Ribosomes
Ribosomes are very small organelles often drawn as dots in diagrams. They are free in the cytoplasm or attached to the rough endoplasmic reticulum (RER). One cell can contain millions of ribosomes if it needs to produce a lot of proteins.
If we zoom right in, we find that ribosomes are actually made up of two subunits – a large subunit and a small subunit. The exact size of the subunits depends on whether the ribosome is from a eukaryotic or prokaryotic cell, which comes in useful for identifying the presence of bacteria in a sample. The subunits are made up of ribosomal RNA (rRNA) and protein. If you remember from the article on the nucleus, it is the nucleolus which makes rRNA.
The function of ribosomes is to carry out translation – the process of translating genetic code (carried to the ribosome by messenger RNA) into a sequence of amino acids. You can read about translation here. Amino acids are the building blocks of proteins, and the ribosomes join them together with peptide bonds to form polypeptide chains. This is the first stage of protein synthesis; the amino acid sequence is the primary structure of the protein. To form the final active protein, the polypeptide chain needs to be folded and processed into its secondary, tertiary, and possibly quaternary structure.
The Rough Endoplasmic Reticulum (RER)
It seems fitting to include the rough endoplasmic reticulum (RER) in this article as it has ribosomes bound to the outside and is involved in those later stages of protein synthesis we just mentioned. The RER is a collection of membranes filled with fluid. Polypeptide chains that have been made at the bound ribosomes can be transported straight into the RER where they are folded and processed. The RER can also do quality checking; proteins that are incorrectly folded are packaged off to get destroyed.
While we will dedicate a separate article to the Golgi apparatus, it’s worth mentioning that this is also involved in processing proteins. Some proteins are transported from the RER to the Golgi, where they are processed further and packaged into vesicles ready to leave the cell.
Summary
- Ribosomes have a large and small subunit made up of ribosomal RNA and protein.
- Ribosomes are involved in translation – the process of creating a sequence of amino acids from genetic code, an important part of protein synthesis.
- The RER has ribosomes bound to it and is important for folding, processing, and quality checking proteins.
