Ribosomes are distributed universally all throughout the kingdom of animals and plants. They're also found in prokaryotes. The mammalian RBC is the only cell types devoid of ribosomes. For any given type, the density of ribosomes per unit area is rather constant. It is high in active m protein synthesis cells and low in cells where synthesis of protein is low.
The ribosomes frequently occur freely in the cytoplasm in prokaryotic cells. The ribosomes occur freely in the cytoplasm in eukaryotic cells or remain attached to the outer surface of the endoplasmic reticulum (ER) membrane. They are called free ribosomes if they are not attached to the ER. Free ribosomes represent as protein synthesis sites needed to maintain the cytoplasmic matrix's enzyme composition.
Ribosomes can be observed in all cells containing endoplasmic reticulum. Nearly 100 ribosomes per μ3 are found in rabbit reticulocytes, which corresponds to 1x105 particle per reticulocyte and approximately. 5% of the total cell mass, or approximately 20,000 to 30,000 cell mass. But, if unfavourable nutritional conditions slow the rate of protein synthesis, the number of ribosomes can drop significantly in protein synthesizing cells and bacteria.
RNA and proteins are the main constituents of the ribosomes. The lipids are completely absent or traceable. E. Coli's ribosomes possess almost 60-65% of RNA and 35-40% of their weight protein. Ribosomal RNA differs from tRNA and other RNA classes of most cells in size and base content. In all ribosomes, two types of RNAs are found. They are an essential component and can't be easily removed.
As compared to eukaryotic ribosomes, the prokaryotic ribosomes are smaller. This is attributed to the association of eukaryotic cell ribosomes with cytoplasmic or endoplasmic reticulum. Prokaryotic ribosomes are called 70S ribosomes and have physical dimensions of approximately 14 to 15 nm by 20 nm, with a molecular weight of roughly 2.7 million, and are made of 50S and 30S subunits. The S stands for Svedberg unit in the measurement of ribosomal sedimentation coefficient unit. This is a measure of the velocity of sedimentation in a centrifuge; the faster a particle travels when centrifuged, the higher its Svedberg value or coefficient of sedimentation. The coefficient of sedimentation is a function of the molecular weight, volume and shape of a particle. Normally, heavier and more compact particles have greater numbers of Svedberg or faster sediments. There are smaller bacterial ribosomes than eukaryotic ribosomes. A prokaryotic cell usually only has a few thousand ribosomes, while there are several million in a metabolically active eukaryotic cell, such as a human liver cell. Proteins that work in the cytoplasm are produced by free ribosomes that are suspended there, while proteins that are bound within membranes or intended for export from the cell are assembled by ribosomes that are bound to rough ER.
The eukaryotic ribosome (i.e., one not found in mitochondria and chloroplasts) is larger than the prokaryotic 70S ribosome. It is a dimer of the 60S and the 40S subunit, about 22 nm in diameter, and has the sedimentation coefficient of 80S and a molecular weight of 4 million. Eukaryotic ribosomes can be either associated with the endoplasmic reticulum or free in the cytoplasmic matrix. When bound to the endoplasmic reticulum to form rough ER, they are attached through their 60S subunits. Both free and ER-bound ribosomes synthesize proteins. Proteins made on the ribosomes of the RER are often secreted or are inserted into the ER membrane as integral membrane proteins. Free ribosomes are the sites of synthesis for nonsecretory and nonmembrane proteins. Some proteins synthesized by free ribosomes are inserted into organelles such as the nucleus, mitochondrion, and chloroplast. They also assist the transport of proteins into eukaryotic organelles such as mitochondria
When cells are synthesizing a large number of proteins, they must first make a large number of ribosomes. To facilitate this, many hundreds of copies of the portion of the DNA encoding the rRNA are clustered together on the chromosome. By transcribing RNA molecules from this cluster, the cell rapidly generates large numbers of the molecules needed to produce ribosomes. At any given moment, many rRNA molecules dangle from the chromosome at the sites of these clusters of genes that encode rRNA. These proteins are associated with the dangling rRNA molecules. These areas where ribosomes are being assembled are easily visible within the nucleus as one or more dark-staining regions, called nucleoli. Nucleoli can be seen under the light microscope even when the chromosomes are extended, unlike the rest of the chromosomes, which are visible only when condensed.
The main function of ribosomes is to translate the messenger RNA (mRNA) encoded genetic information into proteins.