108 Inside the Living Cell 



but one which occurs in a number of stages. This is probably an ad- 

 vantage from the plant's point of view because it has to make large 

 numbers of other compounds besides carbohydrates and the inter- 

 mediates of the carbon dioxide-sugar process are also available for 

 carrying out many other necessary reactions. It is interesting to find 

 that the basic process of photosynthesis is the splitting of water (H2O) 

 into H and OH, and the use of the reactive hydrogen atoms to bring 

 about reactions which result in the production of energy-rich com- 

 pounds, on which all animal life depends. 



Since all life still depends on photosynthesis, it is impossible not 

 to conclude that the earliest manifestations of life made use of the 

 energy of radiation. As we have seen, the ionizing radiations produce 

 highly reactive species of molecules directly and it is quite possible 

 that at the beginning the process which led to living things used the 

 more penetrating radiations. But in the photosynthetic mechanism of 

 the living plant, a means has been found of using the much gentler 

 visible radiations of light, which are harnessed in the mechanism of 

 the chloroplast. This is effected by the chlorophyll molecules which 

 can absorb visible light and use it to split water. The chlorophyll sys- 

 tem acts as an energy transformer, and a very efficient one, for the 

 conversion of light into chemical energy. 



Theoretically the energy of at least three photons of visible light^ 

 should be required to turn one molecule of carbon dioxide into 

 carbohydrate. Many measurements have been made to determine 

 the number actually required in living cells. The results are rather 

 variable owing to the extremely variable circumstances which ob- 

 tain; but under good conditions a value of eight photons per molecule 

 has been observed. This indicates that the efficiency of the photo- 

 synthetic apparatus in converting light energy under the best condi- 

 tions is at least 35 per cent. 



The amount of solar radiation falling on the earth is quite 

 enormous. For example, in the United States, on the average 1,000 

 calories reach each square foot of land during the daytime every 

 minute.^ For an acre of land this equivalent to the output of an elec- 

 trical station generating 2,600,000 kilowatts ! About half of this is 

 made up of heat rays not available for photosynthesis. The total 

 amount of energy which could actually be used for photosynthesis 

 under the best possible conditions is thus about J X 35 per cent X 

 1,000 = 175 calories per sq foot per minute or 3-85 X 10^ calories 



^ The photon is hr where h is the Planck constant and v the frequency of the 

 light, hv varies from 40 to 70 kcals. per molecule according to the wave- 

 length of the visible light. 

 2 This is enough to heat 1 litre of water 1 °C. 



