210 PLANT BIOCHEMISTRY 



dOmM 



-^ 500 rriM 



FIGURE 9-2. A diagram of a lamella (disc) of a granum of tulip thloroplast. 

 Viewed from above instead of edgewise, the lamella is circular. Each circle in the 

 diagram represents a spherical macromolecular component which is aligned with 

 its neighbors to form the indicated flattened envelope. About ten of these lamellae 

 are stacked vertically to form the granum. In the chloroplast itself, the grana are I 

 distributed throughout a matrix called the stroma. 



Absorption of light by the photosynthetic pigments must result in 

 structural changes of some sort in these pigments. It is now believed 

 that these changes involve shifts of electrons to higher energy levels 

 corresponding to excited states. Photosynthesis may require particular 

 excited states reached only by absorption of light having definite 

 energies. One current theory assigns two unpaired electrons to the 

 excited molecule in contrast to the completely paired electrons in 

 the systems of normal molecules. The energy inherent in the unpaired 

 electrons comes from the absorbed light and is transferred along the 

 series of pigments to chlorophyll a. Postulated mechanisms of transfer 

 require a series of pigments placed relatively close to one another, 

 suggesting an organized structure. (See Figure 9-2.) The mechanism 

 of utilization of the energy of excited chlorophyll a is still largely 

 unknown. Somehow the energy is used to dissociate water, leading 

 ultimately to the formation of oxygen and the reduction of carbon 

 dioxide. 



The photochemically active structural unit anticipated above, called 

 the chloroplast, is readily observed in and isolated from green plants. 

 Its structural organization has not been resolved, but it is believed 

 to contain the pigments and enzymes participating in photosynthesis. 

 In fact, isolated chloroplasts supplied with suitable cofactors carry 

 out some of the processes of photosynthesis. 



Reaction of Water 



During photosynthesis, the absorption of light and the transfer of 

 the resulting energy from one pigment to another are followed by 

 chemical reactions leading to all the components of cells. Many of 

 these steps are only remotely concerned with photosynthesis and will 

 be discussed later in the other sections on metabolism. The reactions 



