Polysaccharides
Starch can be digested by humans for energy, whereas cellulose passes through the human digestive tract largely unchanged even though both are glucose polymers. Which structural difference is responsible for this contrast?
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About This Question
- Subject
- chemistry
- Chapter
- biomolecules
- Topic
- polysaccharides
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
Starch has alpha-glycosidic linkages while cellulose has beta-glycosidic linkages
Although starch and cellulose are both polymers of D-glucose, their digestibility difference comes from the stereochemistry of the linkage, a frequently tested point. In starch, the glucose units are connected by alpha-1,4 glycosidic bonds (with alpha-1,6 branches in amylopectin), giving a coiled structure that human amylase enzymes can hydrolyse to release glucose for energy. In cellulose, the glucose units are joined by beta-1,4 glycosidic bonds, producing long straight chains held by extensive hydrogen bonding into rigid fibres; humans lack the enzyme needed to cleave the beta linkage, so cellulose passes through as dietary fibre. Thus the alpha-versus-beta linkage is the key difference. The option claiming starch is made of fructose is wrong, since both are glucose polymers. The branching description is reversed and not the cause, because amylopectin is the branched one. Cellulose contains no nitrogen, so that option is false. This matches the NCERT comparison of starch and cellulose. As a plausibility check, ruminants that host bacteria with cellulase enzymes can digest cellulose, confirming that the obstacle is specifically the beta linkage and the missing enzyme rather than the glucose content.
This medium difficulty chemistry question is from the chapter biomolecules, covering the topic of polysaccharides. It appeared in the 2025 exam.
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