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Cellulose and starch are both glucose polymers, yet humans digest only starch. The structural reason is that in cellulose:
AEvery glucose monomer is flipped relative to the next
BEvery glucose monomer carries an extra phosphate
CGlucose is replaced by fructose along the whole chain
DThe chain is held together by ionic rather than covalent bonds
Answer & Solution
Correct answer: A. Every glucose monomer is flipped relative to the next
1. Both polymers use glucose, so the monomer cannot explain the difference.
2. In starch the way the subunits are joined gives the glucose chains a helical structure.
3. In cellulose every glucose monomer is flipped relative to the next one.
4. That gives a linear, fibrous structure human enzymes cannot cleave.
5. Option B and C invent chemistry that is not present, and option D is wrong because glycosidic bonds are covalent.
_Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 3 'Biological Macromolecules', sections 3.1-3.5_
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