Photosynthesis In Higher Plants

The Calvin cycle, or the C3 biosynthetic pathway, is quantified in NCERT Class 11, Chapter 13 (Photosynthesis in Higher Plants). Each turn of the cycle fixes one molecule of carbon dioxide by attaching it to the five-carbon acceptor ribulose-1,5-bisphosphate. Since glucose is a six-carbon sugar, six molecules of carbon dioxide must be incorporated to build one glucose molecule, and therefore the cycle must turn six times. Each of these six turns consumes ATP and NADPH produced in the light reactions; in total the cycle uses eighteen ATP and twelve NADPH to make one glucose. The option three is wrong because three turns fix only three carbons, enough for one triose, not a hexose. Twelve confuses the count with the number of NADPH molecules required. Eighteen mistakes the number of ATP molecules used for the number of cycle turns. A numerical sanity check confirms the answer: six turns fix six carbon atoms (6 × 1 CO2), which precisely matches the six carbons present in a single glucose molecule, so six turns are needed, and this also accounts for the eighteen ATP and twelve NADPH consumed across those six cycle turns.