Ribosomes: Characteristics, Types, Structure and Functions, Practice Problems and FAQs

How often have you been told by your mother to have fish and eggs and chicken because they are ‘rich in proteins’? In fact you must have seen that people who are very much into fitness and building up their bodies tend to include a high amount of protein in their diet. Do you know why? This is because proteins are extremely essential building blocks of our body which help in building our bones and muscles and are also crucial for all metabolic actions in the body. Did you know that our hair and nails are also primarily composed of proteins?

But do you think the proteins that we eat are directly used by the cells? Well, no they are not. Proteins are first digested and broken into smaller amino acids which can be absorbed by the cells. The cells use these amino acids to synthesise proteins. But how does the cell synthesise proteins? Which part of the cell is directly involved in protein synthesis?

The cells have some non-membrane bound organelles known as ribosomes which act as ‘protein factories’ for the cell. Do all cells have ribosomes? Are the ribosomes in prokaryotes and eukaryotes the same? What are ribosomes made up of? To know the answers to all these questions, continue reading this article.

Table of contents

• Ribosomes
• Occurrence
• Shape, size and number of ribosomes
• Types of ribosomes
• Structure of ribosomes
• Functions of ribosomes
• Practice Problems
• FAQs

Ribosomes

Ribosomes are defined as one of the most important cell organelles that are composed of RNA and protein. These are non membrane bound sub spherical granular organelles present inside living cells which help in the translation of the genetic code transcripted onto a messenger RNA (mRNA) into chains of amino acids. This process is known as protein synthesis or translation.

The important characteristic features of ribosomes are listed below:

• These are the smallest, submicroscopic ribonucleoprotein particles which are also known as palade particles or cell engines. The ribosomal RNA or rRNA is a highly folded RNA filament which can extend to a length of around 7000 Å. The protein molecules in the ribosomes remain attached to the rRNA filament.
• These are membrane-less structures.
• Due to their primary involvement in the synthesis of proteins or polypeptides, they are often referred to as protein factories.
• In comparison to prokaryotic cells, eukaryotic cells have a higher number of ribosomes.
• Ribosomes can exist alone or in groups known as polyribosomes or polysomes.
• In a polyribosome, the many ribosomes are linked by a long string of mRNA.

History

The ribosomes were first observed by Robinson and Brown in the plant cells in the year 1953. In 1955, Palade first observed ribosomes in animal cells. Initially they were named as microsomes by Claude but Palade named them as ribosomes in 1955.

Occurrence

This specialised organelle is found in prokaryotic as well as eukaryotic cells except in mammalian RBCs and mature sperms.

Fig: Ribosome

In prokaryotes, the ribosomes are present freely in the cytoplasm but in eukaryotes, the ribosomes are either freely present in the cytoplasm as cytoribosomes or are embedded on the outer surface of the rough endoplasmic reticulum (RER) or nuclear envelope. In eukaryotic cells, even the matrix of mitochondria and stroma of plastids contain ribosomes.

Fig: Ribosomes on the surface of RER

Shape, size and number of ribosomes

Ribosomes exhibit uniformity in shape irrespective of their type and are mostly oblate spheroid in shape. The diameter of prokaryotic ribosomes ranges between 17-21 nm and the length ranges between 20-29 nm. Eukaryotic ribosomes have a diameter ranging between 20-24 nm in and length ranging between 30-34 nm.

Ribosomes are found in large numbers in cells which are actively synthesising proteins such as liver cells, pancreatic cells, endocrine cells and meristematic cells. They are very few in number in starving or less active cells. The eukaryotic cells may contain up to half a million ribosomes.

