During the second phase of photosynthesis, carbon dioxide fixation, carbon dioxide from the atmosphere unites with the sugar ribulose diphosphate. The product is split into two equal parts, hydrogen from the light reaction is added, and the resulting molecules of PGAL (phosphoglyceraldehyde) are then used as small building blocks for the construction of more elaborate molecular forms, using the stored energy from the light reaction. Although carbon dioxide fixation is slower than the light reaction, millions of PGAL molecules are synthesized within minutes after light enters a leaf’s mesophyll
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