Chapter VIII — 147 — Active Relations 



point determinations, and protoplasmic factors. Forces regulating the state of water 

 in protoplasm may constitute one of the latter. 



Frost resistance in plants may be related to protoplasmic factors including in- 

 creased permeability and decreased viscosity, both of which are attributed to a greater 

 hydrophily of the protoplasm. Such a condition might involve a reversible increase in 

 protoplasmic water at the expense of the vacuole brought about by unfolding of pro- 

 tein chains or opening of ring structures to expose more points for hydration, such 

 structure being maintained by metabolic energy. 



The evidence for active control of water by animal cells and tissues is such that 

 water secretion, or active transport, appears likely in several instances. 



A comparison of solute and water absorption has suggested that aerobic respiration, 

 protein synthesis, water absorption, and salt accumulation are mutually dependent 

 processes. 



Active cell water relations may affect many other plant functions. Transpiration, 

 translocation of salts and water, plant movements, and glandular secretion are a few. 



Several mechanisms have been suggested to account for active water control by 

 the protoplasm. Anomalous osmosis involving the carrying of water of hydration by 

 ions during accumulation is one. Electroosmosis, the movement of liquid along a 

 charged solid surface (pore wall) due to a potential difference across a membrane is 

 another. Differential ion migration and a heterogeneous pore system contribute to 

 such a potential difference. Continuous diffusion of solute may account for a potential 

 difference and provide the driving force for electroosmosis. A continuous cycle in- 

 volving enzymatic conversion of malic acid to carbohydrate in the protoplasm, diffu- 

 sion of the latter into the vacuole and breakdown to malic acid again is one suggestion. 

 Salts probably play an important role in any anomalous movement of water across 

 membranes. The continuous production and removal of H+ and HCO3" ions may be 

 one source of solute for continuous diffusion from the cell. 



Though the argument for active water regulation in plants is supported by an in- 

 creasingly wide array of data, the overall evidence at present is inconclusive. One 

 does, however, gain the impression that the classical views of cell water relations can- 

 not account for many of the recent findings. There may be as yet too many unknown 

 factors related to protoplasmic structure, permeability, and imbibition to allow a simple 

 osmotic explanation of all cell water relations. 



