Paper 1 Q5 — Reading a radioactive-label experiment

Cell biology — interpreting isotope labelling; proportion of labelled monomers matters

What the examiners said

“Candidates considering statement 4 to be correct had not noted that radioactive threonine will only account for a small proportion of amino acids in each glycoprotein… They also did not realise that radioactive glucose will account for a very large proportion of the monosaccharides incorporated into glycoproteins.”

Try the question (reconstruction)

Cells make glycoproteins (a protein core plus large carbohydrate chains). To trace where the carbohydrate part is assembled, which radioactive label gives the strongest, most specific signal?

Examiner report: “Candidates overlooked the proportion of labelled monomer in each part of the glycoprotein.”

Explore — proportion labelled = signal strength

Set how much of each monomer type carries a label. The bars show the resulting signal from the protein core versus the carbohydrate chains.

Label choice: glucose

A good tracer labels a large proportion of the structure you want to follow. Glucose saturates the sugar chains; threonine barely dots the protein.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H2 Biology 9744 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org