# I don't understand osmosis

• theslowblitz

#### theslowblitz

I don't understand osmosis. According to my textbook, water will flow through a partially permeable membrane from a less concentrated solution of sucrose, to a more concentrated solution of sucrose, even if it means one side of a partially permeable membrane has more water than the other.

I understand diffusion as a net movement of randomly moving particles. If one side is all particle A, and the other side is all particle B, then it is almost absolutely certain that some of the particles on each side will move to the other side. If one side is mostly particle A, then chances are more of it will move over to the other side than will come back, over time resulting in equilibrium.

I don't see why it should be different in osmosis. If one side is more concentrated than the other, and the sucrose molecules cannot pass through the partially permeable membrane, I don't see how it makes the water more likely to go to one side. The textbook tells me that water always moves from a solution with higher water potential to a solution with a lower one, but it doesn't tell me why!

So, is anyone kind enough to explain?

More than one way to skin that cat... For example: speed at which water migrates through the membrane must depend on how often molecules hit the membrane. The higher the water concentration, the higher the frequency, so the more water passes the membrane.

Amount of water on the side doesn't matter, it is concentration that is important. Concentration of WATER. The higher the concentration of sucrose, the lower the concentration of water.

I don't understand osmosis. According to my textbook, water will flow through a partially permeable membrane from a less concentrated solution of sucrose, to a more concentrated solution of sucrose, even if it means one side of a partially permeable membrane has more water than the other.

I understand diffusion as a net movement of randomly moving particles. If one side is all particle A, and the other side is all particle B, then it is almost absolutely certain that some of the particles on each side will move to the other side. If one side is mostly particle A, then chances are more of it will move over to the other side than will come back, over time resulting in equilibrium.

I don't see why it should be different in osmosis. If one side is more concentrated than the other, and the sucrose molecules cannot pass through the partially permeable membrane, I don't see how it makes the water more likely to go to one side. The textbook tells me that water always moves from a solution with higher water potential to a solution with a lower one, but it doesn't tell me why!

So, is anyone kind enough to explain?

Think about it like the "ratio" of water and sucrose. If on one side of the membrane that ratio is higher, we say the osmolar concentration is higher. Water needs to move to the other side such that the ratio will be even.

@Borek: Thanks, I get it now!

Forget the membrane. Imagine you have sugar solution and you let drops of it fall into a vessel of water. Or you must have dropped drops of ink into water sometime. What happens? The even without mixing it sugar or ink slowly spreads out. If you wait enough you will have a vessel of uniform concentration of the dissolved stuff. If you wait longer it never unmixes though. That is what is happening in your example, the sugar is just getting maximally mixed with the water, all the membrane does is slow this down but eventually all the water will have gone into the sugar .

Now a different thing. If you had two vessels of water. or make that of sugar solution all of the same concentration connected by a tube at the bottom, but at different levels. You know that when they are connected water will flow from where it's higher into the lower vessel until the level is the same height in both, there's a pressure difference making it flow. But when they are the same level they will never of their own accord rise on one side lowering the other.

Now say you put a membrane like yours in the connecting tube. And you put sugar into just one of the vessels. The water tries to mix in with the sugar like before, so it goes in. But that makes the water height rise on one side and fall on the other - not what it does without the membrane barrier. There is a height difference, i.e, a pressure difference that is trying to drive the water back to the other vessel. The level rises only to the height where this tendency counteracts the mixing tendency. You reach an equilibrium. On the one hand the water is not all mixed with the sugar as it was in the first para, on the other hand the levels on the two sides are not equal as they were in the 2nd para. The water pressure corresponding to this height difference is called the osmotic pressure. Very important! The two tendencies - the mixing tendency and the mechanical (pressure equalisation) one work against each other - but reach a calculable equilibrium.

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Told you there are many ways to skin that cat.

think of water as also having a concentration - the higher the solute concentration - the less the water concentration. Water also moves down its concentration gradient!