Types of ribosomes

Ribosomes are largely classified into two types based on their size and sedimentation coefficient:

• 70S ribosomes
• 80S ribosomes

Significance of ‘S’ in ribosome

The ‘S’ in ribosomes refers to the Svedberg unit, which denotes the sedimentation coefficient. The sedimentation characteristics of a particle depend on various factors, such as molecular weight and geometry of the particle, and physical properties of the solution. It is a measure of how quickly a particle can sediment in an ultracentrifuge:

• Larger and denser the particle - greater its ‘S’ value
• Smaller and lighter the particle - lesser its ‘S’ value

GIF: Ultracentrifugation

70S ribosomes

70S type of ribosomes is found in prokaryotic cells and in the mitochondrial matrix and stroma of plastids of many eukaryotic cells. They are smaller in size as compared to 80S ribosomes. They have a molecular weight of 2.7✕10⁶ Daltons. The 70S ribosome has a larger subunit of 50S and a smaller subunit of 30S. These subunits are synthesised in the cytoplasm of a cell. They unite with the help of Mg²⁺ ions during protein synthesis.

This type of ribosome has three types of RNA molecules namely, 23S rRNA, 16S rRNA, and 5S rRNA. The 23S rRNA and 5S rRNA are found in the 50S subunit while the 16S rRNA is found in the 30S subunit.

The 70S ribosomes consist of around 50-55 proteins known as the split proteins of which around 21 proteins occur in the 30S subunit and around 34 proteins occur in the 50S subunit.

Fig: 70S ribosome

The organelles, such as mitochondria and chloroplasts, synthesise their own ribosomes.

Fig: 70S ribosome in mitochondria and chloroplast

80S ribosomes

80S type of ribosome is found in the cytoplasm or outer surface of the RER of eukaryotic cells. They have a molecular weight of 4.5✕10⁶ Daltons. They are larger ribosomes as compared to prokaryotic ribosomes. The 80S ribosome has a larger subunit of 60S and a smaller subunit of 40S. These subunits are synthesised in the nucleolus and then transported to the cytoplasm via the nuclear pore where they get assembled. Magnesium ions are required for the binding of ribosomal subunits.

This type of ribosome contains four different types of RNA namely; 28S rRNA, 18SrRNA, 5.8S rRNA, and 5S rRNA. The 28S, 5S and 5.8S rRNA molecules lie in the 60S subunit and the 18S rRNA is found in the 40S subunit. The 5S rRNA consists of about 120 nucleotides and usually resembles the Clover leaf structure of a tRNA. The 28S, 18S and 5.8S rRNA are formed within the nucleolus by cleavage of a precursor 45S rRNA strand whereas the 5S rRNA is synthesised outside the nucleolus.

The 80S ribosome consists of around 33 proteins in the 40S subunit and 47 proteins in the 60S subunit.

Fig: 80S ribosome

Structure of ribosomes

A ribosome is also referred to as a ribonucleoprotein since it is a complex of RNA and protein. They also contain metallic ions like Mg ²⁺ and some amount of lipids. Approximately 65% of the ribosomal composition in the 70S ribosome comprises RNA and 35% comprises proteins. In 80S ribosomes, approximately 45% is RNA and around 55% is composed of proteins.

It consists of a smaller and larger subunit. The bigger subunit is where the amino acids are added, and the smaller subunit is where the mRNA binds and is decoded. The larger subunit consists of -

• A Peptidyl site or P site meant for the attachment of the peptide chain carrying tRNA
• An amino acid site or A site meant for the attachment of the charged tRNA carrying the activated amino acid)
• An E site for the free tRNA after it has transferred the peptide chain to the P site.

These subunits exist separated from each other in the cytoplasm and combine only during protein synthesis. The mRNA lies on the cleft between the two subunits and polypeptide chains are synthesised in this cleft. In eukaryotic cells, the polypeptide chains are pushed into the lumen of ER after synthesis through a channel that extends from the cleft between the ribosomal subunits, through the larger subunit into the lumen of the ER.

Fig: The E, P and A sites of ribosome

Functions of ribosomes

The primary function of ribosomes is the formation of proteins from amino acids. This process is known as translation.

Fig: Protein synthesis

The ribosomal subunits assemble during protein synthesis and serve as the site for the attachment of the mRNA and also the attachment of the tRNAs charged with amino acids to the respective codons of the mRNA. The rRNA strand to which the mRNA binds during translation is present on the small ribosomal subunit. The enzyme peptidyl transferase which catalyses the formation of peptide bonds between amino acids to form the polypeptide chain is located in the large ribosomal subunit.

