Photosynthesis: Enzymatic Stage

Enzymatic Stage

The enzymatic stage has many participants. These include carbon dioxide, hy-drogen carrier with hydrogen (NADPH), ATP, ribulose biphosphate (RuBP, or 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 CO₂ as-similation with C₅ into short-living C₆ molecules. Assimilation requires Rubisco as an enzyme. Next, this temporary C₆ breaks into two 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 (though the intermediate stage of PGAL) one molecule of glucose (C₆H₁₂O₆) for every six as-similated molecules of 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 as-similation. Consequently, all reactions described in this paragraph are part of the cycle which has the name “Calvin cycle” or “C₃ cycle” (because the C₃ PGA molecules here are most important).

In all, enzymatic stage starts with CO₂, NADPH, ATP and C₅ (RuBP). It ends with glucose (C₆H₁₂O₆), NADP+, ADP, P_i and the same C₅. With an addition of nitro-gen and phosphorous, glucose will give all other organic molecules (Fig. 2.7).

To summarize, the logic of photosynthesis (Fig. 2.8) 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 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.