sugars. Thus sugar, a photosynthetic product, can supply 

 both the energy and the material for the biosynthesis of fats. 



Photosynthetic organisms achieve energy storage through 

 their ability to convert electromagnetic energy to chemical 

 potential energy. The conversion begins when pigments 

 absorb light energy. The absorbed energy changes the elec- 

 tronic configuration of the pigment molecule (chlorophyll) 

 from its ground energy state to an excited state. The return 

 of the pigment molecule to its ground-state energy level is 

 accompanied by a (chemical) reaction that would not proceed 

 without energy input; i.e., the products of this reaction have 

 a smaller negative free energy of formation from their ele- 

 ments than do the reactants (in the same reaction). Thus some 

 of the light energy is converted to chemical potential. 



The detailed mechanism of all these energy-conversion 

 steps is not known. However, the net result is often formu- 

 lated by two chemical equations. One of these is an oxida- 

 tion-reduction reaction resulting in the transfer of hydrogen 

 from water to triphosphopyridine nucleotide (TPN): 



(1) HOH + TPN+ -^ iOa + TPNH + H+ 



AF' = +52.6 kcal* 



The other reaction is the formation of an anhydride, adeno- 

 sine triphosphate (ATP), from the ions of two phosphoric 

 acids, adenosine diphosphate and orthophosphate: 



(2) ADP3- + HP04= ^^ HOH + ATP^" + H + 



AF' = +11 kcal* 



In each of these reactions some of the light energy is stored 

 as chemical potential, as indicated by the positive quantities 

 for free energy change. 



The structural formulas of these two cofactors are shown 

 in Figure 1. TPNH and its close relative DPNH (reduced 

 diphosphopyridine nucleotide) serve a double function in 

 photosynthesis and in all biosynthesis. Both TPNH and 



* Assuming these concentrations: (TPNH) = (TPN + ), 

 (ATP*-) = (ADP-^-), (H + ) = 10-' M, (HP04=) = 10-=^ M. 



