PHOTOSYNTHESIS OF CARBON COMPOUNDS 



455 



was plotted against time (Fig. 5). The maximum slope of the curve in Fig. 5 is 13 

 ^moles. This is a lower limit for the rate of appearance of i*C in soluble compounds 

 which are also stable to chromatography. It does not take into account other com- 

 pounds, too weakly radioactive to be counted, or "lost" from the front of our chroma- 

 tograms. (In order to obtain good separation of phosphate esters we customarily 

 allow the phenol-water solvent to drip from the ends of the chromatograms. Small 

 amounts of labeled fatty material are lost in this way.) 



After 30 sec, appreciable amounts of radioactivity are passing through the 

 extractable precursor compounds seen on the chromatograms into non-extractable 

 substances, which are not seen on the chromatograms. Consequently the rate of 

 appearance of **C in compounds on the paper decrease. 



During the first ten seconds, the rate of appearance of i*C in stable compounds 

 is less than the maximum rate during the subsequent time. This could be ascribed 

 to mixing time of the added H^CO^with the H^^CO^ present initially, or alternately 

 to the presence of an intermediate pool of either HCO3 or some other unstable or 



10 



20 



30 40 50 60 70 

 Time in Seconds with ^^COg 



Fig. 5. Appearance of '•'C in compounds on chromatograms prepared from Chlorella pyrenoidosa vs. 



time of photosynthesis with ^^COj. 



volatile compound. Such a compound would precede the stable soluble compounds 

 in the fixation pathway. The size of this "pool", if it exists, cannot be greater than 

 the difference between the fixation curve after 10 sec and a line of the maximum slope 

 drawn through the origin (see Figs. 3 and 5). This is no more than i.o to 1.5 /nmoles, 

 which is equal to the carbon fixed in 4 to 6 sec in this experiment. A calculation of 

 the amount of HCOJ which would be found inside algae cells in a volume of i ml with 

 an internal pH of 7 in equihbrium with 1.7 % CO^ gives a value of about i to 1.5 

 /j,moles, depending on the volume available inside the cells. It seems to us to be not 

 unreasonable to suppose that this "pool" is merely intracellular COg and HCO3 

 but it does not matter to the subsequent argument whether it is this or some other 

 unstable or volatile substance. 



From the measured rates of uptake of COj and "C and from the rates of ap- 

 pearance of i*C in stable compounds these experimental findings may be listed: 

 (a) The appearance of ^*C in stable, nonvolatile compounds, after the first 10 sec 

 of exposure of the plant to ^*C02, is equal to the rate of total uptake of "COj within 



111 



