Explain how the structure of the ribosome is related to its role in protein synthesis.

IB DP Biology Standard Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Ribosomes consist of a small and a large subunit that join to hold the mRNA in place during translation.
The small subunit contains the mRNA‑binding site and reads the codons.
The large subunit has the A, P and E sites for tRNA binding, positioning amino‑acyl‑tRNA for peptide‑bond formation.
Ribosomes may be free in the cytoplasm or attached to the rough ER, enabling synthesis of cytosolic proteins or proteins destined for secretion or membrane insertion.

Examiner tips

Use the exact terms ‘small subunit’, ‘large subunit’, ‘mRNA attachment’, ‘A/P/E sites’, ‘free or bound to rER’
Show the link between structure (subunits, sites) and function (reading codons, peptide bond formation, localisation)
Keep answer concise – 4 marks, one point per sentence

Common mistakes

Confusing ribosome subunits with other organelles
Omitting the role of the large subunit’s A/P/E sites
Failing to mention free vs. rER‑bound ribosomes

Mark scheme (4 marks)

Ribosomes are composed of two subunits (large and small) that come together to hold the mRNA in position during translation.
Each ribosome has a small subunit that contains a site for mRNA attachment / reading of codons.
The large subunit contains the A, P, and E sites (tRNA binding sites) allowing tRNA carrying amino acids to be positioned correctly for peptide bond formation.
Ribosomes can be free in the cytoplasm or bound to the rough endoplasmic reticulum (rER), allowing proteins to be synthesised either for use within the cell or for secretion / membrane insertion respectively.

Key terms in this question

ribosome