Question

Which and What proportion of the chemical energy generated in the light reactions of the photosynthesis is utilized in the dark reactions?

Answer 1

During the light-dependent phase, a set of biochemical processes occur in the leaves of the plant through the action of light. The light energy is captured by structures called photosystems. From the breakdown of water molecules, the chemical energy released is used to synthesize two basic molecules of plant metabolism: ATP and NADP. The formation of ATP is called photophosphorylation, and there are two variants of it: cyclic and acyclic. This light phase is necessary for the next phase, the dark phase, which occurs not necessarily in the presence of light. It occurs in chloroplasts and depends directly on the products obtained in the light phase.

In the dark phase, ribulose bisphosphate is added to the gaseous carbon dioxide (CO2) present in the air, resulting in the production of organic compounds, mainly carbohydrates or sugars which are composed by carbon, hydrogen and oxygen . This entire transformation chain is called the Calvin cycle.

The dark phase is composed by three main stages: fixation, reduction and regeneration

Fixation: The first enzyme involved in the Calvin cycle is called RUbisCO, and fixes 3 atmospheric CO2 atoms joining them to 3 units of ribulose bisphosphate. The result of such binding is 6 molecules of 3-phosphoglycerate.

Reduction: The previous molecule is transformed into 1,3 bisphosphoglycerate by the action of 6 units of ATP (generated in the light phase), and that compound is transformed into G3P ​​by action of 6 units of NADPH. One of these two G3P ​​molecules passes to the plant's metabolic pathways to produce higher compounds such as glucose or starch.

Regeneration: Finally, the addition of phosphorus by 3 ATP ends up generating a new molecule of ribulose-1,5-bisphosphate, which will trigger the process again.