Ribosomes

Ribosomes are the protein-synthesizing machines of the cell.

They translate the information encoded in messenger RNA (mRNA) into a polypeptide.

• roughly spherical.
• With a diameter of ~20 nm, they can be seen only with the electron microscope. The image on the right is an electron micrograph showing clusters of ribosomes. These clusters, called polysomes, are held together by messenger RNA (mRNA). (Image courtesy of Alexander Rich.)
• They can make up 25% of the dry weight of cells (e.g., pancreas cells) that specialize in protein synthesis. (A single pancreas cell can synthesize 5 million molecules of protein per minute.)

• Ribosomes that synthesize proteins for use within the cytosol (e.g., enzymes of glycolysis) are suspended in the cytosol.
• Ribosomes that synthesize proteins destined for:
  • secretion (by exocytosis)
  • the plasma membrane (e.g., cell surface receptors)
  • lysosomes
  are attached to the cytosolic face of the membranes of the endoplasmic reticulum. As the polypeptide is synthesized, it is extruded into the interior (lumen) of the endoplasmic reticulum.

  Then, before these proteins reach their final destinations, they undergo a series of processing steps in the Golgi apparatus.

  Link to discussion of Protein Kinesis: the pathways taken by proteins synthesized on the endoplasmic reticulum.

  
  
• Ribosomes that synthesize 13 of the proteins destined for the inner membrane of mitochondria are found within the mitochondrion itself and are quite different in structure from the others.
  
  

  Link to discussion of the evolution of mitochondria.

This table gives some of the data. (S values are the sedimentation coefficient: a measure of the rate at which the particles are spun down in the ultracentrifuge. S values are not additive. nts = nucleotides.)

But despite these differences, the basic operations of bacterial, eukaryotic, and mitochondrial ribosomes are very similar.