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For example, I do know that red blood cells have both the Na+/Cl- co-transporter and Aquaporin (water channel) that allows both Na/Cl and water molecules to pass through. But why in biology textbooks they imply that osmosis occurs instead of the diffusion of Na/Cl molecules, though red blood cells have the channels for them, too? (that leads into crenation or lysis of the cells)

NOT ion PUMPS, but ion CHANNELS (those that don't require energy). The Na+/Cl- co-transporter works along with the gradient (not against!).

If an RBC is to put in a hypertonic solution, why wouldn't it absorb the excess NaCl using this channel instead of letting the water out?

I do know in the case when we put cells in a hypertonic/hypotonic glucose solution, osmosis will occur instead because the cells need insulin to allow glucose in (via GLUT4).

But when does exactly osmosis occurs instead of diffusion?

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But when does exactly osmosis occurs instead of diffusion?

These processes are not mutually exclusive. Both can happen simultaneously. Their rates depend on different parameters such as permeability of the membrane towards water and the solutes. A semi-permeable membrane has different permeability for different molecules. There are cases when one of these two processes may happen but not the other (again, a function of the parameters):

Osmosis+/Diffusion-
When there are no solute transporters expressed in the membrane of the concerned cell. For e.g. RBCs do not express the Sodium-Chloride symporter. It is expressed in the distal convoluted tubule of the nephron.

Osmosis-/Diffusion+
When the osmolarity of the cytosol is same as that of the ECF and the cell expresses ion channels. The action potential in a neuron would be an example of this phenomenon.

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  • $\begingroup$ I think you could address more specifically why some cells lyse in hypotonic solutions when they contain substrate channels. I think the answer is that in most cells the various channels are gated, but I have no idea what "RBC" is. $\endgroup$ – Inhibitor Aug 17 '15 at 16:35

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