CELL DIFFERENTLATION ^ly 



the large flat culture beds that are exposed to sunlight and the tall towers 

 through which ascending CO., gas is bubbled through the descending 

 culture medium that is circulated through the culture beds (Fig. 4). If 

 the bicarbonate ion were equivalent to free CO.,, then the photosyn- 

 thesizing troughs would not have to be covered with a transparent plastic 

 cover that reduces the available light but keeps in the vital CO., gas. All 

 that would be necessary would be to add bicarbonate to the culture media 

 and grow the algae in direct sunlight. 



Jacobs [5] has described an elegant experiment to show that free CO., 

 can selectively penetrate cell walls and produce intracellular acidity e\"en 

 in alkaline solutions. He found a flower whose colour changed reversibly 

 from blue to red when it was dipped in alkaline or acid solutions, an 

 internal type of litmus reaction that could be used to determine intra- 

 cellular pH. When this flower was placed in an alkaline solution that had 

 a pCO., of 5",, atm., the colour of the flower changed from blue to red, 

 indicating that the free gaseous COo in solution had penetrated the fatty 

 cell membranes and had ionized inside these cells to form carbonic acid. 



Accepting pCO.^ therefore, as an important but unfamiliar biological 

 variable, what are the units used to measure it .- The easiest unit I believe 

 is as a percentage of an atmosphere, for almost all biological reactions are 

 carried out near sea-level and hence simple percentages mav be used rather 

 than the more confusing pressure units such as millimetres of mercury. 

 Take for example the syringe described above in which water and pure 

 carbon dioxide gas had been shaken together to equilibrium. The water in 

 this syringe has a pCOo of 760 mm Hg. but it is just as true, and more 

 vivid operationally, to say that it has a pCOo of 100",, of an atmosphere. 

 This latter terminology brings to mind the method of making such 

 solutions, for water shaken with 50",, CO2 gas mixtures has a pCO.^ of 

 5o"o of i^ii atmosphere; that shaken with lo*^,', CO.^ has a pCO., of lo'^'o 

 atm. ; and that shaken with i",, CO., has a pCO._, of i",, atm. 



This, then, is the simplifying principle behind our nomenclature that 

 allows one to handle concentrations of free COo with the same ease as one 

 handles calcium ion concentration. •- 



We have recently devised a rapid means of measuring pCO., that does 

 not involve pH and so avoids many of the complications that ha\ e plagued 

 this field [6]. The essence of the measurement is to shake 10 ml. of the 

 sample of water to be tested with an equal volume of air and then to 

 determine how much CO., enters the gas phase from the solution. The 

 results ot this measurement are simply multiplied bv a factor to obtain the 

 pCOo of the original solution. In our laboratorv this summer, my son and 

 I had side-by-side arrangements for measuring pH and pCOo. This en- 

 abled us to examine all cultures from both these angles, as well as to vary 

 pCO.,, pH and bicarbonate concentration relative to one another. Since the 



