Starch and cellulose are both polymers of glucose, yet humans can digest starch but not cellulose. What accounts for this difference?
ACellulose contains different monosaccharide units than starch
BStarch is shorter than cellulose, making it easier to break down
CThe α-1,4 glycosidic bonds in starch are cleaved by human amylase, while the β-1,4 bonds in cellulose are not recognized by any human digestive enzyme
DCellulose is crystalline and insoluble, preventing digestive enzymes from reaching it
Both starch and cellulose are made entirely of glucose, but the geometry of the glycosidic bond differs: starch uses α-1,4 linkages (with α-1,6 branch points in amylopectin/glycogen), while cellulose uses β-1,4 linkages. Enzyme active sites are exquisitely sensitive to this geometry. Human amylase cleaves α-1,4 bonds; we lack β-glucosidase, the enzyme needed for β-1,4 bonds. Same monomer, different linkage = completely different biological role and digestibility.
Question 2 Multiple Choice
Glycogen branches approximately every 8–12 glucose residues, compared to every 24–30 in amylopectin. What is the primary functional significance of this denser branching?
AMore branches increase total molecular weight, allowing more glucose to be stored in smaller space
BMore branch points reduce the molecule's solubility, allowing it to crystallize inside the cell
CMore branches create more non-reducing ends where glycogen phosphorylase can act simultaneously, enabling rapid glucose mobilization
DMore branches reduce osmotic pressure inside the cell by packing glucose units more tightly
Glycogen phosphorylase removes glucose units from the non-reducing ends of glycogen chains. More branch points means exponentially more non-reducing ends on the glycogen sphere's surface. Because many phosphorylase molecules can act simultaneously on these ends, densely branched glycogen releases glucose far faster than a more linear polymer. This is exactly what muscles and liver need during exertion or hypoglycemia — rapid glucose availability requires architectural redundancy at the polymer's surface.
Question 3 True / False
Cellulose and starch are both polymers built entirely from glucose monomers connected by glycosidic bonds.
TTrue
FFalse
Answer: True
True — and this makes their functional difference all the more striking: it arises entirely from bond stereochemistry. Starch uses α-1,4 (and α-1,6) linkages; cellulose uses β-1,4 linkages. The β configuration in cellulose causes each glucose to flip 180°, creating a straight, ribbon-like chain that hydrogen-bonds with adjacent chains to form rigid crystalline fibers — ideal for plant cell walls. The α configuration in starch allows helical coiling and recognition by amylase.
Question 4 True / False
Sucrose is a reducing sugar because it contains at least one free anomeric carbon available for oxidation.
TTrue
FFalse
Answer: False
Sucrose is a non-reducing sugar — the only common disaccharide with this property. Its glycosidic bond links the anomeric carbon of glucose (C1) directly to the anomeric carbon of fructose (C2 of fructose), locking both in the bond. Since neither anomeric carbon is free to open into the reactive open-chain aldehyde or ketone form, sucrose cannot act as a reducing agent. This contrasts with maltose and lactose, where one anomeric carbon remains free.
Question 5 Short Answer
Why does the α versus β configuration of a glycosidic bond matter so much biologically, even when the monosaccharide units are identical?
Think about your answer, then reveal below.
Model answer: The α and β configurations place the bonding oxygen on opposite faces of the sugar ring, fundamentally changing the three-dimensional shape of the resulting polymer. Enzymes that cleave glycosidic bonds have active sites shaped to recognize one configuration but not the other — so starch (α-1,4) is cleaved by amylase, while cellulose (β-1,4) requires a completely different enzyme that humans don't produce. Bond configuration also determines physical properties: β-1,4 linkages in cellulose create rigid structural fibers, while α-1,4 linkages in starch allow helical coiling for compact energy storage.
Identical monomers, completely different biology, determined entirely by bond stereochemistry. This principle generalizes broadly: it explains why lactase-deficient people cannot digest lactose (β-1,4 galactosidic bond), and why insects with cellulases can digest wood that mammals cannot. The specificity of enzyme active sites for bond geometry is one of the most important themes in carbohydrate biochemistry.