146 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



In Calvin's view, in the organized and oriented system containing chloro- 

 phyll, absorption of light liberates an electron. As depicted in Fig. 28, 

 water is on one side of the photobattery and the disulphide form of thioctic 

 acid on the other. Electrons move towards the thioctic acid layer and the 

 sites which are left empty are immediately filled up by electrons coming 



CO2 COj 



H 



o— H 



o— H 



I 



H 



SH SH 



TPNH 



• 



o + 



+ 



1 o 



o 



o + 



■"0 • 



2H00 



H 



I 

 O— H 



O— H 

 I 

 H 



1^ 

 2H- 



[H2O,] 



\H20 



Fig. 28 (after Calvin) — The role of thioctic acid in the first stage of photosynthesis. 



from the water molecules. In particular, this photobattery produces a 

 high concentration of TPNH. 



For the moment, the scheme in Fig. 28 is sufficient to illustrate the 

 fact that the energy-transformer at the primary stage of photosynthesis is 

 different from the chemical type of transformer. Later, we shall return to 

 the question of photosynthesis. 



VII. THE PYROPHOSPHATE BOND AND CELLULAR WORK 



The pyrophosphate bonds of ATP are the coins which pay for the 

 performance of cellular work, and there are as many examples of their use 

 in this way as there are types of dynamic biochemical reaction. 



A very common type of cellular work is the transport of a molecule 

 against the concentration gradient, for example, the transport of glucose 

 from a region where its concentration is low, through a membrane, to a 

 region where the concentration of glucose is higher. Such a case is 

 represented in Fig. 29. On the left-hand side of the membrane, corre- 

 sponding to the low concentration of glucose, the glucose is phosphorylated 

 in the presence of hexokinase at the expense of a molecule of ATP and 

 G-6-P is formed. The presence of this molecule, which diffuses freely in 

 the thickness of the membrane, does not prevent the diffusion of glucose to 

 continue through the left-hand face. On the right-hand side, this ester is 

 hydrolysed in the presence of a phosphatase, regenerating glucose and 



