What is the role of intrinsic factor, produced by parietal cells?

Intrinsic factor, produced by the parietal cells of the stomach's gastric mucosa, plays a critical role in the absorption of vitamin B12. This glycoprotein binds to vitamin B12 in the stomach, protecting it from degradation and facilitating its absorption in the terminal ileum of the small intestine. Without intrinsic factor, vitamin B12 cannot be absorbed effectively, leading to potential deficiencies and conditions such as pernicious anemia.

The roles of the other options are not related to intrinsic factor. Pepsinogen activation is facilitated by gastric acid and not by intrinsic factor, while stomach acid neutralization occurs in the duodenum through bicarbonate secretion. Gastric motility is influenced by other factors like hormones and nerve signals, rather than by intrinsic factor. Thus, the specific role of intrinsic factor is crucial for vitamin B12 absorption, making the correct answer clear.