What is a way to remember the differences in the structures of glucose, fructose, and lactose?
We are looking at glucose, fructose, and lactose. The easiest difference to initially look for is the fact that lactose is a disaccharide while glucose and fructose are monosaccharides. Disaccharide means that lactose is comprised of two different sugar molecules (glucose and galactose) that are coupled together with a glycosidic bond. So it is essentially twice the size of the other two. Glucose and fructose are both monosaccharides (single sugars) that are composed of a six carbon backbone with numerous oxygen containing groups attached to the carbons. The main structural difference between the two is that fructose has a ketone group on the second carbon while glucose has an aldehyde group on the first carbon. This allows fructose to cyclize to a 5 membered ring while glucose cyclizes to a 6 membered ring. I'm not sure how specific you have to know these structures but other than these things you will simply have to memorize the specific structures.