The tRNAs charged with the amino acids enter the ribosomes at the A site and bind to the respective codon on the mRNA. The tRNA carrying the polypeptide chain is present on the P site and as a new peptide bond is formed between the newly brought amino acid and the existing polypeptide chain, the polypeptide chain is transferred to the tRNA on the A site.

Now the ribosome shifts to the right by three base pairs, such that the tRNA carrying the polypeptide chain shifts from the A site to the P site, the A site becomes free to accept another charged tRNA based on the mRNA codon it lies on and the uncharged tRNA on the P site shifts to the E site to be released. This process continues until the entire mRNA is decoded into a polypeptide chain.

In contrast to ribosomes attached to the ER, which produce proteins to be used outside the cell, free ribosomes can produce structural and enzymatic proteins to be used inside the cell. The newly generated protein is shielded from cytoplasmic enzymes by being enclosed in the groove of the bigger subunit until it reaches the secondary structure.

Practice Problems

1. Which of the following rRNA molecules are found in the smaller subunit of an 80S ribosome?

1. 5S
2. 5.8S
3. 16S
4. 18S

Solution: The 80S ribosome contains four different types of RNA namely; 28S rRNA, 18SrRNA, 5.8S rRNA, and 5S rRNA. The 28S, 5S and 5.8S rRNA molecules lie in the larger 60S subunit and the 18S rRNA is found in the smaller 40S subunit. Hence, the correct option is d.

2. Which of the following options correctly represents the RNA and protein content of 70S ribosomes?

1. 35% RNA and 65% proteins
2. 45% RNA and 55% proteins
3. 65% RNA and 35% proteins
4. 55% RNA and 45% proteins

Solution: A ribosome is also referred to as a ribonucleoprotein since it is a complex of RNA and protein. They also contain metallic ions like Mg ²⁺ and some amount of lipids. 65% of the ribosomal composition in the 70S ribosome comprises RNA and 35% comprises proteins. In 80S ribosomes, 45% is RNA and around 55% is composed of proteins. Hence, the correct option is c.

3. Prokaryotic ribosomes are composed of

1. 60S and 40S units
2. 50S and 30S units
3. 50S and 20S units
4. 40S and 30S units

Solution: 70S type of ribosome is found in prokaryotic cells. The 70S ribosome has a larger subunit of 50S and a smaller subunit of 30S. On the other hand, the eukaryotic ribosome is of 80S type and is composed of the 60S and 40S subunits. Hence, the correct option is b.

4. What is the sedimentation unit of the ribosome?

1. μ (micron)
2. μm (millimicron)
3. A (angstrom)
4. S (Svedberg)

Solution: The ‘S’ in ribosomes refers to the Svedberg unit, which represents the sedimentation coefficient. It is a measure of how quickly a particle can sediment in an ultracentrifuge:

• Larger and denser the particle, the faster it sediments and greater is its ‘S’ value
• Smaller and lighter the particle, the more time it takes to sediment and lesser is its ‘S’ value

Hence, the correct option is d.

FAQs

1. Is survival possible without ribosomes?

Answer: Ribosomes are essential organelles for the survival of an organism because it is a protein factory. Protein synthesis takes place in the ribosomes which are an essential function for living organisms.

2. Do viruses contain ribosomes?

Answer: Since viruses lack ribosomes, mitochondria, and other organelles, they must rely entirely on their cellular hosts for the synthesis of proteins and the production of energy. They can only reproduce inside the cells of the infected host.

3. What is the charge of ribosomes?

Answer: Ribosomes are composed of both positively and negatively charged proteins and negatively charged RNA. But the overall negative charge on ribosomes is more than the positive charge, which makes them negatively charged particles.

4. Why are ribosomes absent in mammalian RBCs?

Answer: The mature mammalian RBCs are biconcave, circular, and non-nucleated cells. Upon maturation, they lose important organelles such as nucleus, ER, Golgi bodies and mitochondria so as to accomodate more oxygen and carry it efficiently without utilising it. In the absence of the nucleus, they are not involved in protein synthesis and hence they lack ribosomes.

YOUTUBE LINK: https://www.youtube.com/watch?v=yxU2SSEbnkM