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High Yield Notes
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Content Reviewers:

Rishi Desai, MD, MPH

Osmosis is a group of people that take complicated medical topics and teach them in an organized and effective way so that the information seeps into your brain and leads to longer retention… oh wait, not that Osmosis?

Well, then, simply put, osmosis is how water molecules move across a semipermeable membrane that separates two solutions.

It can be thought of as passive diffusion of water and it requires no energy.

When water molecules move like this, they end up equalizing the concentrations of the solutions on either side of the membrane.

This is possible because a semipermeable membrane, like the cell membrane for example, is kinda like a sieve with pores that let small molecules like water across, but not larger molecules or ions like sodium and chloride.

So let’s say that we’re looking at a lab beaker that is filled with a salt water solution, and we separate it in two compartments - A and B - with a semipermeable membrane in the middle.

Now, first off - the salt which is sodium chloride will separates out into sodium ions and chloride ions once it’s in the water.

And since the concentration of sodium and chloride ions is the same on either side of the membrane, we say that A and B are isotonic to each other.

Now, inside the two compartments, water molecules and sodium and chloride ions are moving around and bouncing off each other.

It’s a bit like two big dance parties happening in two adjacent warehouses that are connected by doors that are semipermeable - meaning that the water molecules can get through but not the larger sodium and chloride ions.

Now some of the water molecules may go through one of these doors to go from party A to party B, and some water molecules might go the other direction from party B to party A.

But the truth is that the water molecules aren’t particularly drawn to either compartment, because crossing the membrane to go one way is just as easy as crossing the membrane to go the other way.

We call this point equilibrium - and in this particular state the net movement of water across the membrane is zero.

But what if we were to add some additional salt - or sodium chloride - inside only compartment A?

A now has more solute than B, and we would say that A is hypertonic compared to B, and conversely that B is hypotonic compared to A.

And what we’d notice over time is that because A has more sodium and chloride ions its osmotic pressure increases.

The result is that we’d see a bigger net migration of water molecules over to side A.

We say “net” migration, because water molecules are still going back and forth between the two sides, but overall, more water molecules will now end up staying on side A.

Once there enough water that the concentration of salt on the two sides is equal once more, then the net movement of water across the membrane goes back to being zero and the two sides are isotonic to one another again. So far so good.

But now, there’s the bigger question of why water molecules end up staying more on side A, and it has to do with kinetic energy and entropy.

Kinetic refers to the fact that water molecules as well as sodium and chloride ions have a tendency to want to move around.

And entropy plays a role because this movement is disordered or random so that over time the water molecules and ions move every which way.

You can think of each molecule or ion like a toddler - you know it’s going to move but you can’t predict it’s next move or where it might end up two minutes later!

Now, going back to the experiment. When we initially added more salt there were relatively more sodium and chloride ions on the hypertonic side - side A.

Now we know that, sodium and chloride ions are too large to pass through the pores in the membrane.