Final answer:
Glucose and fructose are soluble in water and form cyclic structures, with glucose having a six-membered ring and fructose a five-membered ring. They are isomeric monosaccharides where glucose is an aldose and fructose is a ketose. Letters in their molecular structures typically denote carbon atoms or functional groups.
Step-by-step explanation:
Glucose and fructose are two types of simple sugars, often referred to as monosaccharides. They are both soluble in water and usually exist in their cyclic forms when in aqueous solutions. Glucose forms a six-membered ring, while fructose forms a five-membered ring, even though they both share the same molecular formula, C6H12O6. This phenomenon where compounds have the same molecular formula but different structures is known as isomerism. In particular, these sugars are isomeric monosaccharides; specifically, glucose and galactose are aldoses, while fructose is a ketose. The cyclic structure of glucose or fructose is represented by letters in the diagrams of their molecular structures, frequently denoting carbon atoms or functional groups like hydroxyl (OH) groups.
The primary difference between glucose and fructose, besides the ring size, is the position of the carbonyl group (C=O). In glucose, the carbonyl group is at the end of the molecule, making it an aldehyde, whereas in fructose, it is in the second position from the end, making it a ketone. Although fructose is the sweetest naturally occurring sugar, glucose is more commonly used in cellular respiration to produce energy. Both sugars can combine to form the disaccharide sucrose.