Editing Ribosome (section)

==Ribosome locations==
Ribosomes are classified as being either "free" or "membrane-bound".

[[Image:Protein translation.gif|thumb|200px|'''Figure 6:''' A ribosome translating a protein that is secreted into the [[endoplasmic reticulum]] via [[protein domain dynamics]].]]

Free and membrane-bound ribosomes differ only in their spatial distribution; they are identical in structure. Whether the ribosome exists in a free or membrane-bound state depends on the presence of an [[signal peptide|ER-targeting signal sequence]] on the protein being synthesized, so an individual ribosome might be membrane-bound when it is making one protein, but free in the cytosol when it makes another protein.

Ribosomes are sometimes referred to as [[organelle]]s, but the use of the term ''organelle'' is often restricted to describing sub-cellular components that include a phospholipid membrane, which ribosomes, being entirely particulate, do not. For this reason, ribosomes may sometimes be described as "non-membranous organelles".{{cn|date=May 2025}}

===Free ribosomes===
Free ribosomes can move about anywhere in the [[cytosol]], but are excluded from the [[cell nucleus]] and other organelles. Proteins that are formed from free ribosomes are released into the cytosol and used within the cell. Since the cytosol contains high concentrations of [[glutathione]] and is, therefore, a [[redox|reducing environment]], proteins containing [[disulfide bonds]], which are formed from oxidized cysteine residues, cannot be produced within it.

===Membrane-bound ribosomes===
When a ribosome begins to synthesize proteins needed in certain organelles, the ribosome making this protein can become "membrane-bound". In eukaryotic cells this happens in a region of the endoplasmic reticulum (ER) called the "rough ER". The newly produced polypeptide chains are inserted directly into the ER by the ribosome undertaking [[vectorial synthesis]] and are then transported to their destinations, through the [[secretory pathway]]. Bound ribosomes usually produce proteins that are used within the plasma membrane or are expelled from the cell via ''[[exocytosis]]''.<ref>{{cite book | vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P | chapter = Membrane-bound Ribosomes Define the Rough ER | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26841/#A2204 | title = Molecular Biology of the Cell | edition = 4th | location = New York | publisher = Garland Science | date = 2002 | isbn = 978-0-8153-4072-0 }}</ref>