You can demonstrate osmosis at home using Umibudo (sea grapes.) The umibudo are packed in super-salty water, which makes them shrivel by osmosis. When you rehydrate them in fresh water, you can see them swell up as the water rushes in by osmosis. Osmosis before your eyes!
Water model have moved beyond "molecules bouncing of each other" and "water molecules that (supposedly while flying by Na+) MIGHT stick to Na+". Why is this "billiard ball" model still taught?
I have doubts about the mechanical blocking argument. The big molecules (of the high solute concentration) would equally block input and output which would balance out it's presence. I was happy to hear the charge argument, that made more sense to me. What about capillary action? Is it not possible that the water flows to the high solute concentration caused by adhesion towards the solute molecules? Or even initially adhesion towards the cell membrane, and after that greater adhesion to the high solute molecules? Last but not least, from Tesla's perspective: think of the universe in terms of vibration. Does vibration have a say in this story whatsoever?
You are partially right when the solute particles block the approach to the semipermeable membrane openings.it would equally block particles moving in or out. So there will be no net movement at that moment.but what you have to know is that theses solute particles keep moving around so when the move out of place the water molecules get the chance to diffuse into the membrane from left to right but the particles on the other side do not get the chance because when these solute particles move the y ricochet and Bounce off the water particles pushing them away from the openings so because of this solute hinderance it impedes the movement from right to left.but keep in mind that still some particles will be able to move but if u look at the greater picture there is a net movement from left to right.hope this helps.
In general these are great videos, however, you are mistaken in the forces producing osmosis. First, water molecules are not randomly moving in the same way that the solute particles are. The polarity of water dictates that there will be hydrogen bonds formed between adjacent molecules. In the liquid form, the water molecules "line up nose to tail". A such, it is not the random movement of individual molecules that dictates the probability of crossing from one side to the other. In addition, the solute molecules do not "get in the way" preventing water from moving toward the membrane on the hypertonic side. The force causing diffusion of solutes is the repulsion of solute by other solute molecules increasing the probability that solute will collide and be "pushed" from the hypertonic side to the hypotonic side. As water molecules do not repel each other, the same force is not present for osmosis as diffusion. The forces causing osmosis are not from the water molecules, but from the solute. Solute molecules will be surrounded by water molecules which are hydrogen-bonded in a "shell" around the solute. The solute, along with the shell, are moving with Brownian motion, but can't cross the membrane, so there is a high probability that solute will concentrate at the membrane (they are moving randomly in 3 dimensions, but prevented from progressing across the membrane, so they concentrate there. This relatively high concentration of solute results in repulsion of solute, along with the shell of water, away from the membrane. The movement of solute and shell of water away from the membrane exerts inertia on other water molecule, and the solute, the shell of bound water, and the water molecules in the same general area all move away from the membrane. This creates a small area where water is absent on the hypertonic side of the membrane. As water on the hypotonic side can cross the membrane, water molecules are drawn across the membrane to fill the vacant space. In a simple form, diffusion is a "pushing" force produced by solute molecules repelling other solute molecules. Osmosis is a "pulling" force produced by solute molecules moving away from the membrane.
I don't think this explanation really makes comprehensive sense. Osmosis is said to be colligative, i.e. it occurs regardless of the properties of the solute. Your explanation seems to depend on the properties of the solute. Secondly, why would the solute molecules concentrate near the membrane? I see no reason they should concentrate there any more than any other point. Thirdly, for every solute molecule moving away from the membrane, you should have a solute molecule moving towards the membrane, cancelling the effect. The idea of the formation of the "shell" being the driving force sounds more like the entropy explanation we hear for the hydrophobic-hydrophilic grouping effect.
@@superdog797 do you think Sal's explanation is correct? Because if the solute physically blocked the water molecules from moving to the hypotonic side, it would pretty much do the same to the other side and block water trying to go from the hypotonic to the hypertonic side
@@Pala4765467141 Except that on the side with the solute the solute reflects against the membrane, whereas the membrane, being semipermeable, does not, in effect, reflect the water. In the instant in which the solute particle reverses against the membrane it pushes solution back into the solute side and in that moment its motion is reversed and allows for other water to come in.
@@superdog797 I think the idea is that solute molecules would concentrate near the membrane because they lose momentum after colliding/interacting with the membrane. But I also agree with JV Victor that the charge-less model described in the video suggests that water is just as likely to be blocked by the solute from either direction. Okay, so when some solute has been reflected by the membrane and is now moving right, water can enter the space left between this solute and the membrane. But so what? Moments later, solute will be moving left towards the membrane again, pushing the water back out. And yet, as you say, there must be an explanation here that doesn't depend on the charge of the particles, because osmosis is supposed to be able to occur with uncharged solute as well.
