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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.
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