Final answer:
Ribosomes are structures in cells that are responsible for protein synthesis. Composed largely of rRNA and proteins, they come in two subunits and can be either free in the cytoplasm or attached to the ER. A cell can contain a substantial number of ribosomes, representing a significant portion of its mass.
Step-by-step explanation:
Ribosomes: The Cellular Protein Factories
The subject in question is ribosomes, ribonucleoprotein particles found in the cytosol of cells. These structures are incredibly important as they are responsible for the synthesis of proteins. Ribosomes are composed of approximately two-thirds RNA, specifically ribosomal RNA (rRNA), and one-third protein. They consist of two units, a large and a small subunit, which come together to translate mRNA into a polypeptide chain. In eukaryotic cells, ribosomes are produced and assembled in the nucleolus before being distributed throughout the cell. They can either be found floating freely in the cytoplasm or attached to the endoplasmic reticulum (ER), forming what is known as the rough ER. Their abundance is highlighted by the fact that they can make up to 20% of a cell's mass, with around 20,000 of them present in a typical eukaryotic cell.
Functionally, ribosomes play a critical role in cellular metabolism by interpreting the genetic code held in mRNA - which is transcribed from DNA in the nucleus - to produce proteins that are essential for cell function and structure. Since protein synthesis is vital to all cellular processes, ribosomes are found in practically every cell, but they are smaller in prokaryotic cells compared to eukaryotic cells. For example, in E. coli, a prokaryotic cell, there could be between 10,000 and 70,000 ribosomes present at any given time.