Photosynthesis and the Role of H+ Ions

How does the light-independent stage of photosynthesis get H+ ions?

During the light-independent stage of photosynthesis, also known as the Calvin cycle or the dark reaction, ATP and NADPH from the light-dependent stage are utilized to convert atmospheric carbon dioxide (CO2) into glucose. In this process, carbon fixation occurs via a series of enzymatic reactions.

The first step in the Calvin cycle is the attachment of a CO2 molecule to a five-carbon compound called ribulose-1,5-bisphosphate (RuBP) catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as Rubisco. This reaction yields an unstable six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA)

Next, ATP and NADPH from the light-dependent reactions are used to convert the 3-PGA molecules into a higher energy compound called glyceraldehyde 3-phosphate (G3P). This involves the phosphorylation of 3-PGA by ATP to form 1,3-bisphosphoglycerate (1,3-BPG), which is then reduced by NADPH to produce G3P

To regenerate RuBP, several G3P molecules must be converted back into RuBP through a series of enzymatic reactions. Here is where the H+ ions (protons) come into play. In two of the steps, NADPH is oxidized to NADP+ while releasing H+ ions, which are then used in the reduction of the intermediate molecules

In the first step, two molecules of G3P are converted into one molecule of glucose-6-phosphate (G6P) through rearrangements and phosphorylation reactions. This reaction is catalyzed by the enzyme phosphoglucoisomerase

The second step involves the reversible conversion of G6P into fructose-6-phosphate (F6P) by phosphoglucomutase. During this conversion, a H+ ion is released from G6P and then taken up by an enzyme involved in the reaction

It’s important to note that H+ ions are not directly acquired or consumed during the light-independent stage. Instead, they are released as byproducts of specific enzymatic reactions in the Calvin cycle. These reactions are energetically coupled to the oxidation and reduction of NADPH, allowing the transfer of high-energy electrons and facilitating the conversion of carbon dioxide into glucose

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