Ribosome: Types, Structure, and Function

Ribosomes are small, dense, rounded, and granular particles of the ribonucleoprotein. The word ribosome is derived from the Greek word “ribo” which means ribonucleic acid, and “soma” which means body. In 1953 Robinson and Brown noted the ribosome in the plant cell while they were studying the bean roots using the electron microscope. Likewise, in 1955 Palade observed the ribosome in the animal cell. In 1958, Palade gave the term ribosome. Ribosomes are also called Palade’s granules/ particles.

The ribosome is called the protein factory of the cell because it helps in protein synthesis or polypeptide synthesis. The mass of the ribosome is expressed in the Svedberg unit (S). It is also called the universal cell organelle because they are found in prokaryotic and eukaryotic cells. In the prokaryotic cell, it is found freely floating in the cytoplasm. In the eukaryotic cell, they are found in the, cytoplasm, endoplasmic reticulum, nuclear membrane, and inside the cell organelles of mitochondria and chloroplast.

Table of Contents

Location of Ribosomes

A large number of ribosomes are present in yeast cells, reticulocytes or lymphocytes, meristematic plant tissues, embryonic nerve cells, and cancerous cells. These ribosomes are present freely in the cytoplasmic matrix.
In the actively protein-synthesizing cell, ribosomes are attached to the endoplasmic reticulum as the rough endoplasmic reticulum. Examples: pancreatic cells, plasma cells, hepatic parenchymal cells, Nissls bodies, osteoblasts, serous cells, or the submaxillary gland, chief cells of the glandular stomach, thyroid cells, and mammary gland cells.
A large number of free ribosomes are present in the cell synthesizing specific proteins. Example: erythroblasts, developing muscle cells, skin, and hair. These specific proteins are associated with intracellular utilization and storage.

Number of ribosomes: In E. coli, ribosome covers 25 percent of the total mass of bacteria. It contains 10,000 ribosomes. In the mammalian cultured cell, 10 million ribosomes are present per cell.

Structure of Ribosomes

A ribosome is in the subspherical structure and consists of two unequal parts. One is a larger unit, and the next is a smaller unit. The larger subunit is dome-shaped, whereas the smaller subunit is in the oblate-ellipsoid cap shape. In the larger subunit, protuberance, stalk, and ridge are present. In the larger subunits, two sites, the peptidyl (P) and the aminoacyl (A) site, are present. The smaller subunit consists of the head, cleft, and platform. The smaller unit fits over, the larger subunit like the cap. The cleft provides space for the mRNA in between these two subunits.

These two subunits remain attached to each other due to the higher concentration of the Mg^₊₊ions. When there is a decrease in the concentration of the Mg⁺⁺, then these larger and smaller subunits get separated. In the bacterial cell, these two subunits remain freely in the cytoplasm, and only during the protein synthesis do they unite. During the higher concentration of the Mg⁺⁺ ions, ribosomes known as monosomes get associated with each other forming the dimer. During protein synthesis, many ribosomes are aggregated in the mRNA forming the polyribosomes or polysomes.

Types of ribosomes

Based on the sedimentation coefficient, two types of ribosomes are found.

70S Ribosome

70S ribosome is smaller in size with a molecular weight of 2.7×10⁶ daltons. In the prokaryotic cell, 70S ribosome is found in bacteria and blue-green algae. The eukaryotic cell 70S ribosome is found in the chloroplasts and mitochondria. 70S ribosomes consist of two subunits: 50S (larger subunit) and 30 S (smaller subunit)

80S Ribosome

It is larger in size with a molecular weight of 40×10⁶ daltons. 80S ribosome is absent in the prokaryotic cell. In the eukaryotic cell, 80S ribosome is found either in free form in the cytoplasm or bound with the endoplasmic reticulum and nuclear membrane. 80S ribosome consists of two subunits: 60 S (larger subunit) and 40 S (smaller subunit).

Chemical Composition of Ribosomes

A ribosome is composed of RNA and proteins. In the 70S ribosome, the RNA content is high than that of the protein. E.g., in the ribosome of E. coli, rRNA is 63%, and protein is 37 %. The case is just the opposite in the 80S ribosome. In the 80S ribosome, the RNA content is less than that of the protein. E.g., in the ribosome of a pea seedling, RNA is 40%, and protein is 60%.
70S ribosome consists of three types of rRNA, i.e., 23S rRNA, 16S rRNA, and 5S rRNA. In the larger 50S ribosomal subunit, 23S and 5S rRNA are present. In the smaller 30S ribosomal subunit, 16S rRNA is present.
80S ribosome consists of four types of rRNA, i.e., 28S rRNA, 18S rRNA, 5S rRNA, and 5.8S rRNA. In the larger 60S ribosomal subunit, 28S, 5S, and 5.8S rRNA are present. In the smaller 40S ribosomal subunit, 18S rRNA is present.
The important metallic ions in the ribosome are Mg⁺⁺, Ca⁺⁺, and Mn⁺⁺.

Functions of Ribosomes

Ribosomes are the sites of protein synthesis. So ribosomes are called the protein factories or workbench of proteins.
Free ribosomes synthesize the structural and enzymatic proteins for intracellular use.
The ribosomes, bound with the endoplasmic reticulum, produce secretory, lysosomal, and membrane proteins.
The ribosome provides enzymes (peptidyl transferase) and initiation factors for the condensation of the amino acids to form polypeptides.
Ribosomes contain the rRNA for attachments of mRNA and tRNA.

Difference between the 70S and 80S Ribosomes

The eukaryotic 80S ribosome is larger than the prokaryotic 70S ribosome. Eukaryotic ribosomes contain more proteins (70-80 types ) than prokaryotic ribosomes, consisting of only 53 types of protein. Eukaryotic ribosomes comprise four types of RNA, while prokaryotic ribosomes consist of only three types of RNA. The protein and the nucleic acid of the eukaryotic ribosome are larger than that of the prokaryotic ribosome.

References

Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (First edition). S.Chand and Company Ltd.
Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.