BIO105K a Handout to Help You Understand OSMOSIS on Page 133 of Our Book

BIO105K A handout to help you understand OSMOSIS on page 133 of our book.

All cells are bounded by a CELL MEMBRANE (also known as the PLASMA MEMBRANE) composed of a PHOSPHOLIPID BILAYER.

Imagine a phospholipid bilayer in a beaker of water. This isn’t a living cell. It is just sort of similar to a soap bubble. There is water outside of the membrane “bubble” and also water inside of it.

The membrane itself has a thin hydrophobic core where the tails of the phospholipids are located. Now imagine adding table salt (NaCl) to the beaker. The salt would rapidly dissolve into the water as ions.

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MOLARITY - We focus on the concentration of the solution (how much salt is dissolved in how big a volume of water) and this is given as molarity. The units for molarity are called “molar” and are designated by a big M which stands for “moles per liter”.

The GRAM MOLECULAR WEIGHT of NaCl is about 58. That means one mole of NaCl weighs 58 grams. Let’s say our beaker has two liters of water in it and we dissolve 14.5 grams of NaCl into it. We dissolved one fourth of a mole of salt (14.5 divided by 58 is equal to 0.25). So you might think we have a 0.25 M solution but we don’t because we dissolved the salt into two liters of water. So we have a 0.125 M solution of NaCl in water. Or you could call this 0.125 moles/liter. (0.125 M would often be written in millimolar units as 125 mM).

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If you added salt to the beaker above it would dissolve and DIFFUSE evenly throughout the beaker so that if you took a drop from anywhere inside the beaker, the concentration of salt in the drop would be 125 mM NaCl. EXCEPT FOR THE WATER INSIDE THE MEMBRANE. The salt would diffuse evenly throughout the beaker but it cannot cross the membrane because the membrane has a hydrophobic core. The salt ions are very hydrophilic and won’t stop dancing with water for even a split second and so they can’t cross the membrane. NOTE: Keep in mind we are not talking about a living cell. This is an artificial membrane.

The concentration outside the membrane is 125 mM NaCl and the concentration inside the membrane is 0 mM NaCl. We say there is a CONCENTRATION GRADIENT across the membrane.

Imagine poking a little hole in the membrane. The salt would diffuse in.

The salt would continue diffusing in until the concentration was the same inside and outside. When the concentration is the same inside and outside, there is no concentration gradient and we are at EQUILIBRIUM. At equilibrium, it is important to understand that salt still diffuses in through the hole but it now is also diffusing out of the hole at the same RATE.

*** NOTE: In the drawing, the bubble looks big, but really it would be very small and so the concentration of salt outside the bubble really doesn’t go down much when it is allowed to diffuse through the hole into the bubble.

Now imagine the hole gets closed and we add more salt to give us the following:

Once again there is a concentration gradient. Salt would like to diffuse in but it can’t cross the membrane. Over time, something surprising happens. The membrane is held together by weak forces and is somewhat leaky, but not to the salt. The salt ions can’t cross because they are constantly surrounded by water molecules called the SHELL OF HYDRATION so they are effectively pretty big objects and are too big to leak across. On the other hand, water molecules are small and can slowly leak across the membrane. Some are leaking in and some are leaking out all the time. In our example, because there is a higher concentration of salt outside, the rate of water leaking out will be higher than water leaking in. Over time, bubble will shrink as water leaks out and as it does, the concentration of salt inside will slowly go up.

Equilibrium is reached when the concentration inside has risen to 500 mM NaCl. At equilibrium, water continues to slowly leak across the membrane in both directions and the rate of water leaking in is the same as the rate of water leaking out.

Water leaks across biological cell membranes also. This is called OSMOSIS. OSMOLARITY is the molarity of all the substances (it doesn’t matter what they are) either inside or outside the cell. If the total osmolarity is higher outside the cell, it is said to be HYPERTONIC and the cell will shrink because water will leak out faster than it will leak in. If the osmolarity is lower outside the cell than inside (HYPOTONIC), the cell will swell because water will leak in faster than it leaks out and the cell might even swell so much it bursts. When the osmolarity outside is the same as inside, it is said to be ISOTONIC and is at equilibrium. At equilibrium, water continues to leak across the membrane but it leaks in at the same rate that it leaks out.

Here are some examples that use the sugars glucose and sucrose. Like salt, the sugars are hydrophilic and cannot cross the membrane. Would the bubble shrink, swell, or stay the same? Would the overall net rate of water flow be inward, outward, or neither? What would the concentrations of sugars be at equilibrium? What would happen if there were a hole in the membrane so the sugars could cross?

*** NOTE: Keep in mind that the volume of water inside the bubble is small compared to the total water in the beaker. So if the cell shrinks because water flows out, the concentration of sugars outside the cell will basically stay the same.

ISOTONIC HYPERTONIC

HYPOTONIC ISOTONIC

ISOTONIC

In the previous two examples, the total osmolarity outside of the bubble is 200 mM. Osmolarity is calculated by adding the molarities of everything together. It doesn’t matter what the substances are.

One last point: