PHOTOSYNTHESIS 91 



The effect of oxygen is probably not a simple one, but an effect on a number of 

 enzyme systems in the fermentation chain of reactions. For example oxygen may 

 oxidise SH groups, and these are necessary for the activation of a number of enzyme 

 systems so that in the presence of air such enzyme reactions will be automatically 

 inhibited. Michaelis and Sjnythe (1936) found that some oxidation-reduction 

 potential dyes, by virtue of their oxidising effects, inhibit the conversion of the Neuberg 

 ester to hexose diphosphate and thus bring the fermentation reaction chain to a 

 standstill. On the other hand Dickens (1936) found that some dyes such as phenosa- 

 franine inhibited the Pasteur effect in brain and the inhibition was not due to the low 

 oxidation-reduction potential of the dye. The reason for this appears to be found in 

 the observation by Mcllwain (1949) that the same dyes and nicotinamide which 

 inhibit the Pasteur effect also inhibit the breakdown of Cozymase in brain tissue. 

 The enzyme in brain also breaks down Coenzyme II, but the reduced Coenzymes I and 

 II are not destroyed appreciably. Thus the oxidation-reduction potential of mixtures 

 of coenzymes and reduced coenzymes treated with the brain enzyme is lowered and 

 this affects the many enzymes sensitive to changes in potential. It is evident that if 

 the coenzymes are destroyed many of the glycolytic reactions linked with them will 

 come to a standstill and this accounts at least in part for the Pasteur effect, since 

 under aerobic conditions the coenzymes will be mainly in the oxidised condition and 

 hence susceptible to attack by the enzyme. 



Some of the complicated effects of aerobiosis on fermentation reactions are 

 indicated in a chart by Barron (1943) which indicates oxidation-reduction potential 

 levels at which aerobic interference with fermentation appears likely. 



PHOTOSYNTHESIS BY PLANTS AND BACTERIA 



The abihty to use the energy of light falling on the organisms to effect synthesis 

 is a characteristic of plants containing chlorophyll, certain bacteria containing 

 bacteriochlorophyll, green algae, etc. 



CARBON DIOXIDE FIXATION 



Synthesis of carbohydrate occurs through fixation of carbon dioxide : — 



6(C02 + HaO) -> CeHiaOe + 12 



but this general equation gives no information about the mechanism of the process, 

 which is not yet completely understood although much work has been done. The 

 mechanism appears to be different in the cases of green plants and bacteria, and also 

 between different classes of bacteria. The purple and green sulphur bacteria need 

 hydrogen sulphide or sulphur, which are oxidised in the process whilst the non- 

 sulphur purple bacteria (Athiorhodaceae) oxidise organic acids, hydrogen, etc., during 

 the reduction of carbon dioxide. The efficiency of utilisation of light energy by 

 purple bacteria is over 10 per cent., some 0-12 molecules of carbon dioxide being fixed 

 for each quantum of light absorbed. The first process in photosynthesis may be the 

 reversible conversion of CO 2 to carboxylic acids which are then reduced by the 

 hydrogen liberated by the photolysis of water (see next section). 



Another aspect of the utilisation of light for syntheses is the storage of the 

 energy in the form of energy-rich phosphate bonds and work is already proceeding on 

 this problem. 



