Animated Solution for Chemistry - Biomolecules: Complete hydrolysis of cellulose gives

Welcome to the fascinating world of biomolecules! When we look at the structural integrity of plants, the towering trees, and the crisp leaves, we are essentially looking at the marvel of a specific polysaccharide: Cellulose.

Polysaccharides are massive polymeric carbohydrate molecules composed of long chains of monosaccharide units bound together by glycosidic linkages. They range in structure from linear to highly branched. Cellulose, in particular, is a linear, unbranched polymer. But what exactly is it made of? Let's dive deeper into its molecular architecture.

Imagine you are building a long, sturdy chain. To do this, you need identical, strong links. In the case of cellulose, these links are molecules of $\beta$-D-glucose.

Unlike starch, which is made of $\alpha$-D-glucose and forms a helical structure, cellulose is formed by $\beta$-1,4-glycosidic linkages. This specific $\beta$ linkage means that every alternate glucose monomer is flipped upside down relative to its neighbor. This alternating arrangement allows the cellulose chains to be incredibly straight and rigid. Furthermore, these straight chains align parallel to each other and form extensive intermolecular hydrogen bonds, creating microfibrils that give plant cell walls their immense tensile strength.

Now, what happens if we want to break this chain down? We use a chemical process called hydrolysis. The word itself gives away its meaning: hydro means water, and lysis means to break. By adding water molecules in the presence of an acid catalyst ($H^{+}$) and heat, we can cleave the glycosidic bonds that hold the polymer together.

Complete Hydrolysis

When we subject cellulose to partial hydrolysis, we are essentially chopping the massive polymer into smaller, more manageable fragments. We don't break every single bond. The primary intermediate product of this partial breakdown is a disaccharide known as cellobiose ($C_{12}{H}_{22}{O}_{11}$).

Cellobiose is very similar to maltose (the disaccharide from starch), but with one crucial difference: the geometry of the bond. While maltose has an $\alpha$-1,4 linkage, cellobiose retains the $\beta$-1,4-glycosidic linkage of its parent cellulose.

However, the question asks us about complete hydrolysis. This means we push the reaction to its absolute limit. We continue to apply heat and acid until every single glycosidic bond is shattered. The cellobiose molecules are forced to split apart.

When the dust settles and the complete hydrolysis is finished, what are we left with? Since the entire cellulose polymer was built exclusively from one type of monomer, the final soup contains only that specific monomer.

Therefore, the complete hydrolysis of cellulose yields pure D-glucose.

It is crucial to remember that naturally occurring sugars are overwhelmingly in the 'D' configuration. L-glucose is a synthetic enantiomer and is not found in natural polymers. Thus, the correct answer is unequivocally D-glucose.