Physical Chemistry of Vital Phenomena; For Students and Investigators in the Biological and Medical Sciences (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER VIII PERMEABILITY OF CELLS It was shown by Nfigeli, Pfeffer and de Vries that concentrated solutions of neutral salts cause plant cells to shrink by extracting the water from them. The shrinkage, plasmolysis, of plant cells may be easily detected because, though the cell wall does not shrink, the shrinkage of the protoplasm separates it from the cell wall, leaving a space between the two. Solutions with equal plasmolytic power de Vries (1884) called isotonic solutions, and showed a relation between their percentage concentrations and the molecular weights of the solutes. This was the foundation of van't Hoff's theory of osmotic pressure. The discrepancies in the relation between the concentration of isotonic solutions and the molecular weight of the solutes was explained by Arrhenius on the assumption that electrolytic dissociation increases the number of particles in solution. In this way a .method was developed of measuring the osmotic pressure of plant cells. The osmotic pressure of solutions that just fail to plasmolyze the cells is equal to the osmotic pressure of the cells. If animal cells are used, some method of determining the first decrease in volume must be used. When the cells are in large masses, changes in volume may be determined by weighing the masses. If the cells are free, as with blood corpuscles, their volume may be determined by centrifuging them in graduated tubes (Hamburger, 1893). Some substances do not plasmolyze cells no matter how great the concentration, due to the fact that the cells are permeable to the substances which consequently cannot exert any osmotic pressure on the cells. Others cause only a temporary plasmolysis because they slowly penetrate the cells. When the concentration becomes the same outside and inside they cease exer...