Sizes of Organisms

In terms of biology, life can be defined as a process of growth that allows all living organisms to reproduce and give them the energy to respond to any metabolic changes. Although there is no specific definition of life, most scientists define it as a biological process that is necessary for every living organism for its survival. All these organisms have to undergo this process of growth and reproduction. The process varies depending on the sizes of organisms.  

In this article, we will talk about the variation in the size of microorganisms. We will also discuss the prokaryote size and its differences with the eukaryotic cell size. Students can refer to this article for any kind of exam preparation.

Variation in the Size of Microorganisms

Every living organism in this world consists of a structural and a functional unit of life, i.e., the cell. It plays a major role in performing all the metabolic activities required for the growth of the organisms. But these cells are different in sizes and shapes and vary from organisms to organisms. This difference creates a variation between all the organisms. A cell can be prokaryotic and eukaryotic. The single-celled organisms are called unicellular while those having multiple cells are multicellular. 

Most of the single-celled organisms come under prokaryotes and do not consist of a nuclear membrane and mitochondria. While, on the other hand, eukaryotic cells can be single-celled or multicellular. They are membrane-bound cells possessing mitochondria.

Do you know the size of the smallest microorganism and the smallest living organism in the world? 

The size of a cell may range from 0.0001 micrometres to twelve inches. To date, the smallest microorganism in the world is reported to be Pelagibacter ubique, a type of ultramicrobacteria measuring 370 nanometres to 890 nanometres in length. It is considered to be the smallest bacteria in the world.

One example of a single-celled eukaryotic microorganism is Amoeba. It belongs to the kingdom of protozoa and multiplies by the process of budding. The amoeba size can range from 400 to 600 microns whereas, there have been reports of some amoeba having a size of 20 cm which are visible to the naked eye. 

Prokaryotic Cell Definition

A prokaryotic cell can be defined as unicellular or single-celled microorganisms which lack a nuclear membrane. They mostly consist of bacteria and archaea. The bacteria is considered to be the smallest living organism and cannot be seen by the naked eye. They usually reproduce asexually by binary fission and sometimes sexually through conjugation.

Along with the nuclear membrane, a prokaryotic cell also lacks mitochondria, chloroplast, lysosomes and Golgi bodies. Here, the plasma membrane plays the role of mitochondria and all the genetic information is stored in a single chromosome.

The size of normal single-celled prokaryotes can range from 0.2 microns to 10 microns. 

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Eukaryotic Cell

The cells of a eukaryotic consist of a nuclear membrane and are found mostly in large and complex organisms. For example, plant cells, animal cells, protozoa and fungi. They fall under the Kingdom Eukaryota and are larger than the prokaryotic cell. All the genetic information in eukaryotes are stored in the DNA of the nucleus which is enclosed within the nuclear membrane. 

Some of the characteristics of a eukaryotic cell are as follows:

• A eukaryotic cell multiplies and reproduces by the process of cell division.

• They all consist of an outer membrane i.e., the cell wall.

• They consist of two locomotory organs i.e., the flagella and the cilia.

• Every eukaryotic cell contains mitochondria and a cytoskeletal structure.

• Since the eukaryotic cells are properly compartmentalized, they have various shapes such as spherical, elongated, oval. discoidal and so on. 

• Mostly, eukaryotic cell size can range from 10 micrometres to 20 micrometres depending on the size of microorganisms.

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Eukaryotic Cell Structure

A typical eukaryotic cell consists of the following components:

1. Plasma Membrane: They play a role in protecting the cell from any outside injury by separating the cell from the outside environment. They also help in the exchange of substances from in and out of the cell.

2. Cell Wall: This protective layer helps in cellular interaction and gives a proper shape to it. The cell wall mainly contains cellulose, proteins, pectins, etc. and are absent in animal cells.

3. Cytoskeleton: It consists of microfilaments, microtubules, and fibres which helps in cell anchoring and cell stimulation. It is found in the cytoplasm.

4. Endoplasmic Reticulum: They are the small and tubular structures present in the eukaryotic cells. They are mainly of two types- Rough endoplasmic reticulum and Smooth endoplasmic reticulum.

5. Nucleus: It consists of DNA and proteins and is a hub for ribosome production.

6. Golgi Apparatus: The formation of glycoproteins and glycolipids takes place in the Golgi apparatus.

7. Ribosomes: Consisting of proteins and ribonucleic acids, these ribosomes are the main hub for protein synthesis.

8. Mitochondria: Commonly known as the powerhouse of cells, they help in cell regulation and cell metabolism.

9. Lysosomes: They help in the digestion of proteins, carbohydrates, nucleic acids and lipids with the help of hydrolytic enzymes and are also commonly known as suicidal bags.

10.  Plastids: They are only found in plant cells and are of three types- Chloroplast, Chromoplast and Leucoplasts. These plastids are double-membraned.