Chapter VII 



109 — Osmotic Quantities of Cells 



moved. Priestley (1920) and Crafts and Broyer (1938) have attempted 

 to picture mechanisms to explain the uptake of vi^ater by roots (cf. Chapter 

 IX, pages 149 and 150). 



"V 



O P=/2fltmos. 



WP«6 



CPD'6 



OP^IO/itmoi. 



-A^ 



Direction of water movement 



Fig. 35. — Diagrammatic illustration of water 

 movement along a gradient of DPD. (Meyer, 1938). 



Summary: — The osmotic quantities of plant cells are osmotic pressure, diffusion 

 pressure deficit, and turgor pressure. The dynamics of water movement and retention 

 can be explained in terms of these quantities. 



As with an osmometer also with cells the relation OP = DPD + TP is funda- 

 mental to an understanding of osmotic relations. Under certain conditions of water 

 stress TP may pass the zero level and pressure may become subatmospheric. 



Under OP all forces leading to water absorption by the cell are included. When 

 tension exists, its value must be added to the OP. Increase in hydrophily of the proto- 

 plasm results in water uptake. The OP of the vacuolar sap reflects the integrated re- 

 sultant of the interaction of all forces causing water to enter the cell. Under TP are 

 included forces acting in the opposite direction. 



In order that osmotic pressure of a solution may find expression in turgor a differ- 

 entially permeable membrane must separate the solution from a more dilute solution 

 or from pure solvent. In the cell the protoplasm serves as this membrane. Its selec- 

 tive capacity is believed due to two cytoplasmic membranes, the ectoplast, and the 

 tonoplast. These are pictured as a complex of lipoid and protein. Some physical 

 membranes act as sieves, others as selective solvents. The lipoid-sieve theory com- 

 bines the essential features of both to explain the permeability of protoplasm. 



Active absorption is responsible for most solute uptake by the cells of living 

 plants. Essential to this process are oxygen supply, temperature, organic nutrients, 

 the salt status of the cell, and the nature and concentration of solutes in the external 

 medium. 



Some ions may be taken up from the solid phase of the soil by contact exchange. 

 Polyuronides in the root hair walls make a connecting medium between the soil and 

 the cell protoplasm capable of ion transfer. The passive permeability of cells to ions 

 does not account for active absorption by living cells. 



Cells are generally readily permeable to water. Cells may vary among themselves 

 with respect to water permeability. 



Tugor is contingent upon osmotic water uptake; absorption and movement are 

 dependent upon osmosis. An external solution which reduces turgor to zero is said 

 to be isotonic with respect to the cell sap. Such a solution is used in determining the 

 osmotic pressure of the cell by the plasmolytic method. 



Many objections have been raised to the plasmolytic method, nevertheless it has 

 proved valuable in studies of cell physiology. Some of the more pertinent criticisms 

 of the method involve adhesion of the protoplast to the wall, plastic stretching of the 

 wall, and the technical difficulties in correctly observing the critical state of incipient 

 plasmolysis. By plotting the number of plasmolyzed cells against sugar concentration 

 an average OP value for many cells may be obtained. 



Several forms of plasmolysis have been observed, the more important being con- 

 vex and concave. Plasmolysis may follow chemical or physical stimulation. Vacuole 

 contraction is another type of response. 



The plasmometric method involves strong plasmolysis followed by measurement 

 of cell and protoplast. Osmotic pressure is calculated from these. 



The cryoscopic method is widely used. It is relatively free of errors but involves 

 the expression of sap and this has definite limitations. A differential thermometer is 

 used to determine freezing point lowering of sap; a thermocouple is usually used to 



