SPECTRAL CHANGES IN PHOTOSYNTHESISi 



Bessel Kok and George Hoch 



Photosynthesis Group, 



Research Institute for Advanced Studies 



Baltimore, Maryland 



Energy Transfer in Photosynthesis 



It is well known that in photosynthesis the number of sensitizing 

 pigment molecules is far in excess of the number of catalytic chains 

 \vhich process the photochemical products. In order to absorb most 

 of the light falling onto it, the plant requires a high pigment concen- 

 tration. But even in full sunlight each chlorophyll molecule receives 

 a quantum only a few times per second — a rate which is several fold 

 lower than the turnover rate of most metabolic reactions. However, 

 a mechanism is provided whereby all quanta received by a large 

 number of pigment molecules are funneled into a single reaction 

 center. Flashing light measurements (7, 12) indicate that this num- 

 ber (often called the photosynthetic unit) amounts to between 300 

 and 500 pigment molecules. 



In an array of closely spaced pigment molecules, such as we find 

 in the chloroplast, the quanta received by the various absorbers have 

 been shown to flow towards the longest wavelength absorption band: 

 in aerobic photosynthetic organisms this is the red (675 m^u.) peak of 

 chlorophyll a. In turn, a very small concentration of a pigment ab- 

 sorbing slightly beyond 675 m^u, would suffice to trap efficiently all 

 energy conveyed to its neighboring chlorophyll a molecules. It there- 

 fore could function as the focal point of the light-collecting step, 

 and be the link between photochemistry and dark metabolism. The 

 first evidence for the existence of such a pigment was obtained in 

 fluorescence studies with red and blue-green algae by Duysens (3) . 

 He observed that the fluorescence of chlorophyll a was excited to a 

 larger extent via accessory pigments (phycobilins) than by chloro- 

 phyll a itself. This was explained by assuming two fractions of 



^ The reported work was supported in part by grant No. 6692 of the National 

 Institute of Health. 



397 



