Polysaccharide mechanics — If you could pull on the two ends of each polymer, which of the following polysaccharides would be expected to stretch the most under tension before reaching its limit?

Introduction / Context:
The mechanical response of polysaccharides depends on their primary linkages and higher-order structure. Stretching behavior reflects chain flexibility, hydrogen bonding, branching, and crystallinity. Comparing glycogen, starch, and cellulose illuminates how structure dictates physical properties relevant to food science and materials biology.

Given Data / Assumptions:

• Glycogen: highly branched polymer of glucose with α-1,4 backbones and α-1,6 branches every ~8–12 residues.
• Starch: mixture of amylose (mostly linear α-1,4) and amylopectin (branched), with amylose forming helical conformations that can uncoil.
• Cellulose: linear β-1,4 glucan with extensive inter- and intra-chain hydrogen bonding forming microfibrils (stiff).

Concept / Approach:
A polymer that can uncoil and reorient under force will “stretch” more than one that is heavily cross-branched (glycogen) or locked into crystalline fibrils (cellulose). Amylose’s helical structure allows significant elongation as helices unwind; thus starch (particularly amylose-rich) is the best answer for maximal stretch under tension.

Step-by-Step Solution:

Verification / Alternative check:
Single-molecule force spectroscopy studies show distinct force–extension curves, with helical polymers exhibiting unfolding transitions not seen in rigid, crystalline cellulose microfibrils.

Why Other Options Are Wrong:

Common Pitfalls:
Equating tensile strength with stretchability; cellulose is strong but does not elongate much before failure.

Final Answer:
Starch (amylose-rich).