Ribosomes – Definition, Structure, Size, Location And Function

When examining cells through a microscope, it can be easy to spot numerous organelles coming together to complete the numerous cell activities taking place. 

These help to carry out many of the cellular processes that are necessary for life, with some of the main organelles include:

Endoplasmic reticulum (ER) – Where proteins are made in the cell. It also carries out other functions such as protein folding, lipid synthesis and calcium storage.

Mitochondria – These are the powerhouse of the cell. They produce energy by breaking down nutrients into usable forms.

Ribosomes - Definition, Structure, Size, Location and Function

The mitochondria have their own DNA which helps them replicate themselves. This means they don’t need to rely on the DNA in the nucleus to replicate.

Golgi apparatus – Where proteins produced in the ER are transported to different parts of the cell.

Nucleus – This is where DNA is stored and genes are replicated.

Ribosome is also one of these essential and important cell organelles serving a crucial purpose in the body and being made up of multiple components that allow it to serve its purpose with full efficiency. 


The ribosome is a cell organelle made of protein and RNA (Ribonucleic acid) and is in charge of protein synthesis.

It can essentially be seen as a very complicated but elegant micromachine with the sole purpose of producing proteins. 

Ribosomes are found within all eukaryotic cells from yeast to humans, with a single human cell being able to contain up to 100,000 ribosomes.

They are complex modular machines found inside all living cells and produce the proteins from amino acids. 

The way the ribosome achieves protein synthesis is by reading and decoding the message of the RNA (mRNA) sequence and translating the genetic code into a long specified string of amino acids which then go on to fold and form proteins. 

When observing animal or plant cells through a microscope, you will most probably spot the numerous organelles working together to serve their own primary function.

With the ribosome being one of the most important and involved out of all of these, being a crucial component to keeping us healthy and strong.

These small particles that came to be known as ribosomes were first described by Romanian born American cell biologist George E. Palade and were a huge revelation in terms of biological and cellular science.

The wonder and thrill of finally, after decades of intensive research, understanding how a mass amount of proteins and large and small RNAs fit together into the elegant machines.

Functioning as protein factories for every cell and living organism on the planet makes it quite a revelation.

Because of their essential presence in all living cells, ribosome profiling has even emerged as a powerful and effective method of assessing global gene translation levels across a wide range of species. 

With the technology readily available today, a ribosome profiling protocol can be completed in as soon as 7 days to create a completed entire profiling sequence library, demonstrating how integrated they are into all of us. 

This has made ribosome’s an incredibly vital component that can be used to gain a greater insight into gene regulation.

It can also help give us a better understanding of the rate of protein synthesis and protein abundance in humans and animals and how to counter it.


The basic structure of a ribosome consists of three parts: the small subunit (SSU), the large subunit (LSU) and the peptidyl transferase complex (PTC).

Each of these components work together to create a functional ribosome.

The SSU contains the 23S rRNA and the 5S rRNA. Together they make up the 30S subunit which is responsible for binding tRNAs and translating mRNA into protein.

The LSU contains the 50S subunit which binds aminoacylated tRNAs and mRNA. It also holds the L1 and L12 ribosomal proteins.

The PTC consists of two parts: the A-site and the E-site. They are made up of both the 18S rRNA and the 28S rRNA. The A-site is responsible for holding the amino acid attached to the tRNA while the E-site holds the growing polypeptide chain.

The commonly used names for the smaller subunits are prokaryotic and larger subunits are called eukaryotic.

Prokaryotic ribosomes are much simpler than eukaryotic ribosomes because they only require a single rRNA molecule instead of two. 

However, they are still capable of producing proteins. Prokaryotic subunits are often located at the center of the cell and work to bind to the mRNA and decode the necessary information for making proteins.

Eukaryotic on the other hand have evolved more complicated structures to perform a variety of functions. These include the ability to regulate protein production, repair damaged ribosomes and even remove faulty proteins.

They are much bigger and are usually found spread throughout the cytoplasm and have the job of producing the proteins by adding the amino acids necessary.

Both of the larger and smaller subunits are made up of RNA and proteins, and work together to ensure the protein synthesis process runs smoothly.



In terms of size, ribosomes can vary greatly depending on what type of organism it comes from. For example, bacteria have much smaller ribosomes than those of animals and plants. 

