Rob R. answered • 02/27/20
Former University Professor of Biology, Physiology and A&P
1A: F; Physiological osmolarity is 280-300 mOsm; 100 mOsm is less than 280 mOsm (hypotonic), therefore the cells will swell. Crenation only occurs during cell shrinkage.
1B: T: Under physiological conditions, RBC osmolarity is 280-300 mOsm. Sucrose has one osmotic particle per molecule. Therefore 300 mM sucrose = 300 mOsm, isosmotic to the RBC.
1C: T: 400 mM urea is greater in osmotic strength than a physiological solution (i.e.hyperosmotic). However, a RBC membrane is freely permeable to urea, therefore has much lower tonicity (hypotonic). In 400 mM urea, the cell would swell and lyse as if suspended in a solution of deionized water.
2B: The rate of cell shrinkage and crenation depends on membrane permeability to water; i.e., the faster water can exit the cell, the faster the cell can shrink in response to hypertonic solution. The greater the degree of hypertonicity of the solution (concentration of membrane impermeant solutes), the greater the osmotic force available to drive water out of the cells, causing shrinkage and crenation (force drives flow).
2C: The rate of subsequent swelling after osmotic shrinkage (Regulatory Volume Increase) depends on activation of sodium transport; in the human RBC, this is carried out by the Na-K-2Cl cotransporter, an electrically silent transport that is activated by cell shrinkage. Following transport activation, water osmotically follows Na+, K+ and Cl- out of the cell. Therefore the rate of cell swelling also is limited by membrane permeability to water (i.e., if water can't cross the cell membrane, then the cell volume can't change).
2D: What governs whether the cells swell and lyse is the concentration of membrane impermeant solutes outside the cell relative to those inside the cell. If the outside concentration of impermeant solutes is much less (hypotonic) than that inside the cell, then the cell will swell, and perhaps will lyse if the difference is extreme and osmotic swelling exceeds the mechanical durability of the cell membrane.
3A1: Cane sugar (sucrose) has 1 osmotic particle per molecule, therfore 280 mM sucrose is 280 mOsm (the same osmolarity as physiological solutions), isosmotic. Sucrose is membrane impermeant and therefore the same tonicity as a RBC at 280 mOsm, and hence will not cause lysis.
3A2: 140 mOsm sucrose plus 154 mM NaCl (2 x 154 = 308 mOsm) is 448 mOsm total. 448 mOsm is hyperosmotic to physiological saline; Sucrose and NaCl are membrane impermeant. Therefore 448 mOsm is hypertonic to RBCs (280-300 mOsm at physiolgical conditions). In hypertonic solution, the RBCs will shrink, not lyse = no hemolysis.
3A3. 280 mM urea is isosmotic to physiological saline. Hoever, urea is membrane permeant, and therefore not osmotically active in the presence of RBCs. In this case, the urea solution behaves like water alone, is severely hypotonic to RBCs, causing the cells to swell and hemolysis to occur.
3A4: 1 M urea alone is extremely hyperosmotic to physiological saline. However, because urea is membrane permeant, we can ignore it with respect to tonicity. 154 mM NaCl is (2 x 154) 308 mOsm, which is near to, but only slightly hyperosmotic compared to physiological osmolarity. All the tonicity in this combined solution is due to NaCl, 308 mOsm, which is very slightly hypertonic to RBCs. This may cause slight shrinkage, but certainly will not cause hemolysis.
