â¢ For every three CO2 molecules that enter the Calvin cycle, six G3P molecules are produced, only one of which can be counted as net gain. - The cycle begins with three five-carbon RuBP molecules â a total of 15 carbons. - The six G3P molecules produced contain 18 carbons, a net gain of three carbons from CO2. - One G3P molecule exits the cycle; the other five are recycled to regenerate three molecules of RuBP.