Without further context I would assume the answer here is: Hydroxyl groups.
hope this helps
I do think that they close to not allow too much of either water or air from entering.
the hollow tube allows water to flow easily.
https://www.bbc.co.uk/bitesize/guides/z28g7p3/revision/1
Breastfeeding can reduce the mother's risk of breast and ovarian cancer, type 2 diabetes, and high blood pressure.
