514 W. F. LOOMIS 



the maintenance of oxaloacetate levels as well as playing a vital role in 

 setting the pH of the interior of the cell. In contrast to this high reactivity, 

 molecular oxygen combines almost solely with cytochrome oxidase and 

 that in a manner that is independent of the level of oxygen tension except 

 at the lowest levels [4]. As a possible regulator of cellular differentiation, 

 therefore, CO2 is a more likely candidate than oxygen ; in addition, it has 

 the cybernetic advantage of being actively produced rather than used up 

 with the resulting difference that pCOg increases from nearly zero at the 

 surface to its highest point at the centre, while oxygen tension levels do 

 the reverse and hence are far more at the mercy of the over-all environ- 

 ment. 



Before continuing, certain facts concerning CO2 must be reviewed, for 

 they are vital to any understanding of the subject. Thus, it is well known 

 that pH, or concentration of the hydrogen ion, is something quite different 

 from the total amount of acid present in a buffered solution. In the same 

 way, pCOa, or the partial pressure of dissolved CO., gas, is something 

 quite different from the total amount of CO2 that may be in a solution in 

 such hydrated forms as bicarbonate and carbonate. Only free gaseous CO2 

 is given off by a solution on simple aeration ; both free and bound CO2 are 

 given off following the addition of acid. 



Everyone knows the characteristic taste of dissolved free CO2 gas, for 

 the taste buds of the tongue are uniquely sensitive to this variable as found 

 in beer, champagne and similar carbonated beverages. A vivid demonstra- 

 tion of this fact can be arranged in the laboratory by simply filling a large 

 syringe with gas from a tank of COo and then shaking it with a small 

 amount of water that has subsequently been drawn into the syringe. Since 

 this water sample has been equilibrated with 100% of an atmosphere of 

 COo (760 mm. Hg), it has a PCO2 of 100% atm. The basic fact is that 

 pressures of any dissolved gas equalize whenever gas and water phases are 

 shaken together; if the gas phase has a partial pressure of pCOg of 100% 

 atm., then the water phase within the syringe has an equal PCO2. It should 

 be noted that syringes are extremely useful because they provide the 

 operator with an adjustable volume that is always automatically main- 

 tained at a pressure of i atm. In contrast to this simplicity, the concentra- 

 tioti of free COo dissolved within the water phase changes with both the 

 temperature and the ionic strength of the solution, for Henry's Law states 

 that [COo] = cc pCOo and both temperature and ionic strength affect the 

 solubility coefficient a. 



If the water in such a syringe is now expressed into a small beaker and 

 placed beside a similar beaker of plain water, the demonstrator may 

 remark, "Oh, I have forgotten which is which. I wonder which has the 

 PCO2 of 100% atm. and which has a PCO2 equal to that of air ? " (0-03% 

 atm.). At this point the onlooker may be challenged to figure out some 



