Ribosomes (AQA AS Biology): Revision Note

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

  • 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

  • Condensation enzymes linked to rRNA can then catalyse the formation of peptide bonds 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