388 PLANT METABOLISM 



the action of microorganisms. In addition to tlie reactions shown, carbon 

 dioxide may be added to the cycle from volcanoes and hot springs, and 

 removed from the cycle as carbonates. 



In addition to reducing carbon dioxide, photosynthesis releases oxygen, 

 and this oxygen is used by animals and plants for their energy-releasing 

 respiratory processes. This oxidation destroys the substances produced 

 in photosynthesis, but the energy made available in this way is essential 

 for maintenance of animal and plant life, as is discussed in Chap. 16. 

 Life is dynamic, having both its anabolic and catabolic phases, but only 

 the plants are capable of the net synthesis essential for keeping the overall 

 life processes in balance. 



The simplest formulation of the photosynthetic reaction is: 



n(CO0 + 2n(H,0) ,,,i,lte"ne.gy * (CH2O), + n(0,) + n(H,0) + n(119.6) Cal. 



Carbon dioxide is reduced to an end product with the empirical formula 

 of a carbohydrate (CH20)„. The energy stored in this reduced com- 

 pound is supplied the plant as radiant energy. The oxygen evolved all 

 originates from water, so it is necessary to designate 2 molecules of water 

 on the left side of the equation. 



Photosynthesis can be described as a sensitized, photochemical, oxida- 

 tion-reduction reaction. The sensitizer is chlorophyll, for it captures 

 light and functions in the transformation of radiant into chemical energy. 

 The reaction is in part a photochemical one, for light energy is required 

 to drive it. It is an oxidation-reduction with carbon dioxide serving as 

 the oxidant and water as the reductant. The carbon dioxide is reduced 

 to the level of carbohydrate, that is, to substances containing as many 

 oxygen atoms as carbons and twice this number of hydrogens. This 

 description applies only to monosaccharides; polysaccharides have the 

 same reduction level, but water has been removed from them. 



Chlorophylls 



The green pigment chlorophyll serves as the photosensitizer in the 

 photosynthetic reactions. The first requirement in the utilization of light 

 is that the light be absorbed, and chlorophyll accomplishes this. From 

 the absorption spectrum for chlorophyll a, shown in Fig. 15-2, it is evi- 

 dent that the pigment absorbs strongly in the blue and red regions of 

 the spectrum and transmits much of the green portion of white light. 



Chlorophyll in the cells of higher plants is concentrated in chloroplasts. 

 The chloroplasts usually are discs about 5 microns in diameter, and within 

 them the chlorophyll is further concentrated in minute bodies called 

 grana. Although lacking chloroplasts, the blue-green algae do contain 

 grana, which are distributed throughout the cells. 



The chlorophyll molecule apparently does not occur free in the cell 

 but is bound to proteins and lipides. It is suggested that aqueous organic 



