364 Photosynthesis /20 : 3 



Probably each plate is surrounded by a very thin membrane. The 

 plates are assembled or stacked up in larger ordered structures called 

 grana, each surrounded by its own membrane. The grana in turn are 

 located in an oriented fashion within the chloroplasts, each within its 

 own membrane. The chloroplasts tend to be arranged in a random 

 fashion in the cytoplasm of the cell, although chloroplasts of many cells 

 are oriented in the light. 



3. Basic Chemistry of Photosynthesis 



The over-all reaction of photosynthesis consists of the conversion of 

 C0 2 to carbohydrate at the expense of the energy contained in photons 

 of visible light. The over-all process in green plants may be divided 

 into three parts, each of which can, under suitable conditions, be 

 independently observed. These are (a) the conversion of the carbon 

 dioxide to sugar, (b) the light reaction resulting in the splitting of water, 

 and (c) photosynthetic phosphorylation. Each part has involved 

 studies which fill many books. In this text, only some of the established 

 processes will be mentioned. 



A. C0 2 Conversion 



The fixation of C0 2 and its reduction to sugars is, in one sense, the 

 central, net result of photosynthesis. The simple sugars formed have 

 the general formula C n H 2n O n where n is in the range of three to seven. 

 These sugars may exist in either a straight chain or in one of several 

 ring forms, as discussed in Chapter 15. The six-carbon sugars and their 

 polymers are the ones produced in largest amounts. The various 

 hexoses are all stereo-isomers differing only in the relative locations of 

 the — H and — OH groups. They can be converted from one form to 

 another by suitable catalysts with very little expenditure of energy. 

 Thus, if the cell forms or obtains one hexose, it can, with suitable 

 enzymes, readily convert it to other hexoses. 



Forming the hexose from C0 2 and water requires energy. Specifi- 

 cally, it requires Gibbs' free energy. (This is discussed in the following 

 chapter more fully.) In describing energy changes, it is customary to 

 divide Equation 1 by six, giving 



C0 2 + H 2 -- MCH 2 0) 6 + O s (2) 



The value of the extra Gibbs' free energy per mole, AG , necessary to 

 drive this reaction to the right, has been measured to be 



AG = 116kcal/mole (3) 



