SCOPE OF THE PROBLEM 



move freely into and out of the cell. This is generally taken to be 

 the case, although the view is seriously challenged by some investi- 

 gators (cf. Robinson, 1954). Direct experimental measurements of 

 the osmotic pressure of the cell interior are beset with great diffi- 

 culties, especially in preventing spontaneous formation of osmolites. 

 Means of avoiding these changes have been described, together with 

 evidence that the osmotic pressure lies near that of the extracellular 

 fluid (cf. Conway and McCormack, 1953; Brodsky et al., 1953). 



Obviously, we are not to think of the cell as filled with a 

 homogeneous sap. The nuclear region may or may not be essentially 

 continuous with the bulk of the cytoplasm, as far as the movement 

 of small solute molecules is concerned, but the mitochondria and 

 the endoplasmic reticulum undoubtedly present separate phases. For 

 some cells as much as 60 per cent of the volume may be taken up 

 by reticulum and other organelles. For certain other cells this pro- 

 portion is small. The osmotic and transport behavior of the mito- 

 chondrion is an emerging subject, which we shall encounter briefly 

 in Chapter 6. Electron-microscope studies have suggested to some 

 cytologists (cf. Palade, 1956) that the cisternae formed by the endo- 

 plasmic reticulum may communicate with the exterior of the cell 

 and, therefore, contain a fluid resembling the extracellular fluid (Fig- 

 ure 2). Such an arrangement, if it really exists, could increase the 

 surface available to transport between the cellular and extracellular 

 phases. As will be discussed on page 73, this interpretation has, 

 however, only limited support and acceptance. 



Although a free movement of water into and out of the interior 

 of cells is widely believed to occur, at least some cells must present 

 comparatively effective barriers to water movement; otherwise the 

 urine and other secretions could not become hypo- or hypertonic 

 to the extracellular fluid. This barrier action to water appears to 

 be diminished by vasopressin, one of the polypeptide hormones of 

 the posterior pituitary gland. 



In the bacterial and plant world, the net osmotic movement of 

 water may also be severely limited, apparently because of the pres- 

 ence of a rigid cell wall. Once this has been removed experimentally 

 from bacterial cells, the resulting spheroplast or protoplast (Figure 

 3) becomes osmotically sensitive. The experiments of A. Fischer 

 showed long ago (1897, 1900) that bacterial cells also are only selec- 

 tively permeable and that they also possess a degree of osmotic 

 sensitivity. It has been held in recent years that cells of coliform 



9 