Bacteria’s ribosomes are less than 10nm wide while eukaryotes’ are around 40nm.

The reason behind this is because bacteria don’t need as many proteins to survive as we do, so they use fewer ribosomes to help them carry out the same task.

Because of the uneven pairing between prokaryotic and eukaryotic, ribosomes can take on various different sizes though the average size of a eukaryotic ribosome is around 2.5 micrometers in length and 0.8 micrometer in width.

The size of a prokaryotic ribosome can vary but is usually around 3 micrometers in length.

This means that there are many more eukaryotic ribosomes per unit volume than prokaryotic ribosomes.

Additionally, in terms of weight, for the smaller prokaryotic with its 70S overall subunit it has a molecular weight of around 2.7 X 106 Daltons.

In contrast, the eukaryotic with its subunits is about equal to the molecular weight of 4 X 106 Daltons. 

Ribosomes themselves are not too hard to spot when looking under a microscope, but if you want to see them in action, you’ll need to look at an electron micrograph.

This is done by using a sample that has been fixed and stained with heavy metals such as uranium or lead.

These heavy metals cause electrons to be attracted towards the nucleus of the cell, making the ribosomes visible.


Ribosomes will be located inside all animal, human and plant cells and are often situated in the cytosol, with others being free to roam to the membrane and endoplasmic reticulum.

This is where yet another difference occurs between prokaryotic ribosome and eukaryotic ribosome.

While the prokaryotic ribosomes are mostly found in the center of the cell, the eukaryotic ribosomes tend to be distributed throughout the cytoplasm.

This is due to the fact that the eukaryotic needs to produce more proteins than the prokaryotic does and therefore requires more space.

They can also be located outside of the cell as well, especially in cases where the cell is dividing.

When this happens, one half of the cell will contain ribosomes while the other half will not. This allows for the production of new ribosomes without having to start over again.


There are two main functions of ribosomes: translation and degradation.

Translation is the process of turning mRNA into protein. It involves taking the information stored in the mRNA and translating it into a sequence of amino acids. 

Each amino acid consists of a carbon backbone (amino group) attached to a carboxylic acid (carboxylate).

There are 20 types of amino acids used in protein synthesis, which are suitably named the twenty canonical amino acids. 

Proteins are composed of chains of amino acids linked together by peptide bonds. Once these amino acids are made, they are then folded together to make the final product, which is a polypeptide chain.

Degradation is the process of breaking down proteins once they have served their purpose. This is done through either autophagy or proteasomal digestion.

Autophagy is the breakdown of cellular components within the cell, while proteasome digestion is the breakdown of proteins via the ubiquitin-proteasome system.

The proteins themselves which ribosomes are responsible for creating are large molecules that are responsible for carrying out most of our bodily functions including digestion, respiration, muscle movement, etc. 

They are produced through the process of transcription and translation. Transcription is the process of copying DNA into mRNA, which is then used to create proteins.

Protein is incredibly important for cell functions, being vital in directing chemical processes or fixing cell damage.

To create the proteins, ribosomes require three components:

  1. Amino Acids (AAs)
  2. Nucleotides (NTPs)
  3. Ribonucleic acid (RNA).

It is through the translation machinery, which is made up of tRNAs and elongation factors, that the process can work.

tRNAs are responsible for bringing the correct amino acids to the ribosome. They bind to the mRNA strand and then attach to the corresponding AA.

Elongation Factors are then used to add onto a peptide chain that is ever-growing. Once the peptide chain reaches its full length, the ribosome releases the completed protein.

A cell cannot function in the absence of ribosomes, so this process is necessary for a cell to function, with proteins needing to constantly be produced. 


Ribosomes are essential for life on Earth, ever since their discovery they have been emphasized to be one of the most crucially important and vital parts of our biological makeup. 

Research has not stopped however, over time biologists and researchers are finding new ways of utilizing ribosomes to assist living organisms with their protein levels, as well as other interesting ventures such as gene expression. 

When using a microscope next to analyze the cells of a living organism, try and look out for the ribosomes.

They can be quite easy to spot when looking in the right areas and will always be present serving their purpose of creating protein and keeping us strong and healthy.

Jennifer Dawkins