Question 23M.1.HL.TZ1.30

This was another question which was found much more difficult than expected. Only a third of candidates answered it correctly and the discrimination index was very low. The expected answer was worded rather awkwardly, which may have caused some of the stronger candidates to think it must be incorrect. The most popular answer was B, which stated that rubisco catalyses the hydrolysis of ribulose bisphosphate. Perhaps candidates thought that the conversion of a five carbon sugar to two three carbon sugars reduces the size of the sugar molecules so must be a hydrolysis reaction. Although the name 'rubisco' is briefer and therefore easier to remember than 'ribulose bisphosphate carboxylase oxidase', it does not indicate what types of reaction the enzyme catalyses. It is worth emphasising that rubisco carboxylates RuBP by addition of carbon dioxide. The subsequent splitting of the six-carbon molecule produced does not involve the lysis of water, so it is not hydrolysis.