ISOLATED CHLOROPLASTS 125 



in the presence of 5% carbon dioxide (pH 7-14) malic 

 acid was produced in a concentration of 3-5 iq-^M. after 

 120 minutes illumination. Other workers (Tolmach, 195 1; 

 Arnon, 195 1) used 'malic' enzyme from wheat germ and 

 from sugar beet and sunflower leaves. With unwashed 

 chloroplast preparations addition of TPN resulted in a con- 

 siderable oxygen evolution far in excess of the amount of 

 TPN added. This indicates a coupling of the reduced TPN 

 with other reactants present in the unwashed suspension 

 which have not yet been identified. 



We have seen in the previous chapter that if the carbon 

 dioxide were first changed to a carboxyl phosphate, TPN 

 could then eflfect its reduction to carbohydrate. We have 

 already mentioned that if these *high energy' phosphate 

 compounds were to be directly produced from light energy 

 the process would be likely to be one of low efficiency. It 

 is therefore more probable that they would arise from the 

 oxidation of some intermediate, possibly the reduced pro- 

 duct of the light reaction. The uptake of inorganic phos- 

 phate in the light for both Chlorella and the photosynthetic 

 bacterium Chromatium is not very different from that in the 

 dark. There is some evidence for a change in distribution 

 between different phosphate compounds with an increase in 

 ester phosphate which is less in the presence of carbon 

 dioxide or of glucose. More recently Strehler (1953) has 

 demonstrated marked changes in ATP in the green cell upon 

 illumination. The significance of such changes will arise in 

 the discussion of the mechanism of photosynthesis. 



