Action of Light Rays on Organic Compounds. 



173 



energy of formation, or combustion, of two gramme-molecules of any 

 hexose is almost equivalent to that of one gramme-molecule of a biose or 

 disaccharide, and all sugars and starches possess almost equal stores of 

 chemical energy in equal masses. 



As has been pointed out by Moore,* it is under such conditions that the 

 typical reactions in living systems occur. When the total chemical energy 

 of the reacting substances on either side of the equation is large, but the 

 difiference on the two sides is small, then typical reversible reactions are seen 

 with the equilibrium point a considerable distance removed from either 

 end-point. 



This condition of affairs so characteristic of bio-chemical reactions is due 

 to a labile balance between osmotic energy and chemical energy, and it is 

 for this reason that conjugation with formation of more complex molecules 

 is favoured by higher concentration, whereas dilute solution favours cleavage 

 into simpler molecules. This point is of great importance in biological 

 synthesis, and for understanding reversal of action within and without the 

 cell, and as it is not yet sufficiently appreciated, an example illustrating it 

 may be adduced. 



Taking the heats of combustion as determined by Thomsen, the following 

 equation gives the heat of reaction in the change between glucose and 

 maltose (K = 100 gramme-calories) : — 



Thus the reaction is apparently a slightly endothermic one, as it runs from 

 glucose into maltose (a rule which holds for all the similar reactions), but 

 observe that as the reaction runs in this direction the osmotic energy of the 

 system diminishes, for every two molecules of glucose passing out of the 

 system are replaced by only one of maltose. Hence for every gramme- 

 molecule of glucose passing into maltose a definite amount of osmotic energy 

 is set free, which can be converted into chemical energy, and hence make an 

 apparently endothermic reaction run without added energy from without. 

 The amount of osmotic energy set free by the disappearance of the molecules 

 of a constituent varies with the pressure at which it disappears, and there- 

 fore increases with the concentration of the solution,f and hence it came 

 about that the first successful experimental proof of reversibility was given 



* 'Recent Advances in Physiology and Bio-clieniistry,' edited by Leonard Hill, 

 pp. 19 -40. Arnold, London, 1906. 



t The thermodynamic proof of this is given by Moore in the articles quoted previously. 

 VOL. XC. — B. P 



C6H12O6 + C6H12O6 ^ 



(Glucose) + (Glucose) 

 6737K -f- 6737K ^ 



Ci2H220n-t-H20-33K, 

 (Maltose), 



13,507K -33K. 



