<P> Thus, of six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15 carbons), with only one G3P available for subsequent conversion to hexose . This requires nine ATP molecules and six NADPH molecules per three CO molecules . The equation of the overall Calvin cycle is shown diagrammatically below . </P> <P> RuBisCO also reacts competitively with O instead of CO in photorespiration . The rate of photorespiration is higher at high temperatures . Photorespiration turns RuBP into 3 - PGA and 2 - phosphoglycolate, a 2 - carbon molecule that can be converted via glycolate and glyoxalate to glycine . Via the glycine cleavage system and tetrahydrofolate, two glycines are converted into serine + CO . Serine can be converted back to 3 - phosphoglycerate . Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3 - PGA . It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing CO, this process leads to loss of CO . C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates--corn, for example . </P> <P> The immediate products of one turn of the Calvin cycle are 2 glyceraldehyde - 3 - phosphate (G3P) molecules, 3 ADP, and 2 NADP . (ADP and NADP are not really "products ." They are regenerated and later used again in the Light - dependent reactions). Each G3P molecule is composed of 3 carbons . For the Calvin cycle to continue, RuBP (ribulose 1, 5 - bisphosphate) must be regenerated . So, 5 out of 6 carbons from the 2 G3P molecules are used for this purpose . Therefore, there is only 1 net carbon produced to play with for each turn . To create 1 surplus G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle . To make one glucose molecule (which can be created from 2 G3P molecules) would require 6 turns of the Calvin cycle . Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs . </P> <P> These reactions do not occur in the dark or at night . There is a light - dependent regulation of the cycle enzymes, as the third step requires reduced NADP . </P>

Can the dark reactions of photosynthesis happen in the light