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Osmosis - cell transport

Osmosis is the movement of water from an area of high concentration to an area of low concentration across a semi permeable membrane, but how does it link to the surrounding water being Hypertonic, Hypotonic and Isotonic?
 
In other words, how do Hypertonic, Hypotonic and Isotonic water have to do with Osmosis?
 
Thanks in advance :)
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3 Answers

Osmosis is the movement of WATER molecules across a semipermeable membrane (such as the cell membrane) from where there is a high concentration of water to where there is a low concentration of water. The interior of a living cell consists of cytoplasm, which is a solution of mostly water with dissolved substances such as salt.  
 
Now for the fun stuff!  An Isotonic solution is a solution that has the same concentration of dissolved substances as is found inside the cell.  If a cell is surrounded by isotonic solution, then there is no net movement of water across the membrane by osmosis, because the concentration of water is the same on both sides of the membrane.
 
A hypertonic solution is a solution with a higher concentration of dissolved substances than is found inside the cell.  If a cell is surrounded by hypertonic solution, then water will move OUT of the cell by osmosis because there is a higher concentration of water inside the cell compared to outside (where there is lots of dissolved salt, etc.)  A cell in hypertonic solution will shrink.  
 
A hypotonic solution is a solution with a lower concentration of dissolved substances than is found inside the cell.  If a cell is surrounded by hypotonic solution, then water will move INTO the cell by osmosis because there is a higher concentration of water outside the cell compared to inside.  A cell in hypotonic solution will swell up and, if there is no cell wall, may burst.  A silly but simple way to remember that water moves into a cell in hypotonic solution is that "hypo causes the cell to swell up like a hippo"
Water diffuses across most cell membranes fairly easily, but many dissolved substances do not.  Water is moving in and out rapidly and constantly.  The NET movement of water is from an area of lower dissolved material to an area of higher dissolved materials.  Or, if you phrase it as you have, from an area of high WATER concentration to an area of low WATER concentration.  The terms isotonic, hypotonic, and hypertonic refer to the concentration of dissolved material.  "Iso-" means equal; "hypo-" means low; and "hyper-" means high--all in reference to dissolved material (solutes).  So, the net movement of water is from LOW solute concentration to HIGH solute concentration.  If a solution surrounding a cell has a solute concentration equal to the solutes in the cell, then there will be no net movement.  The following chart might help:
Concentration of solutes     Concentration of water     Direction of water        Term applied to solution
compared to the cell            compared to the cell         movement                   surrounding the cell
       HIGH                                     LOW                        OUT OF CELL                HYPERTONIC
       LOW                                      HIGH                       INTO CELL                    HYPOTONIC
       EQUAL                                   EQUAL                     NO NET MOVEMENT         ISOTONIC
As an example, a solution of about 0.85 g/100 milliliters of sodium chloride or solution of about 5 grams/100 milliliters of glucose is isotonic and can be infused into the veins without problems.  Various other solutions are made for intravenous (within the veins) infusion.  Not all are isotonic, but for various reasons are not damaging.  Whatever the case, these principles must be kept in mind for IV (intravenous) solutions. 
   
 
Hi Aimee,
 
Hypertonic solution has a higher concentration of salt molecules than water molecules.
 
In contrast, hypotonic solution is just reverse; it has more water molecules than salt molecules. 
 
Isotonic solution is balanced with similar concentration of salt and water molecules and has neutral pH. 
 
Osmosis is the movement of molecules across semi-permeable cell membrane; it is true for both salt and water molecules.  Therefore, a cell would only survive briefly in hypertonic or hypotonic solution since natural osmosis would create pH imbalance and disturb homeostasis  in the cell thereby perturbing cellular function of survival.
 
Hope it helps.