In describing a cell as being in a hypotonic or hypertonic solution, what is the point of reference?

1. In living systems the point of reference is always the cytoplasm. 2. So hypo- means the extracellular fluid has a lower osmolarity than the cell cytoplasm. 3. Likewise hyper- means the extracellular fluid has a higher osmolarity than the cytoplasm. 4. Taking the extracellular fluid as the reference would flip every one of these labels, which is why the convention matters. 5. Pure water is not the baseline, since the comparison is always between two real solutions. 6. The membrane is the barrier across which the comparison is made, not one of the two things compared. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 5 "Structure and Function of Plasma Membranes", section 5.2 Passive Transport_