• Ribosomes?are found in cells
  • Either?freely?in the cytoplasm (of all cells)
  • Or bound to the?endoplasmic reticulum?(ER) to form?rough ER?(only in eukaryotic cells)
• Ribosomes are the?site of protein synthesis
• They consist of a?large?and a?small subunit?composed of?protein?and?ribosomal RNA?(rRNA)
  • Protein provides?structure?to the ribosome
  • rRNA?facilitates the binding of mRNA?and tRNA?and catalyses the formation of peptide bonds between amino acids
• Ribosomes have?three tRNA binding sites?and?one mRNA binding site
• mRNA sits in a?groove?between the two subunits and the?ribosome moves along, forming a polypeptide as it travels

A diagram of a ribosome, showing the small and large subunits

• In eukaryotic cells, protein synthesis commonly occurs at?free ribosomes?in the cytoplasm
• Free ribosomes can?move?within the cytoplasm and synthesise proteins for use primarily?within the cell
  • As opposed to proteins destined to be secreted extracellularly
  • Proteins synthesised on free ribosomes are destined for use?within the cytosol?(the fluid part of the cytoplasm)
    • And within?large organelles?such as mitochondria and chloroplasts

• Eukaryotic cells make thousands of proteins that need to be?delivered to the correct location, sometimes in different tissues/organs altogether
• When free ribosomes make proteins destined for lysosomes, or secretion from the cell, the ribosome becomes?bound to the endoplasmic reticulum (ER)
• Signal sequences in the growing polypeptide chain dictate whether the free ribosome needs?to move to the ER
  • The signal sequence occurs at the beginning polypeptide
  • Signal recognition proteins bind to the polypeptide, pausing translation
  • The free ribosome binds to a receptor on the ER,?forming rough ER
  • Translation is re-initiated?and the polypeptide chain?moves inside the ER
• The synthesised protein can be?carried via a vesicle?to the?Golgi?apparatus before being?secreted out?of the cell

Proteins destined for lysosomes or secretion out of the cell are synthesised by ribosomes bound to the endoplasmic reticulum