86 



BIOLOGICAL CHEMISTRY 



The observation [of Warburg that sodium and potassium 

 hydrates do not change the reaction of the cells of sea-urchin 

 eggs yet they cause an increased oxidation* may indicate that 

 the cell prevents their entry by expending chemical energy. 



The discussion on the exchange between cells and their 

 surroundings applies just as well to surfaces of separation 

 inside the individual cells. Such surfaces are present and 

 they may be of considerable importance for biological processes. 



FIG. 



17. Pulvinus 

 Mimosa. 



FIG. 1 6. Drawing showing an amoeba mov- 

 ing in the direction of the arrow. 



The round anterior end clear of granules flows out whilst 

 the posterior end is drawn in as the animal moves on- 

 wards (after Verworn). 



TRANSFORMATION OF CHEMICAL ENERGY 

 INTO MECHANICAL ACTIVITY 



Returning to the problem of the pro- 

 duction of mechanical energy by cells 

 we believe that amoeboid movement and 

 cell division are probably due to changes 

 in surface tension. 



A localised change in surface tension 

 produced either by some localised 

 change inside the cell or by the action 

 of some substance in the vicinity of the 

 cell produces amoeboid movements. It 

 must not, however, be forgotten that 

 localised changes in osmotic pressure 

 inside the cell can produce and may 

 aid such movements. A zone of in- 

 creased surface tension will cause a constriction which may 

 go on to complete separation of the cell into two daughter cells. 



Movements of leaves are produced by turgor or plasmolysis 

 of the cells of the pulvinar region. Drooping of the petiole 

 is accompanied by an escape of liquid from the cells ; the most 

 probable explanation of the escape of liquid is a change in the 

 osmotic concentration of the cell contents. 



Contraction of striated muscle is explicable by a change in 

 vapour pressure (osmotic pressure) in the anisotropic material 



* O. Warburg, Zeitschr f. physiol. Chem. f 1910, vol. 66, p. 305. 



li ,the cells at B increase in 

 size, the leaf is raised, but 

 if they shrink the leaf falls. 

 In some cases, the cells at A 

 change reciprocally to those 

 at B. The vascular tissue is 

 narrowed at the pulyinus to 

 allow greater flexibility. 



(From " Vegetable Physiology" 

 J. R. Green (Churchill). 



