PHOTOSYNTHESIS OF CARBON COMPOUNDS 453 



Rate measurements 



Gas exchange: Measurements of the rates of COj uptake, ^*C uptake and O^ 

 evolution by the photosynthesizing algae are made by taking the slopes of the three 

 traces on the recorder. In order to obtain accurate readings in 10 min or less, the 

 total effective gas volume of the closed circulating system is made small, about 

 435 ml. With 80 ml of 0.5 % algal suspension in the system the resulting change in 

 Oj or COg pressure is about 0.5 % in 10 min in a typical experiment. This corresponds 

 to a rate of 22 /umoles of gas exchange/min/ml of wet packed algae. The response of 

 the Beckman Infrared Analyzer, model 15 A, used in these experiments is not 

 completely linear in the range used (0 to 2.0 °o CO,) so that a correction based on a 

 previously obtained calibration curve is applied to the COg uptake curve plotted 

 on the recorder. The response of the A. O. Beckman oxygen analyzer is essentially 

 linear in the range used (19 to 21 °o). The level of '■*€ is plotted on the recorder as 

 millivolts response of the Applied Physics Corpn.'s Vibrating Reed Electrometer to 

 the ionization chamber (volume 118 ml, R = 10' ohms). From the known calibration 

 of the ionization chamber this reading can be directly converted to /xC of "C. From 

 the ^^COg reading and the ^*C reading the specific radioactivity of the CO2 may be 

 calculated at all times during the experiment. This specific radioactivity is used to 

 convert the rates of change of radioactivity in the system to rates of change of what 

 we shall call ""C" throughout this paper. For convenience of expression and calcula- 

 tion, this '^C will be expressed in ;umoles and represents the amount of ^^C and "C 

 corresponding to a given measured amount of radioactivity in the CO 2 administered 

 to the algae at any time during the experiment. 



Total fixation in algae: In some experiments, small aliquot portions of each 

 sample of algal material, taken and killed in alcohol during the course of the experi- 

 ment, are spread in a thin layer on planchets with acetic acid, dried, and counted. 

 The amount of '^C found at each time of exposure of the algae to "CO 2 is plotted 

 and the slope of the curve drawn through these points gives the rate of appearance 

 of "C in stable compounds in the plant. 



Fixation of ^*C in compounds found on the paper chromatograph : After the '^C 

 in individual compounds found on the paper chromatogram has been measured, the 

 amounts are sometimes totaled for each sample up to one minute, and a rate of ap- 

 pearance of "C in these compounds is calculated. 



RESULTS 

 Steady state Expt. 18 



The rates of exchange of gases before, during and at the end of the experiment 

 are shown in Table IV. We shall take 15.5 ^moles/min as an average value for uptake 

 of carbon during the experiment. 



Ahquot portions of the samples were dried on planchets and their radioactivity 

 was counted. When results of these counts were plotted versus time of sampling, the 

 rate of fixation of "C into compounds stable to drying on the planchets was found 

 to be about 15 /xmoles/min (Fig. 3). 



After chromatographic separation of the compounds, radioautographs, of 

 which Fig. 4 is typical, were obtained. The radioactivity of each compound in each 

 sample was determined and the total radiocarbon found in the various compounds 



109 



