6o PHOTOSYNTHESIS 



supplied. This immediate response occurs before any increase 

 in chlorophyll concentration takes place. Similarly during 

 the growth of Chlorella an increase in magnesium concen- 

 tration in a range which does not appreciably increase the 

 chlorophyll content, results in an increase in the rate of 

 photosynthesis. The 'internal' factors important in photo- 

 synthesis may be related to the presence or absence of indi- 

 vidual enzymes or enzyme systems but there is also the 

 possibility that the rate of photosynthesis may be deter- 

 mined by factors affecting the 'architecture' of the cell. The 

 structure and biochemical organization of the cell will deter- 

 mine the degree of mutual accessibility of the chlorophyll and 

 other components of the photosy nthetic system and studies of 

 the quantum efficiency of photosynthesis might be of value 

 in further investigations of chlorophyll deficient plants. 



Intermittent illumination. Quantitative study of the internal 

 factors in photosynthesis has been shown to be possible by 

 the use of intermittent illumination. Brown and Escombe 

 and later Warburg measured photosynthesis in intermittent 

 light and found that with high light intensities when cells 

 were rapidly alternated between light and dark the amount 

 of oxygen produced was greater than that produced in a 

 period of continuous illumination of length equal to the 

 light time. More refined experiments were possible in 1932 

 when Emerson and Arnold (1932) illuminated Chlorella with 

 short intense flashes whose duration was about io~^ to io~® 

 seconds from a neon discharge tube. The dark time between 

 successive flashes was varied and it was found that the 

 yield per flash increased with increase in length of dark time 

 until the dark period was y^t^^ second at 1° C. (Fig. 4.10). 

 Increase in dark time beyond this value did not appreciably 

 increase the yield per flash and this maximum yield corre- 

 sponded to the evolution of i molecule oxygen per 1,000 

 to 2,000 chlorophyll molecules. This result indicates that 

 there must be some factor other than chlorophyll which 

 limits the amount of energy which can be accumulated by 

 the photosynthetic system during the flash. At first it was 

 suggested that the 2,000 (or so) chlorophyll molecules were 

 arranged in a physical structure, called the 'photosynthetic 



