AQA AS Biology

Revision Notes

1.5.5 Ribosomes

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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While studying Biochemistry at Oxford University, Amelia started her own tutoring service, helping to connect Science tutors with students in her local area. Amelia has experience teaching the sciences and Maths at all levels to UK and international students and, as well as being our Biology Lead, designs revision resources for Chemistry.
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