Sucrose-phosphate synthetase is detectable only in intact chloroplast preparations of Phaseolus aureus. In contrast, sucrose synthetase and uridine diphosphate glucose (UDPglucose) pyrophosphorylase activities are low in extracts of photosynthetic tissues of P. aureus but are high in extracts of nonphotosynthetic tissues. Activities for ADP-, dTDP-, CDP-, and GDP-glucose pyrophosphorylases are generally higher in extracts of photosynthetic tissues of P. aureus than in extracts of nonphotosynthetic tissues. The high levels of sucrose synthetase and of UDP-glucose pyrophosphorylase found in dark-grown hypocotyls begin to decline about 4 hours after exposure to light at a rate of 50% every 3 hours.The data suggest that sucrose-phosphate synthetase and sucrose phosphatase are the enzymes responsible for the biosynthesis of sucrose from photosynthetically fixed CO2, and that the major function of sucrose synthetase is to catalyze the synthesis of UDP-, ADP-, dTDP-, CDP-, and GDP-glucose from translocated sucrose in nonphotosynthetic tissues; in photosynthetic tissues the pyrophosphorylases may replace sucrose synthetase in catalyzing the synthesis of these nucleoside diphosphate glucoses. We offer the suggestion that sucrose synthetase and UDPglucose pyrophosphorylase play a major role in the uptake and metabolism of sucrose in nonphotosynthetic tissues. Results are presented from preliminary studies on the conversion in vitro of sucrose to glucose 1-phosphate by the coupled reactions of sucrose synthetase and UDP-glucose pyrophosphorylase with highly purified preparations of these enzymes.The following alternate mechanisms for the biosynthesis of sucrose and nucleoside diphosphate glucose have been demonstrated in plants:UDP-glucose + fructose-6-P sucrose-P synthetase sucrose-P + UDP (la) where N = uridine, adenosine, thymidine, cytidine, or guanosine. The equilibrium constant of reaction la in the direction of sucrose-P synthesis has been reported to be 3250 at 38 C and pH 7.5 (11). When this reaction is coupled with sucrose phosphatase, the biosynthesis of sucrose can be considered essentially irreversible. Sucrose-P synthetase has been demonstrated in a variety of plants (3,12,14,17) and has been suggested to be the enzyme responsible for the biosynthesis of sucrose from photosynthetically fixed CO2.In contrast, the reaction catalyzed by sucrose synthetase is freely reversible. This enzyme has also been demonstrated in a variety of plants (4,10,15,16,18). Grimes et at. (10) have partially purified and characterized sucrose synthetase from extracts of etiolated Phaseolus aureus seedlings. This enzyme can function with either UDP-, ADP-, dTDP-, CDP-, or GDPglucose as substrate. The Km for UDP-glucose is 10-fold lower than for any of the other nucleoside diphosphate sugars, although all five nucleoside diphosphate glucoses are efficient substrates.The pyrophosphorylase reactions are also freely reversible. In contrast to sucrose synthetase, where a single enzyme can catalyze the biosynthesis of five differen...