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Photosynthesis（CO2 Reduction)

 Let's consider the reaction from the plant to CO2 reduction based on the book*1).

Before moving on to the reactions, the chloroplast of a plant is shown in the figure below. Photosynthesis consists of reactions that occur in thylakoids and reactions that occur in the stroma*2).

Reactions occurring in thylakoids

　Chlorophyll in chloroplasts is activated by the absorption of light energy. From this absorbed energy, H2O is decomposed into [H]*3) and oxygen (O2), and the oxygen is discharged from the cell. The hydrogen H is divided into hydrogen ions H+ and electrons e-, and only the electrons are transferred between enzymes. In this process, ATP (adenosine triphosphate) is synthesized. The electron e-, which loses its energy, combines with the hydrogen ion H+ to form hydrogen [H], which is used in the rest of the reaction.


12H2O → 6O2 + 24[H] ④


Reactions occurring in the stroma (Calvin-Benson circuit)

　Carbon dioxide CO2 taken in from the outside is taken up by ribulose (pentose sugar) in chloroplasts, which immediately decomposes into glyceric acid (C3). Glyceric acid is activated by the energy of ATP produced by thylakoids, and then reduced by hydrogen [H] to glyceraldehyde (tricarbohydrate). Some of the glyceraldehyde becomes glucose, while the rest returns to ribulose.


6CO2 + 24[H] → C6H12O6 + 6H2O ⑤


Putting together ④ and ⑤, we get

6CO2 + 6H2O → C6H12O6 + 6O2 ⑥


The previous reaction ② is the reaction that occurs in the thylakoids (i.e., 4), and the reaction that occurs in the stroma is ⑥, so the overall reaction equation for plant photosynthesis that reduces CO2 is ⑥. The diagram below is very easy to understand, so please refer to it.

                                Mechanism of plant photosynthesis

 

*1) New Research on Chemistry I and II for Science University Examinations, by Yoshinobu Urabe, 2005, 6th printing, p680

*2) The green, flattened, bag-like substance inside the chloroplast is called a thylakoid, in which various photosynthetic pigments, including chlorophyll, are present. The rest is a colorless substratum called the stroma.

*3)[H] denotes hydrogen in the bound state with the coenzyme NADP (nicotinamide adenine dinucleotide phosphate).

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