THE MECHANISM OF PHOTOSYNTHESIS 329 



in the photosynthetic process. After passing into solution in the water of a 

 mesophyll cell wall part of the carbon dioxide reacts with water forming 

 carbonic acid (H2CO3) and diffuses to the chloroplasts as this compound. 

 Some of the carbon dioxide, however, diffuses to the chloroplasts in true 

 solution. 



Over a temperature range of about 10-25° O., if light intensity and 

 carbon dioxide concentration are relatively high, the temperature coefficient 

 (Chap. VII) of photosynthesis is approximately two. Strictly chemical reac- 

 tions characteristically have a temperature coefficient of from two to three. 

 This fact indicates that at least one of the reactions involved in photosynthesis 

 is of a purely chemical type. Since this fact was first pointed out by Black- 

 man, this reaction is often called the Blackman reaction. It is also frequently 

 referred to as the dark reaction, since it does not require light, and therefore 

 may take place in either the light or the dark. It is possible that more than 

 one such "dark reaction" may be involved in photosynthesis. 



A chemical reaction which proceeds only at the expense of absorbed light 

 is called a photochemical reaction. That photosynthesis involves such a reac- 

 tion can be inferred from the fact that it occurs only in the light. The tem- 

 perature coefficient of photochemical reactions is approximately one. Under 

 low light intensities, even with a relatively high carbon dioxide concentration 

 and other conditions favorable for photosynthesis, the temperature coefficient 

 of the process is about one, indicating that under such conditions the rate of 

 photosynthesis is limited by its photochemical phase. 



That photosynthesis involves both a photochemical and a chemical reaction 

 is also shown by the results of investigations in which plants are exposed to 

 intermittent light. The most recent experiments of this type have been per- 

 formed by Emerson and Arnold (1932). When cultures of Chlorella (an 

 alga) were exposed to the intermittent illumination at the rate of 50 flashes 

 per second, the periods of illumination being much shorter (0.0034 sec.) than 

 the intervening dark periods (0.0166 sec.) the photosynthetic yield per unit 

 of light was increased about 400 per cent as compared with the rate in con- 

 tinuous light. 



Assuming, as most evidence indicates, that the photochemical reaction 

 comes first, the results just described can be explained as follows. When 

 illumination is continuous the products of the light reaction are formed faster 

 than they can be utilized in the relatively slower dark reaction. When the 

 light is intermittent, however, all or most of the products of the photochemical 

 reaction are removed by the dark reaction during the intervening dark period, 

 and the photosynthetic output per unit of light is considerably greater. 



By experimenting to find out how long a dark period was required for 



