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?