Cyclic structure of Monosaccharides 

We know that aldoses (and ketoses) react with alcohols to give first hemiacetals (and hemiketals) and then acetals (and ketals), i.e.

Since monosaccharides contain a number of hydroxyl groups and an aldehyde or a keto group, therefore, any one of the –OH groups (usually C4 or C5 in aldohexoses and C5 or C6 in ketohexoses) may combine with the aldehyde or the keto group to form intramolecular hemiacetal or hemiketal.      

As a result, the open chain formulae do not represent the actual structures of the monosaccharides. Their actual structures are cyclic involving five or six membered rings containing an oxygen atom. The five membered ring containing one oxygen atom because of its similarity with furan is called the furanose form and the six membered ring containing one oxygen atom because of its resemblance with pyran is called the pyranose form. In nut shell, all the monosaccharides (pentoses and hexoses) in the free state always exist in the pyranose form. However, in the combined state some monosaccharides such as ribose, 2-deoxyribose, fructose etc., usually exist in the furanose form.