AQA A Level Biology

Revision Notes

1.5.5 Ribosomes


  • Ribosomes are small organelles that are either free in the cytoplasm (of all cells) or are attached to the rough endoplasmic reticulum (only in eukaryotic cells)
  • Ribosomes are the site of protein synthesis (where proteins are made)
    • They ‘read’ RNA to make polypeptides (proteins) in a process known as translation
  • Ribosomes are themselves formed from RNA and proteins
    • The RNA that forms part of the structure of ribosomes is a specific type of RNA known as ribosomal RNA (rRNA)
    • The rRNA in ribosomes has enzymatic properties that catalyse the formation of peptide bonds between amino acids
    • Each ribosome is a mixture of ribosomal RNA and proteins
  • Ribosomes in eukaryotic cells are larger than those in prokaryotic cells. In both cell types, ribosomes are composed of a small subunit and a large subunit
    • 80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells
    • 70S  ribosomes (composed of 50S and 30S subunits) are found in prokaryotic cells, as well as in the mitochondria and chloroplasts of eukaryotic cells
  • The large subunit is the site of translation
    • The rRNA and proteins of the large subunit hold tRNA molecules (with their attached amino acids) in place
    • rRNA can then catalyse the condensation reactions between amino acids
  • mRNA sits between the two subunits and the ribosome moves along it as it translates it into a polypeptide
  • Unlike some organelles, ribosomes are not surrounded by a membrane

Cell components_Ribosome, , downloadable AS & A Level Biology revision notes

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

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