Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups — monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as $ ext{D}(+) ext{--glucose}$. It was assigned the open structure on the basis of its reactions with $ ext{HI}$, $ ext{NH}_2 ext{OH}$, $ ext{Br}_2 ext{ water}$, $( ext{CH}_3 ext{CO})_2 ext{O}$ and nitric acid. Despite having the $- ext{CHO}$ group, glucose does not give Schiff's test and hydrogen sulphite addition product with $ ext{NaHSO}_3$. It was found that glucose forms a six-membered ring in which one of the $- ext{OH}$ groups add to the $- ext{CHO}$ group and form a cyclic hemiacetal structure. Answer the following questions : (a) How do you explain the following ? (i) Presence of a carbonyl group in glucose (ii) Presence of five $- ext{OH}$ groups in glucose which are attached to different carbon atoms.