Ribosomes: Definition, Structure, Composition and Types

Ribosomes are essential organelles in living cells and are often called the protein factories of the cell because they produce proteins needed for growth, repair, and metabolism. As a result, all living organisms, from bacteria to humans, depend on them for survival.

In most cases, ribosomes are non membrane bound and are found in both prokaryotic and eukaryotic cells. In addition, they translate genetic information from messenger RNA (mRNA) into proteins through protein synthesis.

In this article, you will explore their structure, composition, types, location, functions, importance, and the steps involved in protein synthesis.

Table of Contents

• What are Ribosomes
• Structure of Ribosomes
• Types of Ribosomes Based on Their Location
• Types of Ribosomes- n Cells
• Functions of Ribosomes
• Difference Between Types of Ribosomes
• Frequently Asked Questions on Ribosomes

What are Ribosomes

Every cell needs proteins to grow, repair itself, and carry out essential life processes. This is where ribosomes play a vital role. Ribosomes are small cellular structures made of ribosomal RNA (rRNA) and proteins. Their main function is to manufacture proteins by reading the genetic instructions carried by messenger RNA (mRNA).

The proteins they produce support almost every cellular activity. They act as enzymes, hormones, antibodies, transport molecules, and structural components, helping cells function properly.

Because proteins are essential for life, ribosomes are equally important. Without them, protein production would stop, preventing cells from growing, repairing themselves, and carrying out their normal functions.

To perform this role, ribosomes are made up of approximately:

• 60-65% ribosomal RNA (rRNA)
• 35-40% proteins

Here, rRNA forms the catalytic core, where peptide bonds are formed, while the proteins help maintain the ribosome's structure and support efficient protein synthesis.

Structure of Ribosomes

To understand how ribosomes make proteins, it is first important to look at their structure. Although they are microscopic, ribosomes are highly organised and designed to carry out protein synthesis with remarkable accuracy.

Each ribosome consists of two unequal subunits that remain separate when inactive. These subunits come together only when the ribosome begins translating genetic information into proteins.

The two subunits are:

• Large subunit
• Small subunit

Together, these subunits are made up of:

• Ribosomal RNA (rRNA)
• Ribosomal proteins

While rRNA forms the catalytic core where peptide bonds are formed during protein synthesis, the ribosomal proteins help maintain the ribosome's structure and support the translation process. 

Although all ribosomes perform the same basic function, their size differs between prokaryotic and eukaryotic cells. This difference helps distinguish the two types of cells.

Cell Type	Ribosome Size
Prokaryotic cells	70S (50S + 30S)
Eukaryotic cells	80S (60S + 40S)

Note: Here, the "S" stands for the Svedberg unit, which measures how quickly ribosomes settle during centrifugation. Rather than indicating their actual size, it reflects their sedimentation rate, which depends on factors such as shape, mass, and density.

Types of Ribosomes Based on Their Location 

Now that we've explored the structure of ribosomes, let's look at where they are found within the cell. Their location is closely linked to their function, as it determines the type of proteins they synthesise and where those proteins are ultimately used.

1. Free Ribosomes

Some ribosomes remain freely suspended in the cytoplasm. These are known as free ribosomes, and they produce proteins that are required for use inside the cell. Such proteins support essential cellular functions, including metabolism, growth, and other internal activities.

2. Bound Ribosomes

Not all ribosomes float freely. Some attach themselves to the surface of the rough endoplasmic reticulum (RER), where they are known as bound ribosomes. Their role is to synthesise proteins that are transported to other parts of the cell or outside it.

These proteins may be:

• Secreted outside the cell
• Incorporated into the cell membrane
• Transported to lysosomes and other membrane-bound organelles

3. Ribosomes in Organelles

Besides being present in the cytoplasm and on the rough endoplasmic reticulum, ribosomes are also found inside mitochondria and chloroplasts. These specialised ribosomes produce proteins required for the normal functioning of these organelles, helping mitochondria generate energy and chloroplasts carry out photosynthesis.

