisms

Some of the examples of multicellular organisms are as follows:

All invertebrates

They lack a notochord and vertebral column in their life cycle which include phylum Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata and Hemichordata.

Fig: Invertebrates

Invertebrate chordates

They possess notochord and lack vertebral columns. It includes subphylums Urochordata (Ascidia) and Cephalochordata (Branchiostoma).

Fig: Invertebrate chordates

All vertebrates

They possess notochord which is replaced by the vertebral column in adults. Examples include superclass Pisces (Chondrichthyes and Osteichthyes) and Class Amphibia, Aves, Reptilia and Mammalia.

Fig: Vertebrates

Angiosperms

The subdivision Angiospermae includes the flowering plants. Examples include coconut trees, mango trees etc.

Fig: Angiosperms

Gymnosperms

The subdivision Gymnospermae includes plants with naked seeds. Examples include Cycas, Pinus etc.

Fig: Cycadophyta

Pteridophyta

The division Pteridophyta includes the ferns. Examples include Pteris, Adiantum etc.

Fig: Pteridophytes

Bryophyta

The division Bryophyta includes the moss plants. Examples include Riccia, Marchantia etc.

Fig: Bryophytes

Algae

It includes all multicellular algae except the unicellular algae like Chlamydomonas, Chlorella, and Spirulina. Examples include Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus etc.

Fig: Multicellular algae

Fungi

It includes all the multicellular fungi except yeast. Examples include mushrooms.

Fig: Fungi (Mushroom)

Differences between unicellular and multicellular organisms

Some of the major differences between the unicellular and multicellular organisms are as follows:

Unicellular organisms

Multicellular organisms

Body is made up of single cell

GIF: Unicellular

Body is made up of more than one cell

GIF: Multicellular

They shows simple body organisation

They shows complex body organisation

They are visible only under microscope (microscopic)

Most of them are visible to naked eyes (macroscopic)

They possess irregular structure

They possess a well defined structure

Both prokaryotes and eukaryotes are included

Only eukaryotes are included

Simple diffusion is the major mode of transportation in them

Major mode of transportation includes simple diffusion, active transport and passive transport

Mostly heterotrophs exception include unicellular algae, cyanobacteria etc

Both heterotrophic and autotrophic nutrition are shown

The single cell performs all metabolic activities

GIF: Single cell

Differentiation of cells into tissues, organs and organ system for specialised functions

Fig: Levels of organisation

Death of the organism happens with the death of the cell

Death of the cell will not cause the death of the organism

Major asexual reproductive method is binary fission

GIF: Binary fission

Asexual reproduction occurs through mitosis

Parasexual reproduction happens through conjugation, transformation and transduction

GIF: Conjugation

Sexual reproduction happenes through fusion of gametes

Fig: Fertilisation

Oldest form of organisms

Comparatively newly formed organisms derived from prokaryotes

True regenerative capacity is more

GIF: True regeneration

True regenerative capacity is comparatively less. Exceptions include earthworms and Planaria

Fig: True regeneration in Planaria

Short life span

Comparatively long life span

Examples include Amoeba, Paramecium, Plasmodium, Yeast, Algae, Euglena Fungi, Protozoans etc.

Examples include vertebrates, invertebrates, invertebrate chordates, gymnosperms, angiosperms, pteridophytes, bryophytes etc.

GIF: Euglena

GIF: Chelone

Practice Problems

1. Which of the following is not correct about a unicellular organism?

  1. Body is made up of a single cell
  2. They are microscopic organisms
  3. All prokaryotic organisms are unicellular organisms
  4. Eukaryotes does not include unicellular organisms

Solution: Organisms with a body made up of single cells are called unicellular organisms or single celled organisms. The first protocells were thought to have emerged 3.8 to 4.0 billion years ago and they are considered as the oldest form of life and the first unicellular organisms. Unicellular organisms are very hard to see with naked eye. Since they can be only visible through a microscope, unicellular organisms are also called microscopic organisms or microbes. All prokaryotic organisms are unicellular organisms and they can be classified into two as Eubacteria and Archaea. Most of the eukaryotes are multicellular, but some organisms are unicellular and they include the members of the Kingdom Protista like Protozoa (Paramoecium), Chrysophytes (Diatoms and desmids), Dinoflagellates (Gonyaulax), Euglenoids (Euglena) and Slime moulds (Physarum). It also includes unicellular algae like Chlamydomonas, Chlorella, and Spirulina and unicellular Fungi like yeast. Hence the correct option is d.

