The ribosome is the site of polypeptide synthesis (protein assembly) within the cell 

• It is composed of protein (provides stability) and ribosomal RNA (responsible for catalytic activity)

• Eukaryotic ribosomes are larger in size (80S) compared to prokaryotic ribosomes (70S)

Ribosomes are comprised of two distinct subunits:

• The small subunit is responsible for binding to mRNA, while the large subunit binds to tRNA
  

• When the two subunits form a complex, translation of an mRNA sequence can occur
  

In eukaryotes, ribosomes can either be located freely within the cytosol or embedded within the rough endoplasmic reticulum

• Free ribosomes synthesise proteins for use within the cytosol (i.e. intracellular proteins)

• Ribosomes embedded within the rough ER synthesise proteins that will be packaged into vesicles and transported to other organelles

• If the vesicles are transported to the Golgi apparatus, then the proteins will be secreted from the cell for extracellular use

Structure of the Ribosome