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Unicellular and Multicellular Organisms

The structural and functional units of all living organisms are cells. Cells are the building blocks of life. All living organisms are made up of cells. The cells were discovered by Robert Hooke in 1665. In the human body there are about 100 trillion to 1014 cells. The size of the cells is about 10 micrometer. The cell contains cellular organelles that control the activity of the cell. Organisms can be classified as unicellular and mutlicellular organisms. Unicellular organisms are made up of a single cell like most bacteria, while the multicellular organisms include plants and animals. The number of cells varies between species. The size of most plant cells and animal cells is between 1 to 100 micrometers; hence they are visible under microscope. The cell theory developed by Schwann and Schleiden states that all organisms are made up of one or more cells. Cells emerged on Earth about 3.5 billion years ago. 


Unicellular Organisms

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Unicellular organisms are known as single-celled organisms. They are made up of a single cell. Organisms like the amoeba, Paramecium are single-celled organisms, they are  the oldest forms of life, they existed about 3.8 million years ago. Bacteria, archaea, protozoa, unicellular algae and unicellular fungi are the main groups of unicellular organisms. The single-cell regulates all the activity of the organism. Unicellular organisms are small are mostly invisible to the naked eye.

There are two general categories of unicellular organisms: prokaryotic and eukaryotic organisms. Prokaryotic unicellular organisms are protists and some fungi. Some of these unicellular prokaryotes live in colonies. They live together and all the cells of the colony is the same. All the process of life is
 carried out in each cell in order for the cell to survive. Simplest multicellular organisms are made of cells that dependent on each other for their survival. Most of the multicellular organism are microscopic and are known as microscopic organisms. 

Unicellular organisms vary in size. The smallest organism a bacteria is only 300 nanometers and range upto 20cm. These organisms usually posses cilia, flagella or pseudopidia that help them in locomotion. They have simple body with basic features. Reproduction is both by asexual and sexual means. Nutrition is usually by the process of phagocytosis, where the food particle is engulfed and stored in vacuoles present in the organism.

Unicellular Organisms

Multicellular Organisms

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Organisms that consist of more than one cell are known as multicellular organisms. Multicellular organisms are made up of more than one cell. These cells identify and attach to each other to form a multicellular organism. Most of the multicellular organisms are visible to the naked eye. Organisms like plants, animals and some algae arise from a single cell and they grow up into a multi-celled orgainsm. Both prokaryotes and eukaryotes show multicellularity. True multicellular organisms regenerate a whole organism from germ cells. 

Depiction of Multicellularity in Multicelluar Organisms

There are three theories to discuss the mechanisms by which multicellularity could have evolved.

Symbiotic Theory - Symbitoic theory states that the first multicellular organism arose from symbiosis behaviour of different species of a single-celled organism, each performing different functions. Such symbitoic relation ship is seen between clown fish and Riterri sea anemones. 

The Cellularization Theory or The Syncytial Theory - The cellularization theory states that a unicellular organism would have developed from membrane boundaries/partitions around each nuclei from a single celled organism with multiple nuclei. Protists like ciliates and slime molds have multiple nuclei supporting this theory. 

The Colonial Theory - The colonial theory sttes that he symbiosis of many organisms of the same species led to the evolution of mutlicellular organism. This theory was proposed by Haeckel in 1874. Majority of multicellularity occurs as a consequence of the cells failing to separate following the process of division. Examples of this theory can be seen in multicellular protists like Volvox, Eudorina. 

Advantages of Multicellularity in organism are that multicellularity allows the organism to exceed the size limits. Multicelluarity also permits in increasing the complexity of the organism by allowing differentiation of numerous cellular lineages in an organism.   

Reproduction in multicelluar organism is by sexual means. Anatomy of multicellular organisms is complex due the various types of cells control the functioning of the organism. The process of cell division is well coordinated in mutlicellular organisms to prevent abnormal cell division and cell growth. 

Examples of Unicellular Organisms

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Unicellular organisms are of two types Unicellular prokaryotic organisms and unicellular eukaryotic organisms 

Unicellular prokaryotic organisms - they are unicellular in nature and they do not have membrane bound nucleus and membrane bound cellular organelles. These organisms are usually bacteria and cyanobacteria. Example: E.coli, Salmonella, Nostoc, etc.

Unicellular eukaryotic organisms - these organisms are unicellular and are eukaryotes. They have membrane bound true nucleus and other membrane bound organelles. These are mainly free living or aquatic parasites like the protozoans, some fungi and algae or some protists. 

Examples of Multicellular Organisms

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Multicellular organism are of two types Multicellular prokaryote and multicellular eukaryote organisms. 

Multicellular prokaryotes are mostly multicellular bacterial species like myxobacteria. Some cyanotbacteria like Chara, Spirogyra, etc are also multicellular prokaryotes. Sometimes these bacteria are considered as colonial instead of multicellular. 

Multicellular eukaryotes - Most of the eukaryotic organisms are multicellular. These organisms have a well-developed body structure and they have specific organ to perform specific function. Most of the well developed plants and animals are multicellular. Examples are almost all species of gymnosperm and angiosperm plants and almost all animals are eukaryotic and mutlicellular. 

Differences Between Unicellular and Multicellular Organisms

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The main differences between unicellular and multicellular organisms are : 

                       Unicellular Organisms                                Multicellular Organisms
 Body of the organism is made up of a single cell.   The body of mutlicellular organism is made up of  numerous cells.
 Body organization is simple. Organization is complex.
 The function of the whole organism is carried out by a single cell.   Specialized functions are performed by different cells, tissues, organs or organ systems. 
 Division of labor in the organism is at organelle level.   Division of labor in the organism may be at cellular level, tissue level, organs and organ system level. 
 Usually prokaryotic in nature.  They are mostly eukaryotic in nature.
 The body of the cell is exposed to the environment on all sides.   Outer cells face the environment. 
 Any injury to cell can cause death of the organism.   Injury or death of some cells does not affect the organisms, the affected cells are replaced.
 A limit is imposed to the size of the cell by the surface area to volume ratio and hence it can attain large size.   Due to multicellularity the organism can attain large size.
 Lifespan of the organism is usually short.   Organisms have a longer lifespan.
 Reproduction is by vegetative/asexual methods.
 Reproduction is sexual type.
 Genome has a few introns.   High introns are present in the genome. 
 Has good capacity of regeneration and power of division.   Capacity of regeneration decreases with increase in specialization and certain cells that are specialized loose the power of division. 
 There is no cell differentiation process.   Cell differentiation is evident.
 Nutrition is by engulfing food.   Nutrition is by specific organs or by food production. They can be autotrophs or heterotrophs.
 They are microscopic in nature.   They are macroscopic in nature.

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