As you can see, whether ribosomes are free, bound, or located within organelles, each group contributes to protein production in a different way, ensuring the cell functions efficiently.

Types of Ribosomes in Cells

Although all ribosomes perform the same basic function of making proteins, they are classified into different types based on the cells in which they are found. 

The two main types are prokaryotic ribosomes and eukaryotic ribosomes, and they differ in both size and structure.

1. Prokaryotic Ribosomes

Prokaryotic cells contain 70S ribosomes, which are made up of:

• 50S large subunit
• 30S small subunit

These ribosomes are found in bacteria and other prokaryotes, where they carry out protein synthesis. An important feature of bacterial ribosomes is that many antibiotics are designed to target them specifically. Since human ribosomes have a different structure, these medicines can stop bacterial growth without significantly affecting human cells.

2. Eukaryotic Ribosomes

In contrast, eukaryotic cells contain 80S ribosomes, which consist of:

• 60S large subunit
• 40S small subunit

These ribosomes are present in the cells of plants, animals, fungi, and protists. They synthesise proteins needed for normal cell growth, repair, metabolism, and other essential cellular functions, helping the cell carry out its day-to-day activities efficiently.

Functions of Ribosomes

Now that we have explored the structure, types, and location of ribosomes, it's time to understand their functions. 

1. The primary function of ribosomes is protein synthesis. They read the genetic instructions carried by messenger RNA (mRNA) and join amino acids together to form proteins.

2. These proteins are also essential for cell growth and division, enabling cells to develop, multiply, and replace old or damaged cells as the body grows.

3. In addition, ribosomes play an important role in tissue repair by producing the proteins needed to repair damaged cells and support the healing process after an injury.

4. Another important function of ribosomes is the production of enzymes. Since most enzymes are proteins, ribosomes synthesise them to regulate the chemical reactions that keep cells functioning properly.

5. Ribosomes also contribute to hormone production by synthesising many protein and peptide hormones. These hormones help regulate vital processes such as growth, metabolism, and communication between different parts of the body.

6. Their role extends to immune defence as well. Ribosomes help produce antibodies, which recognise and destroy harmful bacteria, viruses, and other disease-causing microorganisms, protecting the body from infections.

7. Finally, ribosomes support cellular metabolism by continuously producing the enzymes required for metabolic reactions. 

In this way, they help ensure that cells have the proteins they need to function efficiently and remain healthy.

Difference Between Types of Ribosomes

Some ribosomes are classified based on their location within the cell, while others are classified based on the type of cell in which they are found. 

The table below summarises the key differences to help you compare their structure, location, and functions at a glance.

Feature	Free Ribosomes	Bound Ribosomes	Organelle Ribosomes	Prokaryotic Ribosomes	Eukaryotic Ribosomes
Found In	Cytoplasm of eukaryotic cells	Attached to the rough endoplasmic reticulum (RER)	Mitochondria and chloroplasts	Prokaryotic cells	Eukaryotic cells
Location	Free in the cytoplasm	Attached to the RER	Inside specific organelles	Cytoplasm	Cytoplasm or attached to the RER
Ribosome Size	80S	80S	Similar to 70S	70S (50S + 30S)	80S (60S + 40S)
Main Function	Produce proteins used within the cell	Produce proteins for secretion, membranes, and lysosomes	Produce proteins required by mitochondria and chloroplasts	Synthesize proteins for prokaryotic cells	Synthesize proteins for eukaryotic cells
Proteins Produced	Cytoplasmic proteins	Secretory, membrane, and lysosomal proteins	Organelle-specific proteins	Proteins needed for bacterial cell functions	Proteins needed for growth, repair, metabolism, and other cellular activities
Special Feature	Support internal cellular functions	Help produce proteins exported from the cell or inserted into membranes	Resemble bacterial ribosomes	Targeted by many antibiotics	Generally not affected by antibiotics that target bacteria

Till now, we have explored the structure, composition, types, location, and functions of ribosomes. These tiny organelles convert genetic information into proteins, making them essential for cell growth, repair, metabolism, and other vital life processes.