Learning (by) osmosis: an approach to teaching osmolarity and tonicity

Abstract

Understanding osmolarity and tonicity is one of the more challenging endeavors undertaken by students of the natural sciences. We asked students who completed a course in animal physiology to submit an essay explaining what they found most perplexing about this subject, and what in-class activities proved most useful to them. Students had difficulty distinguishing osmolarity from tonicity and determining tonicity based on the solution’s composition. The most useful activities were questions requiring simultaneous consideration of both osmolarity and tonicity. Problems that require calculating osmotic concentration and the volumes of body fluid compartments after administration or loss of various solutions emphasize the significance of osmolarity and tonicity in the context of systemic homeostasis and clinical medicine. We hope that our approach to teaching osmolarity and tonicity will prove useful to physiology lecturers who are looking for new ways of introducing this complicated topic to their health professions students.

Hyperosmotic stress:in situchromatin phase separation

“…In growth medium+300 mM sucrose the mean peak cell volume shrinks to~61% of the cell volume in isosmotic medium (Table 2), due to the dehydration effect. [Similar measurements of HL-60/S4 cells in medium ± 150 mM NaCl yielded slightly less shrinkage (~73-76%, Table 2), possibly due to the incomplete dissociation of NaCl in aqueous solution [1]]. Besides the increased 'crowding effect', we would expect that intracellular ionic and nonionic solutes would increase in concentration during sucrose-induced dehydration, possibly increasing the internal ionic strength tõ 500 mM (i.e.,~300 mM x 1/0.61) and weakening macromolecular interactions that have a significant electrostatic component.…”

“…Hyperosmotic solutions can be defined as (aqueous) buffers with higher solute concentrations than those of living cells; isosmotic solutions possess solute concentrations comparable to living (mammalian) cells (~280-295 milliOsmolar, 'mosM' [1]). Hyperosmotic solutions tend to dehydrate cells; i.e., remove intracellular water through the (semipermeable) cell membrane.…”

“…Mammalian tissue culture media and phosphate buffered saline (PBS) are regarded isosmotic to mammalian cells. 150 mM NaCl is isosmotic and isotonic (i.e., containing comparable levels of cations and anions, as in living cells); whereas 300 mM sucrose is isosmotic (but not 'isotonic', since it is not ionic) [1,2]. Sucrose and NaCl do not penetrate into live mammalian cells and therefore, when added to tissue culture media, generate hyperosmotic solutions and dehydrate the cells.…”

The Role of Water Homeostasis in Muscle Function and Frailty: A Review

“…Therefore, any pure glucose solution will be hypotonic, whatever its concentration or osmolarity. Moreover, since water passes through the cell membrane more rapidly than solutes, a hyperosmotic (and hypotonic) glucose solution will cause an initial loss, immediately followed by a gain, in cell volume so that glucose can be transported into the cell [36,37].…”

Using Visuals to Help Explain Tonicity to Introductory Biology Students

“…Misconceptions surrounding osmosis have persisted for decades, specifically those involving tonicity and water movement (Odom, 1995;Fisher et al, 2011). Definitions of osmolarity and tonicity differ among textbooks (Vujovic et al, 2018), and while any of these definitions may be sufficient for instructor understanding, this can add to student confusion. We clarify the two concepts and their distinctions for students as follows.…”