Although linked at the same carbons as starch and glycogen, cellulose is indigestible to humans. This is a result of the:
- alpha-linkage from carbons 2 to 6 of D-glucose.
- beta-linkage at carbons 2 to 6 of D-glucose.
- alpha-linkage at carbons 1 to 4 of D-glucose.
- beta-linkage at carbons 1 to 4 of D-glucose.
Click for Explanation
In humans, glycogen is broken down by debranching enzyme (alpha 1,6 linkages) and phosphorylases (alpha 1,4 linkages). Starch is broken down at alpha 1,4 linkages by alpha-amylase found in saliva and pancreatic secretions. Cellulose cannot be broken down by humans because it is beta-linked at carbons 1 to 4. For the MCAT: Alpha-1,4 linked glucose indicates digestible glycogen and starch, and beta-1,4 linkage indicates indigestible cellulose.
A. alpha-linkage from carbons 2 to 6 of D-glucose, incorrect.
B. beta-linkage at carbons 2 to 6 of D-glucose, incorrect.
C. alpha-linkage at carbons 1 to 4 of D-glucose, incorrect.
D. beta-linkage at carbons 1 to 4 of D-glucose, correct.
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