Final answer:
The monosaccharide that accompanies glucose in the UDP-glucose structure is galactose, which is an aldose isomer of glucose and differs in functional group arrangement.
Step-by-step explanation:
In addition to glucose, the other monosaccharide that is part of the UDP-glucose structure is galactose. While both glucose and galactose have the same chemical formula C6H12O6, they are structural isomers, which means they have different arrangements of functional groups around their asymmetric carbons. Moreover, glucose and galactose are classified as aldoses, whereas fructose, which is found in sucrose and is another common monosaccharide, is classified as a ketose due to a ketone group at its second carbon.