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Chapter: Introduction to Botany: Photosynthesis

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Photosynthesis: Enzymatic Stage

Photosynthesis: Enzymatic Stage
The enzymatic stage has many participants. These include carbon dioxide, hydrogen carrier with hydrogen (NADPH), ATP, ribulose biphosphate (RuBP, or C5), and Rubisco along with some other enzymes.

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 Pi 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, Pi 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.



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