Why is cellulose not digested by humans but starch is?

Both cellulose and starch are glucose polymers, but they differ in the glycosidic bond: Starch uses α-1,4 glycosidic bonds; cellulose uses β-1,4 bonds. Human digestive enzymes (amylases) can only break α-1,4 bonds. We lack the enzyme cellulase that breaks β-1,4 bonds. Cows and termites have symbiotic bacteria in their gut producing cellulase, so they can digest cellulose (grass/wood).

Why is sucrose a non-reducing sugar while maltose is reducing?

A reducing sugar has a free aldehyde or ketone group (hemiacetal) that can reduce Fehling's/Tollen's reagents. In maltose (glucose+glucose), the glycosidic bond is between C-1 of one glucose and C-4 of the other, leaving the C-1 of the second glucose free (anomeric carbon free) — so it's reducing. In sucrose (glucose+fructose), both anomeric carbons (C-1 of glucose and C-2 of fructose) are involved in the bond — no free aldehyde/ketone → non-reducing.

Carbohydrates, Proteins, Enzymes, and Vitamins

Biomolecules • Class 12 Chemistry • NCERT • CBSE

Carbohydrates: general formula Cₙ(H₂O)ₘ. Glucose: C₆H₁₂O₆ (aldohexose). Proteins: polymers of amino acids linked by peptide bonds. Enzymes: biological catalysts (proteins). DNA has deoxyribose sugar; RNA has ribose.

Key Formulas

General formula of carbohydrates: Cₙ(H₂O)ₘ
Peptide bond: −CO−NH− (condensation of −COOH and −NH₂)
Mnemonic fat-soluble vitamins: ADEK

Frequently Asked Questions

Why is cellulose not digested by humans but starch is?: Both cellulose and starch are glucose polymers, but they differ in the glycosidic bond: Starch uses α-1,4 glycosidic bonds; cellulose uses β-1,4 bonds. Human digestive enzymes (amylases) can only break α-1,4 bonds. We lack the enzyme cellulase that breaks β-1,4 bonds. Cows and termites have symbiotic bacteria in their gut producing cellulase, so they can digest cellulose (grass/wood).
Why is sucrose a non-reducing sugar while maltose is reducing?: A reducing sugar has a free aldehyde or ketone group (hemiacetal) that can reduce Fehling's/Tollen's reagents. In maltose (glucose+glucose), the glycosidic bond is between C-1 of one glucose and C-4 of the other, leaving the C-1 of the second glucose free (anomeric carbon free) — so it's reducing. In sucrose (glucose+fructose), both anomeric carbons (C-1 of glucose and C-2 of fructose) are involved in the bond — no free aldehyde/ketone → non-reducing.

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