THE KINETICS OF PENETRATION

Abstract

The accumulation of ammonia in Valonia macrophysa takes place more rapidly in light than in darkness. It appears to go on until a steady state is attained. The steady state conc. of ammonia in the sap is about twice as great in light as in darkness. Both effects are possibly due to the fact that the external pH (and hence the conc. of undissociated ammonia) outside is raised by photosynthesis. Certain "permeability constants" have been calculated. These indicate that the rate is proportional to the conc. gradient across the protoplasm of NHX which is formed by the interaction of NH or NH0H and HX, an acid elaborated in the protoplasm. The results are interpreted to mean that HX is produced only at the sapprotoplasm interface, and that on the average its conc. there is about 7 times as great as at the sea-water protoplasm interface. This ratio of HX at the 2 surfaces also explains why the conc. of undissociated ammonia in the steady state is about 7 times as great in the sea-water as in the sap. The permeability constant P''" appears to be greater in the dark. The pH of sap has been detd. by a new method which avoids the loss of gas (CO2), an important source of error. The results indicate that the pH rises during accumulation but the extent of this rise is smaller than has hitherto been supposed. As in previous expts., the entering ammonia displaced a practically equivalent amt. of K from the sap and the Na conc. remained fairly constant. It seems probable that the pH increase is due to the entrance of small amts. of NH3 or NH4OH in excess of the K lost as a base.