In order to survive, all organisms need to move molecules in and out of their cells. Molecules such as gases (e.g., O2, CO2), water, food, and wastes pass across the cell membrane. Cells of all types are constantly challenged by their osmotic environments, what can be referred to as tonicity, which is the amount of solute in a solution that cannot penetrate a cell membrane. We can describe tonicity in terms of relative concentrations of solute inside versus outside the cell. Water can move freely across the plasma membrane of all cells, either through protein channels called aquaporins or by slipping between the lipid tails of the membrane itself. The movement of water molecules is not itself regulated by cells. If the concentration of water on one side of a cell membrane is different than on the other side, water will move through the membrane seeking to equalize the concentration of water on both sides.  When water concentration outside a cell is greater than inside (hypotonic), the water moves into the cell faster than it leaves, and the cell swells and may burst, killing the cell.  When the concentration of water inside of a cell is greater than outside (hypertonic) water moves out of the cell faster than it enters, and the cell shrinks. If a cell becomes too dehydrated, it may not be able to survive. Under ideal conditions, the water concentration outside is nearly identical to that inside (isotonic). Human cells are isotonic to 0.9% NaCl (saline) solution. Surrounding a blood cell with a 0.5% NaCl solution would cause the cell to ____________________, because this NaCl solution is ____________________ to the cell. Cells in a ___________________ solution will shrivel as water leaves the cell via osmosis. Osmotic pressure is the pressure that would be required to stop water from diffusing through a barrier by osmosis. It is how hard the water would “push” to get through the barrier to diffuse to the other side. Hydrostatic pressure is the pressure that any fluid in a confined space exerts. If we picture a column shaped container, we can see that the pressure pushing against its wall is greater at the bottom, than it will be at the top. This is partly related to the force of gravity. Fluid will rise up a column until osmotic pressure and _____________________________ equalize.