alleryStart learning →Vitamin B12 binds intrinsic factor. Where is the resulting complex taken up, and by what mechanism?
AIn the terminal ileum, by endocytosis
BIn the jejunum, by simple diffusion
CIn the duodenum, by active transport
DIn the stomach, by facilitated diffusion
Answer & Solution
Correct answer: A. In the terminal ileum, by endocytosis
1. Most water-soluble vitamins are absorbed by simple diffusion, but vitamin B12 is a very large molecule.
2. Intrinsic factor secreted in the stomach binds it and protects it from being digested.
3. The complex then binds to mucosal receptors in the terminal ileum.
4. There it is taken up by endocytosis rather than by diffusion or a carrier.
5. Binding happens in the stomach, but absorption does not, which is the trap in the last option.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 23 "The Digestive System", section 23.7 Chemical Digestion and Absorption: A Closer Look_
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