GLUCOSE METABOLISM IN HYDRODICTYON AFRICANUM IN RELATION TO CELL ENERGETICS

Abstract

SUMMARYWhen ATP is supplied by oxidative phosphorylation alone, cyclic photophosphorylation alone or from both photosynthetic and oxidative phosphorylations, exogenous glucose is metabolized to a similar range of non‐volatile products of which a major one is starch. When only fermentative ATP supply can occur, much less starch is produced. Cyclic photophosphorylation alone supports a rather lower rate of starch synthesis from exogenous glucose than does oxidative phosphorylation alone. This is apparently due to rate‐limitation by glucose influx, since cyclic ATP production supports a faster rate of starch synthesis than does oxidative ATP production when endogenous substrate is used.Production of CO2 from specifically labelled glucose is consistent with the operation of the EMP, PPP and TCAC in both light and darkness in the presence of oxygen. Fermentation of exogenous glucose is inhibited by light absorbed by photosystem one, probably via cyclic photophosphorylation. Respiratory CO2 production is also inhibited to a small extent by photo‐system one, although most of the light inhibition needs photosystem two. This DCMU‐reversed inhibition of CO2 production in the light is due to a direct inhibition of decarboxylation reactions as well as to reassimilation of respired CO2.The relatively small randomization of C1 and C6 labelled glucose during assimilation into starch suggests that conversion to triose phosphate may not be essential for transport of hexose across the outer chloroplast membranes.