TRANSPORT OF WATER FROM CONCENTRATED TO DILUTE SOLUTIONS IN CELLS OF NITELLA

Abstract

The transport of water from concentrated to dilute solns. which occurs in the kidney and in a variety of living cells presents a problem of fundamental importance. If the cell acts as an osmometer we may expect to bring about such transport by creating an inwardly directed osmotic drive which is higher in one part of the cell than in other regions of the same cell. The osmotic drive is defined as the difference between internal and external osmotic pressure. Expts. with Nitella show that this expectation is justified. If water is placed at one end of the cell (A) and 0.4 [image]_ sucrose with an osmotic pressure of 11.2 atmospheres at the other end (B) water enters at A, passes along inside the cell and escapes at B leaving behind at B the solutes which cannot pass out through the protoplasm. Hence the internal osmotic pressure becomes much higher at B than at A. When 0.4[image] sucrose at B is replaced by 0.3 [image] sucrose with an osmotic pressure of 8.1 atmospheres, we find that water enters at B, passes along inside the cell and escapes at A so that water is transported from a concentrated to a dilute soln. although the difference in osmotic pressure of the 2 solns. is more than 8 atmospheres. The sucrose soln. at B thus becomes still more concentrated. It is evident that, if metabolism produces a higher osmotic pressure and consequently a higher inwardly directed osmotic drive in one region of the cell as compared with other parts of the same cell, water may be transferred from a concentrated to a dilute soln. so that the former soln. becomes more concentrated.