2. Assertion: All the multicellular organisms are eukaryotes.

Reason: Bacteria forming colonies or biofilms can be considered as a multicellular organism.

  1. Both the assertion and the reason are true and the reason is the correct explanation of the assertion
  2. Both the assertion and the reason are true, but the reason is not the correct explanation of the assertion
  3. Assertion is true but the reason is false
  4. Both the assertion and the reason are false

Solution: Organisms with a body made up of more than one cell are called multicellular organisms. All the multicellular organisms are eukaryotes. Bacteria which form large interlinked structures like colonies or biofilms can not be considered as a multicellular organism. Bacteria produce biofilms in response to environmental stresses. For example, Spirulina. Hence the correct option is c.

Fig: Filaments of Spirulina

3. Which of the following is a eukaryotic multicellular organism?

  1. Selaginella
  2. Chlamydomonas
  3. Chlorella
  4. Yeast

Solution: Most of the eukaryotic organisms are multicellular, but some organisms are unicellular and they include the members of the Kingdom Protista like Protozoa (Paramoecium), Chrysophytes (Diatoms and desmids), Dinoflagellates (Gonyaulax), Euglenoids (Euglena) and Slime moulds (Physarum). It also includes unicellular algae like Chlamydomonas, Chlorella, and Spirulina and unicellular Fungi like yeast. All pteridophytes (ferns) are multicellular organisms. Selaginella is a pteridophyte and it is also known as spike moss or club moss. It is the only living genus of family Selaginellaceae and it is also the largest genus. Hence the correct option is a.

Fig: Selaginella

4. Which of the following characters differentiates the multicellular organism from a unicellular organism?

A) Complex body organisation

B) Heterotrophic mode of nutrition

C) Eukaryotic organisms

D) High true regenerative capacity

  1. A and C
  2. A and D
  3. B and D
  4. All the above

Solution: The body of a unicellular organism is made up of a single cell and they have a simple body organisation. The body of a multicellular organism is made up of more than one cell and it has a complex body organisation. Both prokaryotes and eukaryotes are included in the unicellular organisms, but only eukaryotes are included in the multicellular organism. Since eukaryotes can be seen in both unicellular and multicellular organisms, it is not a unique characteristic of a multicellular organism. Heterotrophic and autotrophic nutritions are seen in both unicellular and multicellular organisms. True regenerative capacity is more in unicellular organisms and it is comparatively less in multicellular organisms. Exceptions include Planaria, earthworm etc. So this character is also a differentiating character. Hence the correct option is b.

FAQs

1. Why are viruses not included in unicellular or multicellular organisms?

Answer: Viruses are not considered as a cellular organism. They are the packets of genetic material and proteins which get a life only inside other living organisms. Hence they are called obligate intracellular parasites. They do not have any characteristic features that distinguish them into a prokaryote or eukaryote. Hence viruses are not considered as a unicellular or multicellular organism. Examples include Human Immunodeficiency Virus (HIV).

Fig: Virus

2. What are Xenophyophores?

Answer: Xenophyophores are macroscopic unicellular organisms, which have more than one nucleus in their cell. Xenophyophores can reach upto 20 cm in diameter. Henry Bowman described Xenophyophores for the first time in 1883.

3. What is the predation hypothesis?

Answer: According to the predation hypothesis, the evolution of multicellular organisms occurred from unicellular organisms in order to avoid the predation by their predators. In an experiment done by Herron et al., the unicellular alga Chlamydomonas showed some multicellular features when they had grown with its predator Paramecium, a unicellular ciliate protozoan.

4. How do unicellular organisms move?

Answer: The major locomotor structures present in unicellular organisms are cilia, flagella and pseudopodia. The cilia and flagella move by creating currents in the surrounding environment. Cilia are absent in bacteria, but present in Paramecium. Pseudopodia is also called as cytoplasmic projections and it forms on the forward edge of the cell. Pseudopodia is a characteristic feature of microscopic unicellular Protozoa like Amoeba.

Fig: Locomotory structures in unicellular organisms

 

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