Principles Of Biology (POB) Exam 2 Practice

The Calvin Cycle is the process that fixes carbon from CO2 into a five-carbon sugar, RuBP, to ultimately form 3-phosphoglycerate (3-PGA). In the chloroplast stroma, CO2 is carboxylated by the enzyme RuBisCO to create an unstable six-carbon intermediate that immediately splits into two molecules of 3-PGA. This reaction is the first stable product of carbon fixation, and it sets the stage for building sugars using energy from ATP and reducing power from NADPH produced by the light-dependent reactions. This cycle does not generate oxygen; oxygen is produced during the light reactions when water is split in photosystem II. It also does not occur in the mitochondrion, nor does it run before the light-dependent steps—those steps must supply the ATP and NADPH the Calvin Cycle uses. It continues by regenerating RuBP to keep fixing CO2 in a loop. So the statement that CO2 is fixed onto RuBP to form 3-PGA is the true description of what the Calvin Cycle does.