The photosynthetic process can be divided into two parts, the light reactions and the dark reactions. In the light reactions, which take place in the thylakoid membrane system, hydrogen is withdrawn from water and passed along a series of hydrogen carriers to NADP, so that NADPH2 is formed and oxygen is liberated. Associated with this hydrogen (or electron) transport there is a conversion of ADP (adenosine diphosphate) and inorganic phosphate to ATP (adenosine triphosphate), probably two (or, on average, some fractional number between one and two) ATP molecules being formed for every two electrons transferred or molecule of NADP reduced. These chemical changes are associated with a considerable increase in free energy: this is made possible by the light energy absorbed by the chloroplast pigments. Thus we may summarize the light reactions by the equation
In the dark reactions, which take place in the stroma of the chloroplast, the NADPH2 produced in the light reactions is used to reduce CO2 to the level of carbohydrate. This too is associated with an increase in free energy, the energy being supplied by the concomitant breakdown of ATP produced in the light reactions. The dark reactions can be summarized by the equation
Thus, the overall photosynthetic process can be represented by CO2 + 2H2O (CH2O) + H2O + O2
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