How many molecules of 3-PGA form from a single turn of the cycle, and how is each one built?
ATwo molecules, each of three carbons and one phosphate
BOne molecule, made of five carbons and two phosphates
CThree molecules, each of two carbons and one phosphate
DSix molecules, each of three carbons and two phosphates
Answer & Solution
Correct answer: A. Two molecules, each of three carbons and one phosphate
1. Each turn of the cycle involves only one acceptor molecule and one carbon dioxide.
2. For each carbon dioxide that reacts, two molecules of 3-PGA form.
3. Each 3-PGA has three carbons and one phosphate.
4. The carbon count balances, since five carbons from the acceptor plus one from carbon dioxide give six across two molecules.
5. Six molecules of 3-PGA appear only after three turns, so that answer counts a full round rather than one turn.
6. Naming a single five-carbon product describes the acceptor before the reaction rather than the product after it.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 8 "Photosynthesis", section 8.3 Using Light Energy to Make Organic Molecules_
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