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The determination of the activities of the enzymes which could be involved in the starch-sucrose conversion showed that the first step of this transfer is a phosphorolysis of starch. Both phosphorylases are able to attack the starch granules. The glu-l-P thus formed can be further metabolized to UDPG and fru-6-P, which are the substrates for the reaction catalysed by the sucrose-6-phosphate-synthetase. The activity of this enzyme increases about 4-fold during the period from immaturity to the first day of germination. The synthesis of sucrose-6-phosphate is inhibited by sucrose. This inhibition (causing an UDPG accumulation) provides a control for further starch breakdown.

Isolated chloroplasts from the bundle sheath cells show considerable activity of the ADPG- and UDPG-pyrophosphorylase (EC 2.7.7.9), ADPG- and UDPG-transglucosylase (EC 2.4.1.21), and the starch phosphorylase (EC 2.4.1.1). In chloroplasts of the palisade cells, on the other hand, only the UDPG-pyrophosphorylase is remarkably active.

The determination of the activities of the enzymes which could be involved in sucrose-starch conversion showed that the first step of this transfer is performed by the sucrose-synthetase. In this reaction UDPG and almost no ADPG is formed. However, under the given conditions the concentration of UDPG is low, and with the weak activity of the starch synthetase present no starch synthesis takes place.In a further step Glu-1-P is formed from UDPG, a reaction catalysed by the UDPG-pyrophosphorylase. Glu-1-P can also be furnished from the fructose liberated in the first step. In the immature cotyledons of Vicia faba there is sufficient fructokinase, phosphoglucose isomerase and phosphoglucomutase for these transformations. Most of starch synthesis proceeds catalysed by the electrophoretically slower migrating phosphorylase which can be adsorbed on starch granules. The fact that phosphorylase could only solubilize the radioactivity incorporated from Glu-1-P-C(14) (due to phosphorylase) and not the one from ADPG-C(14) (due to synthetase) shows that the two pathways are different. This sucrose-starch conversion is controlled by the concentration of glucose, glu-1-P, glu-6-P and fru-1,6-diP, which inhibit the formation of UDPG from sucrose.