in the dark indicates the presence of some reducing cofactors, 

 either remaining from the light or derived from some other 

 metabolic reaction. 



Before we discuss the evidence for the remainder of the 

 carbon reduction cycle, we must describe another type of 

 experiment with C^^Oa and photosynthesizing algae. In these 

 experiments, algae are first permitted to photosynthesize for 

 20 minutes or more in the presence of a constant supply 

 of C^*02. During this time environmental conditions are 

 maintained nearly constant (temperature, CO2 pressure, 

 light intensity, etc.). After about 10 minutes of exposure to 

 C^*02, so much radiocarbon has passed through the various 

 biochemical intermediate compounds on its way to end 

 products that each carbon atom of each intermediate com- 

 pound contains, on the average, the same percentage of C^^ 

 atoms as the CO2 being absorbed. In other words, the specific 

 radioactivities of all the carbon atoms of all the early inter- 

 mediates are the same as the specific radioactivity of the 

 entering radiocarbon, which can be measured. 



At this point samples of the algae are removed without 

 disturbing the rest of the algae, and these samples are killed 

 and subsequently analyzed by the methods described. The 



MALIC ACID 



ALANINE 



SERINE ASfWlTIC AQD 



PEPA 



i 



20 SEC DARK C'^Oj FIXATION ^0 ^''^ 



AFTER PREILLUMINATION 

 CHLOiELLA SUGAR PHOSPHATE^ 



Figure 5. Radioautograph of chromatogram of products of 

 20 seconds C^^Oa fixation by Chlorella pyrenoidosa in the dark 

 following a period of photosynthesis. 



18 



