noun, singular: ribosome
Ribosomes, being entirely particulate, are not considered organelles when the term organelle is strictly used to refer to membraned structures. Although in some literature they are referred to as "non-membranous organelles". Ribosomes are typically composed of two subunits: the large and small subunits. They join as one during translation; together, they catalyze the translation of mRNA into a polypeptide chain during protein synthesis, and since their active sites are made of RNA, ribosomes are also referred to as "ribozymes."
Ribosomes are formed in the cytoplasm of prokaryotic cells. In eukaryotic cells, they are formed most often in the nucleolus. Another difference between ribosomes of prokaryotes and eukaryotes is the structure of the ribosomes. Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large (50S) subunit. Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit. However, the organelles like chloroplasts and mitochondria that are present only in eukaryotic cells also consist of 70S ribosomes resembling those in prokaryotes (e.g. bacteria), indicating that these eukaryotic organelles have descended from their ancestral bacteria (see Endosymbiotic theory).
In eukaryotes, the ribosomes may be classified as either ‘free’ or ‘bound’. Free ribosomes may be found suspended in the cytosol whereas bound ribosomes are attached to endoplasmic reticulum (as such called rough endoplasmic reticulum). Free ribosomes are involved in the synthesis of proteins that will function in the cytosol while bound ribosomes in the synthesis of proteins that are to be exported or used within the cell membrane. The two types of ribosomes have similar function and structure, and in fact, are interchangeable.