PHOTOSYNTHESIS 



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(Readings: Weisz, pp. 241-263. S.P.T., pp. 95-100. Vill^e, pp. 94-103. E. I. 

 Rabinowitch, "Photosynthesis," Sci. Am. 179, No. 2, pp. 24-34, Aug. 1948, 

 Reprint No. 34. D. I. Arnon, "The Role of Light in Photosynthesis," Sci. 

 Am. 203, No. 5, 104-118, Nov. 1960, Reprint No. 75.) 



The energy that supplies all life on the earth 

 comes ultimately from sunlight, through the 

 process of photosynthesis. Each year plants on 

 the earth reduce about 550 billion tons of carbon 

 dioxide, using about 25 billion tons of hydrogen, 

 and releasing about 400 billion tons of oxygen 

 into the atmosphere. About nine-tenths of this 

 activity goes on in the surface layers of the 

 oceans. 



No industrial process yet invented converts 

 light economically into useful forms on a large 

 scale. For this reason our economy still depends 

 largely upon the combustion of fossil fuels, 

 themselves the products of photosynthesis in 

 past ages. We have only recently begun to 

 understand how plants accomplish this feat. 



For light to be used, it must be absorbed; 

 and substances which absorb visible light are 

 by that token pigments. The pigments which 

 absorb the light used in photosynthesis are 

 found in the chloroplasts of green plants, and 

 in similar particles called chromatophores in 

 photosynthetic bacteria. The principal pigment 

 of chloroplasts is chlorophyll a. Chlorophyll b 

 and the yellow carotenoids play secondary roles, 

 transferring the energy they absorb as light to 



chlorophyll a for use in photosynthesis. Photo- 

 synthetic bacteria possess a special bacterio- 

 chlorophyll, and also a number of specific 

 carotenoids. 



The net action of light in photosynthesis is to 

 split water, thus providing hydrogen for reduc- 

 tions and eliminating oxygen as a by-product: 



I2H2O 



light 



chloroplasts 



^ 24H + 6O2. 



The H atoms supplied in this way are used to 

 reduce CO2 to carbohydrate and water: 



6CO2 + 24H 



18 ATP 



CeHiaOe + 6H2O. 



Thus the overall reaction is 



light 



6CO2 + I2H2O 



chloroplasts 



C6H12O6 + 6H2O + 6O2. 



To fix one molecule of CO2 in the form of 

 carbohydrate requires not only 4 H atoms but 

 also 3 "high-energy" phosphate bonds of 

 adenosine triphosphate (ATP). The structure 

 of ATP and some of its sources are discussed 

 in Exercise XI. It is now recognized that the 



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