@@OlleLindestad The membrane is in a fixed position and being a solid means its molecules are in a much more cohesive state than the solvent and solute ones, so any momentum these lose from colliding against the membrane is elastically restored. Also, to block a water molecule coming from the other side of the membrane, the solute needs to be completely covering the opening, whereas a water molecule from the other side can be blocked by the solvent at any point as long as it's between it and the opening. Another factor to take into consideration is pressure, as in the force water molecules exert on the solute molecules by bouncing on them. This causes the solute molecules to occasionally act in a similar way to valve checks, where pressure from one side causes a ball bearing to get pushed against an opening, sealing it, and pressure from the other side causes it to move away, allowing water to flow but only in a single direction. It's no coincidence that in this case the direction matches the concentration gradient.
I feel like nobody really knows why osmosis occurs and it's best to just remember that adding solute to solution lowers the hydrostatic pressure. The amount of pressure drop is the "osmotic pressure" on the solvent. That's it. Diffusion really doesn't explain it because diffusion is just a statistical property - it can't work against any force that acts on every molecule (such as the force of gravity). The interference explanation doesn't make sense to me because if a solute molecule blocks a solvent molecule from crossing the membrane from high to low concentration solution, then you should equally have a blocking motion on the opposite side of the solute molecule occurring at the same moment. I read somewhere that what is "actually" happening is that the solute molecules are actually preventing the solvent from interacting with themselves and thus preventing them from exerting pressure on themselves, thus decreasing the hydrostatic pressure, thus accounting for the fluid pressure difference that we call the osmotic pressure. That doesn't really make intuitive sense to me though - why doesn't the solute molecule exert pressure also? If anybody knows how it really works mechanistically I'd like to know. Please cite your source, by the way, because I've heard a lot of people try to explain it thinking they understand it and I point out something to them and they're wrong or incomplete. Even people who are knowledgable...
THANK YOU. I've been driving myself nuts researching osmosis for the past few days. It's such a bizarre subject. On the one hand, it's described in nearly all lower-level sources (school textbooks, youtube explainers, etc.) as a simple and settled issue, usually with a reference to water diffusing towards lower water concentration. On the other hand, there appears to be conclusive evidence that osmosis is NOT simple diffusion (experiments with isotope-labeled water, for example, show that osmosis can move water into a compartment faster than the water can diffuse passively), and that it is NOT driven by differences in water concentration (some solutes actually increase the number of water molecules per volume, but additional water will apparently still osmose into such a solution). The explanation involving interactions between molecules based on electric charge, which is also mentioned in the second half of the video, also can't seem to cover every case, because uncharged solutes can also drive osmosis - and the myth-debunking articles I've read point this out, too. Then the myth-debunking articles typically go on to describe a mechanism similar to the one outlined in the first half of the video, where solute molecules block the way of solvent molecules as they bounce off the membrane - but fail to explain, as you point out, why they don't block the way of water molecules headed into the hypertonic solution as well. Usually when there's a physical phenomenon that is poorly understood, there is at least widespread acknowledgement that it IS poorly understood. And I would be fine with that! There's much in the universe we don't understand yet But osmosis isn't talked about in this way. It's either described as a settled issue, or it's described as "okay, biology textbooks get it wrong, there are widespread misconceptions, but the true explanation is known and here it is", and then another seemingly flawed explanation is provided. So yeah, I am losing my mind over here. :P
@@OlleLindestad I agree with you and think you are correct. The phenomenon is a complex set of thermodynamic phenomenon at the end of the day. However, the notion I mentioned about the solute molecules being reflected _backwards_ and thereby creating the flow tendency seems to be the primary driving mechanism. One question, and the reason thermodynamics must be invoked to give a complete explanation, is _why_ the solute molecules are able to _keep_ on bouncing around without losing energy and thereby create the energy input in the system to generate the osmotic flow. The short answer is that a complex system of equations can be used to model the energy input into the fluid system. Part of this involves terms such as brownian motion, enthalpy, entropy, gibbs energy, etc. One simplified way of thinking bout it is that, basically, the fluid takes in heat from the surrounding environment, which keeps it in motion, and thus ultimately you are able to get the energy needed to drive osmosis. I think that if you comprehend, however, the idea that the solute molecules are changed in direction by the membrane, and then they create the flow tendency of the water, you should have a pretty good intuitive understanding of why osmosis works. It makes sense to me at least and I find it unfortunate many biologists and teachers of that subject don't discuss some of the simple physics involved. It leads to confusion and frustration and even misunderstanding. It sounds to me like you are on the right track, however, so I wish you luck.
What is exactly meant by the net movement of water molecules in osmosis? Water molecules pass in both directions randomly not down a concentration gradient as well, so is the net movement the movement of water molecules that do move down a concentration gradient?
Mousehead2000 it’s the diffusion of water through a semi permeable membrane. There are three different variants hypertonic, hypotonic and isotonic. Think of the semi permeable membrane as a filter for water.
Omar Khattab it’s the diffusion of water through a semi permeable membrane. There are three different variants hypertonic, hypotonic and isotonic. Think of the semi permeable membrane as a filter for water.
