UDP Glucose.

The transfer of glucose from uridine diphosphate glucose to fructose 6-phosphate to form sucrose shows the activation of the C-1 of glucose for nucleophilic substitution by the phosphate leaving group. UDP-sugar (UDP glucose, for example) is a common motif in the synthesis of glycosidic bonds. Nucleoside diphosphate sugars are intermediates in nearly all glycosidic linking between sugars. This is a recurring theme. Many biosynthetic reactions are driven by the hydrolysis of pyrophosphate.

If you understand the above paragraph fairly easily, you are making good progress.