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This recent paper in Cell describes a cancer cell using osmotic pressure to move in confined spaces. The cell preferentially inserts Na+/H+ antiporters in the leading membrane. I want someone to double-check my osmosis before I make students reason it out:

  1. The normal Na+/K+ pumps cause an increase in Na+ ions outside the cell, but little change in osmotic pressure across the membrane.
  2. Increased Na+/H+ antiporters will allow Na+ in while pushing H+ out at the "front" end of the traveling cell.
  3. While this causes little net solute change, it increases water flow into the cell via aquaporins (?? this part I find mysterious)
  4. Na+ continues to be pumped out of the cell by the Na+/K+ pump at the "back" end of the cell
  5. Water follows the Na+ through aquaporins out the "back" of the cell, moving the cell forward

I don't feel convinced that moving ions around like this is causing osmotic gradients. Can anyone convince me?

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Why don't you think that moving ions builds up osmotic gradients? –  Chris May 12 at 12:22
    
Because the same number of ions cross in and out - Na+ in, H+, out. –  Adrienne May 12 at 13:00
    
This means you are not building up apotential across the membrane (which would happen if you only transport ions in one direction), but you build up osmotic gradients. This is the reason why glucose is phosphorylated when it has been taken up by cells to remove it from the glucose gradient. –  Chris May 12 at 13:06
    
Sorry, I still don't understand how water flow can be triggered if there is not a net change in solute molecule number across the membrane... can you tell me more? –  Adrienne May 12 at 13:47
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Since posting, I did some more research using background refs from the intro (who knew?). The trick seems to be that when protons are pumped out, the cell regenerates them from water in order to maintain pH. So the solute concentration DOES increase inside the cell with more Na/H transport. –  Adrienne Jun 3 at 19:07

1 Answer 1

Since posting, I did some more research using background refs from the intro (who knew?). The trick seems to be that when protons are pumped out, the cell regenerates them from water in order to maintain pH. So the solute concentration DOES increase inside the cell with more Na/H transport.

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