Which absorbs least water?

Explanation

## Correct Answer: B. Cellulose Cellulose is a linear homopolysaccharide of glucose units linked by β-1,4-glycosidic bonds, forming a rigid, crystalline structure with minimal hydrophilic groups exposed on its surface. This structural rigidity and hydrophobic character make it inherently poor at absorbing water. Unlike gums, mucilages, and pectins—which are branched or contain uronic acids and other polar substituents—cellulose lacks the branching and charged groups necessary for extensive hydrogen bonding with water molecules. The tight, parallel arrangement of cellulose chains in fibrils creates a densely packed matrix that water cannot easily penetrate. In the Indian dietary context, cellulose from vegetables, whole grains, and plant cell walls passes largely unabsorbed through the GI tract, contributing to stool bulk without significant water retention. This property is why cellulose is used as a non-absorbent filler in pharmaceutical formulations and why it does not contribute to osmotic diarrhea—it simply adds mechanical bulk. ## Why the other options are wrong **A. Gums** — Gums are branched polysaccharides (e.g., gum arabic, guar gum) with multiple hydroxyl groups and often uronic acid residues. These polar, hydrophilic substituents form extensive hydrogen bonds with water, allowing gums to absorb and retain large quantities of water—up to 100 times their own weight. They swell significantly and form viscous solutions, making them highly water-absorbent. This is why gums are used as thickening and stabilizing agents in food and pharmaceuticals. **C. Mucilage** — Mucilages are highly branched polysaccharides rich in uronic acids and other polar groups (e.g., from psyllium husk, isabgol—commonly used in Indian Ayurvedic medicine). They are extremely hydrophilic and can absorb 20–30 times their weight in water, forming a gelatinous mass. This property makes mucilages effective as bulk laxatives in Indian clinical practice for constipation management, as they absorb intestinal water and increase stool volume. **D. Pectin** — Pectin is a complex polysaccharide containing galacturonic acid residues and methoxyl groups, making it highly polar and water-soluble. It absorbs significant amounts of water and forms gels, which is why it is used in jam-making and as a thickening agent. Pectin also acts as a prebiotic in the colon and is used therapeutically in diarrhea management in Indian pediatrics, demonstrating its water-absorbing capacity. ## High-Yield Facts - **Cellulose** is a β-1,4-linked glucose polymer with a rigid, crystalline structure that repels water and absorbs minimal amounts. - **Gums, mucilages, and pectins** are branched or substituted polysaccharides with uronic acids and polar groups that absorb 20–100 times their weight in water. - **Isabgol (psyllium husk)** is a mucilage-based bulk laxative widely used in Indian clinical practice for constipation due to its high water-absorbing capacity. - **Cellulose** contributes to stool bulk without osmotic effect because it does not absorb water; other polysaccharides cause osmotic diarrhea if consumed in excess. - **Dietary fiber classification**: insoluble fiber (cellulose, lignin) vs. soluble fiber (gums, mucilages, pectins)—soluble fibers absorb water; insoluble do not. ## Mnemonics **GUMP = Gums, Uronic acids, Mucilage, Pectin (Water-absorbers)** Gums, Mucilages, and Pectins all contain uronic acids or branching that makes them hydrophilic. Cellulose is the odd one out—linear, rigid, hydrophobic. Use this to remember that the first three are water-loving; cellulose is water-repelling. **Cellulose = Crystalline Cage (water cannot enter)** Picture cellulose chains as tightly packed, parallel fibrils forming a rigid cage. Water molecules cannot penetrate this dense matrix. Gums, mucilages, and pectins are branched and floppy—water flows in easily. ## NBE Trap NBE may pair cellulose with "dietary fiber" and expect students to assume all fiber absorbs water. The trap is confusing soluble fiber (gums, pectins, mucilages) with insoluble fiber (cellulose, lignin)—only soluble fibers absorb significant water. ## Clinical Pearl In Indian clinical practice, isabgol (psyllium mucilage) is prescribed for constipation because it absorbs water and increases stool bulk, whereas cellulose-based fiber supplements (like those in whole wheat) provide bulk without water absorption—a key distinction when counseling patients on dietary fiber intake and laxative choice. _Reference: Harper's Biochemistry Ch. 15 (Carbohydrate Metabolism); KD Tripathi Pharmacology Ch. 47 (Laxatives and Bulk Agents)_