"Of all the biochemical inventions in the history of life, the machinery to oxidize water — photosystem II — using sunlight is surely one of the grandest." (Sessions, A. et al, 2009)
We have just seen how we can transduce the chemical potential energy stored in carbohydrates into chemical potential energy of ATP - namely through coupling the energy released during the thermodynamically favored oxidation of carbon molecules through intermediaries (high energy mixed anhydride in glycolysis or a proton gradient in aerobic metabolism) to the thermodynamically uphill synthesis of ATP. There is a situation that occurs when we wish to actually reverse the entire process and take CO2 + H2O to carbohydrate + O2. This process is of course photosynthesis which occurs in plants and certain photosynthetic bacteria and algae. Given that this process must by nature be an uphill thermodynamic battle, let us consider the major requirements that must be in place for this process to occur:
- A strong oxidizing agent must be formed which can take water and oxidize it to dioxygen. We know that redox reactions occur in the direction of stronger to weaker oxidizing agent (just as acid base reactions are thermodynamically favored in the direction of strong to weak acid). Somehow we must generate a stronger oxidizing agent than dioxygen, which often has the most positive standard reduction potential in tables.
- Plants must have high concentrations of a reducing agent for the reductive biosynthesis of glucose from CO2. The reducing agent used for most biosynthetic reactions in nature is NADPH, which differs from NADH only by the addition of a phosphate to the ribose ring. This phosphate differentiates the pool of nucleotides in the cells used for reductive biosynthesis (NADPH/NADP+) from those used for oxidative catabolism (NADH/NAD+)
- Finally, plants need an abundant source of ATP which will be required for reductive biosynthesis.
We will discuss only the light reaction of photosynthesis which produces these three types of molecules. The dark reaction, which as the name implies can occur in the dark, involves that actual fixation of carbon dioxide into carbohydrate using the ATP and NADPH produced in the light reaction.