106 MINERAL SALTS ABSORPTION IN PLANTS 



membrane may be carried about by membrane flow as was suggested 

 by Bennett (1956) and transported into the bulk of the cytoplasm 

 by vesiculation (Fig. 12e, p. 43). Ions are thought to enter animal 

 cells by a similar mechanism (Holter 1959). Evidence for invagina- 

 tion of the surface membrane in plants comes from electron 

 microscopy of meristematic cells (Buvat, 1958; Whaley et al. 1960), 

 and it is not possible at present to judge the situation in vacuolated 

 parenchyma. During cell expansion, the cytoplasm is probably 

 stretched as vacuolar volume increases, and invaginations of the 

 surface membrane m.ay tend to disappear in the mature turgid cell. 

 This might account to some extent for the reduced rate of absorption 

 of salts in turgid tissues (p. 63). 



c. Mitochondria. Mitochondria, chloroplasts, nuclei and 

 vacuoles presumably compete with one another for salts transferred 

 into the cytoplasm across the plasmalemma. Direct evidence that 

 cytoplasmic particles accumulate salt was obtained by Mullins 

 (1940). He allowed Nitella plants to absorb salt containing '♦^K for 

 a short time, and then centrifuged them so that the particles were 

 moved to one end of each plant. It was found that the half of each 

 cell which was richer in particles also showed the greater 

 radioactivity. The particles in question were probably those which 

 we now call mitochondria. These are easily recognizable cyto- 

 plasmic organelles, about 0-5/^ in diameter, 10/i or more in length 

 (Plate I). Each mitochondrion is bounded by two parallel 

 membranes, each 40-60A thick, separated by a distance of 60-90A. 

 The inner membrane is invaginated to give rise to a system of 

 internal membranes, the "cristae mitochondrials". Biochemically, 

 mitochondria are recognized to be the sites of a number of important 

 enzymes, including those involved in the Krebs cycle, and in 

 oxidative phosphorylation. 



Considerable advances in knowledge of biochemistry have 

 followed the discovery that mitochondria extracted from cells, with 

 suitable precautions remain metabolically active. One of the 

 necessary requirements is that the particles should be suspended in 

 a medium of high osmotic pressure. If the osmotic pressure of the 

 extracting medium is too low the mitochondria swell and burst. 

 This is taken as evidence that the mitochondrial membranes are 

 differentially permeable to water and solutes. The observation that 

 mitochondria tend to congregate at secretory surfaces in animal 



