Ribosomes and Plastids

Ribosomes are small, non-membrane-bound particles composed of ribonucleoprotein -- a combination of ribosomal RNA (rRNA) and protein -- and they are, without exception, the sites within any cell where proteins are actually synthesised, by translating the genetic code carried in messenger RNA into a specific sequence of amino acids. The ribosome was first described by the cell biologist George Palade, who also carried out much of the pioneering work identifying and describing the rough endoplasmic reticulum. Ribosomes are found in every living cell, but they differ in size, and hence in sedimentation coefficient, between prokaryotic and eukaryotic cells. Prokaryotic cells contain 70S ribosomes, made up of a large 50S subunit and a small 30S subunit. Eukaryotic cells contain larger 80S ribosomes in their general cytoplasm, made up of a large 60S subunit and a small 40S subunit -- though, as noted in the sections on mitochondria and plastids, the ribosomes found inside these two organelles are themselves smaller 70S ribosomes, matching those of prokaryotes rather than the surrounding eukaryotic cytoplasm.

Within a eukaryotic cell, ribosomes may occur in either of two locations. Many lie freely scattered through the cytoplasm, where they typically synthesise proteins that will remain and function within the cytoplasm itself. Others are attached to the outer, cytoplasm-facing surface of the endoplasmic reticulum (forming what is described elsewhere in this chapter as the rough endoplasmic reticulum), where they synthesise proteins that will be secreted from the cell, inserted into membranes, or delivered into other organelles of the endomembrane system. In both locations, several ribosomes very often attach simultaneously to a single strand of messenger RNA, forming a chain-like cluster called a polyribosome or polysome; because each ribosome in the cluster is independently translating the same mRNA strand at a slightly different stage, this arrangement allows the cell to produce many copies of the same protein simultaneously and efficiently from one mRNA molecule.

Plastids are a second group of organelles found characteristically in plant cells (and in the single-celled euglenoids), entirely absent from animal cells, distinguished by the particular pigments -- and consequently the particular colours -- they contain. Plastids are classified into three main types according to this pigment content and their associated function. …