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MCAT Biology Question — Cellulose

  • by John
  • May 01, 2018
  • MCAT Question of the Day

Although linked at the same carbons as starch and glycogen, cellulose is indigestible to humans. This is a result of the:

 

  1. alpha-linkage from carbons 2 to 6 of D-glucose.
  2. beta-linkage at carbons 2 to 6 of D-glucose.
  3. alpha-linkage at carbons 1 to 4 of D-glucose.
  4. 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.

 

  1. alpha-linkage from carbons 2 to 6 of D-glucose, incorrect.
  2. beta-linkage at carbons 2 to 6 of D-glucose, incorrect.
  3. alpha-linkage at carbons 1 to 4 of D-glucose, incorrect.
  4. beta-linkage at carbons 1 to 4 of D-glucose, correct.

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