Humans can digest starch but not cellulose because:

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Starch and cellulose are both polysaccharides composed of glucose monomers, yet they exhibit dramatically different digestibility in humans due to the specific configuration of their glycosidic bonds. Starch, a plant storage polysaccharide, consists of α-glucose monomers joined by α-1,4-glycosidic linkages (in amylose) and both α-1,4 and α-1,6-glycosidic linkages (in amylopectin). The alpha configuration creates a coiled, helical three-dimensional structure that is accessible to hydrolytic enzymes. In contrast, cellulose, a structural polysaccharide in plant cell walls, consists of β-glucose monomers linked exclusively by β-1,4-glycosidic bonds. This beta configuration causes every other glucose molecule to flip 180 degrees, producing a rigid, linear chain that forms extensive hydrogen bonds with neighboring cellulose chains, creating tightly packed microfibrils.

Why Other Options Are Wrong

Enzyme specificity determines digestibility. Human salivary amylase and pancreatic amylase possess active sites precisely shaped to accommodate the α-glycosidic bond geometry and hydrolyze these linkages into maltose and glucose units. Humans completely lack cellulase, the enzyme capable of catalyzing the hydrolysis of β-1,4-glycosidic bonds. This enzymatic limitation exists because the active site of amylase cannot physically accommodate the three-dimensional orientation of β-linked glucose monomers, rendering cellulose indigestible.

PILLAR 2 — STEP-BY-STEP LOGIC

The correct answer (c7a) states that the difference in digestibility arises from different glycosidic bond configurations between starch and cellulose. Because both molecules are glucose polymers, their monomer composition is identical. The critical distinction lies in stereochemistry: α-glycosidic bonds in starch versus β-glycosidic bonds in cellulose. Because enzyme active sites exhibit strict specificity for particular molecular geometries, amylase recognizes and cleaves only α-linkages. Because humans lack cellulase entirely, β-1,4-glycosidic bonds in cellulose remain intact throughout digestion, and cellulose passes through the human digestive tract undegraded. This stereochemical difference directly determines enzymatic accessibility, which establishes the fundamental reason humans can digest starch but not cellulose.

PILLAR 3 — DISTRACTOR ANALYSIS

Option c7b is incorrect because it claims cellulose contains different monosaccharides than starch. Both polymers consist exclusively of glucose monomers; they differ only in bond configuration. A student selecting this option likely confuses structural differences in bonding with compositional differences in monomers.

Option c7c is incorrect because it suggests starch is more soluble in digestive fluids. While starch does exhibit greater water solubility, this physical property does not determine enzymatic digestibility; enzyme-substrate specificity at the active site drives hydrolysis regardless of solubility.

Option c7d is incorrect because it claims cellulose molecules are too large for enzyme access. Cellulose chains and starch polymers both contain thousands of glucose units. Size does not prevent enzymatic hydrolysis, as enzymes cleave bonds sequentially at chain ends or internally. The bond geometry, not polymer length, determines digestibility.

A student choosing incorrect options may conflate physical properties with chemical reactivity or fail to recognize that identical monomers can produce functionally distinct polymers through different bonding arrangements.