Osmosis

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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 is 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. But in addition to that, and here’s the important part, the ions actually end up slightly blocking or interfering with water molecules that want to go from one side to the other.
That’s because the ions are so large that as they’re bouncing around, they literally get in the way of the door. It’s like a partygoer that is unable to leave a party but also blocks the five foot area around the door - making it hard for others to leave the party as well.
But remember that these ions are not physically stuck to the hole between the two sides, they’re just knocking water molecules in that area out of the way through their random bouncing movements.
To make matters worse, the sodium ions are positively charged and they actually attract the slightly negative charge of the oxygen atom in a water molecule.
Similarly, chloride ions are negatively charged and they attract the slightly positive charge of the hydrogen atoms in a water molecule.
If these water molecules are somewhat attached to an ion that can’t get through the membrane, they will also be hindered from getting through the membrane.
So in short, the ions that can’t get through interfere with the water molecules from leaving the party. And since the concentration of ions is higher on side A, then side B, more of this interference is happening on side A.
As a result more water molecules are ultimately able to get from B to A, then the other direction. All in all, this means that when there’s a difference in concentrations, water molecules move from an area of low solute concentration to an area of high solute concentration.
Put differently, they move from the hypotonic side, where there’s relatively more water, to the hypertonic side, where there’s relatively less water.
Now, it just so happens that osmosis is super important for our cells. For example, let’s say that we put a red blood cell, in a hypertonic solution like super salty water.
Right away, there would be a net movement of water molecules out of the cell, leaving our cell dried out and shrivelled.
On the other hand, let’s say we put a red blood cell in a hypotonic solution like pure water. This time, there would be a net movement of water molecules into the cell, making it swell up and eventually burst.
Again, not a good way to go. Like goldilocks, our cell would prefer something in the middle - an isotonic solution - like the intravenous fluids that are used in hospitals.
These fluids have sodium chloride, in a concentration that’s similar to the solute concentration inside the cell. In this situation, the net movement of water inside and outside of the cell is zero, so the cell continues to live comfortably.
Alright, as a quick recap: osmosis refers to the movement of water molecules across a semipermeable membrane that separates two compartments of fluid.
It can be thought of as passive diffusion of water and it requires no energy. The side that has more solutes will prevent water molecules from leaving, and as a result there will be a net movement of water to the hypertonic side.
Over time the movement of water molecules from the hypotonic to the hypertonic side will equalize the concentrations.