Ribosomes are present in both prokaryotes and eukaryotes. Although it does not have any membrane, it is generally considered a cell organelle. They are not found in mature sperms and mature mammalian red blood cells (RBCs).
They look like tiny particles suspended in the cytoplasm or attached to or embedded on the outer membrane of the rough endoplasmic reticulum and nucleus. A ribosome is basically a complex made of protein and ribosomal RNA. It contains or made of around an equal amount of ribosomal RNA and proteins. Due to the high content of ribonucleic acid (RNA), it is called ribosome. They are also called ribonucleoprotein particle. Besides this, they are also known as protein factories of the cell as proteins are formed on its surface.
The ribosomes were first isolated by Claude from the cytoplasm. They were studied for the first time by Palade in 1955. So, they are also called Palade's particles. The term 'ribosome' was given by Roberts. The ribosome is the smallest organelle in the cell. It has a granular structure. Its size ranges from 150 Angstrom to 230 Angstrom. They were discovered by using the electron microscope as they were very small as compared to other organelles.
Apart from the endoplasmic reticulum, they are also found in the nucleus, mitochondria and plastids. Due to this reason, it is also called organelle within the organelle. In mitochondria, they are known as mitoribosomes, and in plastids, they are called plastidosomes. In the cell, they can be located in the cytoplasm or attached to the membrane. In the cytoplasm, they are known as free ribosomes. Whereas on the membrane, such as on the surface of the ER and nucleus, they are called bound ribosomes.
Structure of Ribosome
They have two major parts or made of two subunits, which include small and large ribosomal subunits. These subunits are assembled or joined together by proteins and ribosomal RNA (rRNA). During protein synthesis, mRNA binds at the smaller subunit where information contained in mRNA is decoded, whereas, in the larger subunit, the amino acids are transferred by tRNA and arranged or added to previous amino acids to form polypeptide chain or proteins.
The large subunit remains attached to the membrane with the help of a protein called ribophorin. So, it is always a bound ribosome. The small subunit remains freely in the cytoplasm and gets attached to the large subunit when proteins are synthesized. When they come together, this process is called association. The Mg2+ helps in this association. When the protein synthesis gets completed the subunits get separated from each other which is known as dissociation. They generally form helical groups during active protein synthesis to produce more proteins. The ribosomes in a polysome are held together by mRNA.
The smaller subunit has a depression, which is an mRNA binding site as the mRNA binds here during protein synthesis. Whereas, the large subunit has a tunnel through which the newly synthesized proteins are passed to Endoplasmic Reticulum (ER). Thus, the large subunit of the ribosome pushes the newly synthesized polypeptide chain to protect it from cellular enzymes.
Also, the large subunit has three sites which are A-site (Amino acyl site or acceptor site) where tRNA carrying amino acid is landed, P-site (peptidyl site or donor site) where peptide bond is formed between amino acids and E-site (exist site) from where the empty tRNA leaves the ribosome.
Types of ribosomes
Based on the size and sedimentation coefficient, ribosomes are of two types; 70S ribosomes and 80S ribosomes.
i) 70S ribosome: The 70s found in prokaryotic cells and in cell organelles like mitochondria and plastids. The letters S (Svedberg's Unit) denotes the density and size of the ribosome. It is a non-SI metric unit for sedimentation coefficient. It measures the size of a particle, based on its sedimentation rate under acceleration which means how fast a particle settles to the bottom of a solution. The 70S ribosome has a sedimentation coefficient of 70S. They are comparatively smaller than the 80S ribosomes. In prokaryotic cells, they are freely located in the cytoplasm and are called monosomes.
The sub-units of 70S in prokaryotes are 30S (small subunit) and 50s (large subunit). Furthermore, the 70S ribosomes have 60 to 65 % rRNA and 35 to 40 % proteins. So, it has more rRNA than protein. Besides this, it has three types of rRNA that include 23S, 5S and 16S. The 23S and 5S are located in its large subunit, whereas, the 16S rRNA is present in the small subunit of the 70S ribosome.
ii) 80S ribosomes: The 80s ribosomes are found in the eukaryotic cells of eukaryotes. Its sedimentation co-efficient is 80S. It has 40S as a small subunit and 60S as a large subunit. The 80S ribosome has or made of 45% rRNA and 55 % proteins. It has four types of rRNA that include 28S, 5S, 5.8S and 18S. The first three are present in the larger subunit (60S) and the last one (18S) is present in the small subunit (40S) of the 80S ribosome. The two subunits normally remain separated and join when protein is to be synthesized.
In eukaryotes, multiple ribosomes may be attached to mRNA to form polyribosomes or polysomes. So, the rate of protein synthesis is high in polyribosomes.
Where are ribosomes formed?
Ribosomes are formed in the nucleolus and come out through the nuclear pores and get attached to the surface of the endoplasmic reticulum. They are present in large number in cells that produce more proteins such as pancreatic cells in the body that produce lots of digestive enzymes, which are basically proteins.
Functions of Ribosomes
Their main function is protein synthesis. It is a site or place where protein synthesis takes place or proteins are synthesized. They make proteins by using their RNA and amino acids. They decode information in the mRNA and accordingly produce proteins by using amino acids from the cytoplasm. So, they translate mRNA into polypeptide chains that are folded and assembled into proteins.
Furthermore, the ribosomes take part in the metabolism of lipids and produce cytochrome for electron transport during cellular respiration. They also offer binding sites for mRNA and tRNA loaded with amino acids.