9.4: Enzymatic Stage
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The enzymatic stage has many participants. These include carbon dioxide, hydrogen carrier with hydrogen (NADPH), ATP, ribulose biphosphate (RuBP, or \(\ce{C5}\)), and Rubisco along with some other enzymes. Everything occurs in the matrix (stroma) of the chloroplast.
The main event of the enzymatic stage is \(\ce{CO2}\) assimilation with \(\ce{C5}\) into short-living \(\ce{C6}\) molecules. Assimilation requires Rubisco as an enzyme. Next, this temporary \(\ce{C6}\) breaks into two \(\ce{C3}\) molecules (PGA). Then, PGA will participate in the complex set of reactions which spend NADPH and ATP as sources of hydrogen and energy, respectively; and yields (through the intermediate stage of PGAL) one molecule of glucose (\(\ce{C6H12O6}\)) for every six assimilated molecules of \(\ce{CO2}\). NADP\(^+\), ADP and P\(_i\) will go back to the light stage. This set of chemical reactions returns RuBP which will start the new cycle of assimilation. Consequently, all reactions described in this paragraph are part of the cycle which has the name “Calvin cycle” or “\(C_3\) cycle” (because the C\(_3\) PGA molecules here are most important).
In all, enzymatic stage starts with \(\ce{CO2}\), NADPH, ATP and \(\ce{C5}\) (RuBP). It ends with glucose (\(\ce{C6}\)H\(_{12}\)O\(_6\)), NADP+, ADP, P\(_i\) and the same \(\ce{C5}\). With an addition of nitrogen and phosphorous, glucose will give all other organic molecules (Figure \(\PageIndex{3}\)).
To summarize, the logic of photosynthesis (Figure \(\PageIndex{4}\)) is based on a simple idea: make sugar from carbon dioxide . Imagine if we have letters “s”, “g”, “u”, and “a” and need to build the word “sugar”. Obviously, we will need two things: the letter “r” and the energy to put these letters in the right order. The same story occurs in photosynthesis: it will need hydrogen (H) which is the “absent letter” from \(\ce{CO2}\) because sugars must contain H, O and C. NADP\(^+\)/NADPH is used as hydrogen supplier, and energy is ATP which is created via proton pump, and the proton pump starts because light helps to concentrate protons in the reservoir.