See [2003 Philip Nelson: Biological Physics: Energy, Information, Life, Chapter 7.3 Osmotic Flow]. "Now I can see the actual mechanism of force generation. When a membrane is impermeable to solute particles, then those particles bounce off the membrane when they approach it. Because of viscous friction, the particles entrain some water as they move, and so water too, is pulled away from the membrane. But water can pass through pores in the membrane, so some is also swept through it [note: what is meant is some water is pulled through the membrane to replace the other water that was pulled away from the membrane]. That's osmotic flow; a backward pressure is needed to stop it." "But wait. Even when the particles are free (no membrane), their Brownian motion disturbs the surrounding fluid! [...] That's true, but the effect you mention is random and averages to zero. In contrast, the membrane [...] create a net motion in one direction." "[...] where the net momentum flow into the fluid comes from. Particles constantly impinge on the membrane in Figure 7.6a from the right, never from the left. Each time the membrane is obliged to supply a kick to the right. Each kick delivers some momentum; these kicks don't average to zero. Instead, they pull fluid through the channel until equilibrium is reached." "Our discussion makes clear how misleading it can be to refer to 'the osmotic pressure'. Suppose we throw a lump of sugar into a beaker. Soon we have a very nonuniform concentration c of sugar. Yet the pressure is everywhere constant, not equal to kTc as we might have expected from a naive application of the van 't Hoff relation. After all, we know that osmotic pressures can be huge; the fluid would be thrown in to violent motion if it suddenly developed such big pressure variations. Instead it sits there quietly, and the concentration spreads by diffusion. The flaw in the naive reasoning is the assumption that concentration gradients themselves somehow cause pressure gradients. But pressure can only change if a force acts. Thus osmotic pressure can only arise if there is a physical object (the semipermeable membrane) present to apply force to the solute particles. In the absence of such an object (for instance, if we just throw a lump of sugar into the water) there is no force and no pressure gradient. [...] Only when solute molecules have had a chance to diffuse from the initial lump of sugar to the membrane will the latter begin to rectify their Brownian motion [note: by rectify the Brownian motion Nelson means to create a net motion in one direction] and so transmit force to them, and thence to the fluid." Also [2013 Kramer, Myers: Osmosis is not driven by water dilution] "The key interactions take place in the small region of space adjacent to a pore aperture that allows water molecules to pass but repels solute [...]. Each time a solute molecule enters this region, it is repelled. That is, the aperture gives to the solute molecule a small amount of momentum directed away from the membrane. Due to viscous interactions between solute and water, this momentum is rapidly shared among all nearby molecules, including both solute and water [...]. Thus, although the pore aperture repels only the solute, the net effect is a force directed away from the membrane acting on the solution as a whole. This is the counterintuitive idea at the center of osmotic theory: a pore that lets water molecules pass freely will effectively repel the water if solute is present."
OK I think I figured out a mechanism for osmosis. Sal's explanation is kind of correct but doesn't quite express it right. The gist of it is that there is a net momentum vector for all the matter in the system that sits on the solute-solvent mixture side of the membrane. If you break the system down into two masses, the mass of water, and the mass of solute, we see that the mass of water's (solvent's) center of momentum movement is directly in the middle of the system over the membrane. However, when we look at the mass of solute's center of momentum, we see that it's in the middle of only the solute-solvent side. When you take the average of these two momentum vectors you get a net momentum vector that has a center somewhere between the two in physical space, so the tendency overall is for the water to move in the direction of the solute-solvent side toward the center of mass of the system. Another way to think of it is that the barrier imparts energy to the system only on the side in which it is capable of deflecting matter (solute side). The Brownian motion of the molecules is the driving energy of the movement of molecules in the system. Where does the energy come from from the Brownian motion? Well, perhaps there is some internal energy at the subatomic/nuclear level, but I suspect it's more driven by the addition of heat from the environment and the transfer of kinetic energy to the particles from the barrier and walls. If a molecule hits the membrane, it is accelerated in the opposite direction. Energy is imparted to the molecule from the wall, and the wall gains energy from the particle. With each exchange, some kinetic energy is lost due to friction. Because the membrane is, on net, only interacting with the solute particles, any kinetic energy that the solute particles lose to the membrane barrier is lost only in that side of the system, but not the other half. This would imply the overall kinetic energy of the solute-solvent system is less than the pure-solvent side, which would obviously lower the water pressure and thus move water, on net, into the solute-solvent mixture side. But, you might ask, osmosis is powerful enough, apparently, to work against gravity. This requires work, so energy LOSS doesn't seem to really explain how it can do work. Well, like I said, the Brownian motion of the particles is constant overall, so whatever inputs to the Brownian motion of the particles are, it must be the energy into these inputs that osmotic energy is driven by. It must be the case that the heat of the environment is going into one side of the system at a higher right than the other. I suppose that the solution must have the same temperature throughout on both sides of the membrane (does it? I suppose this could be measured). The order of energy seems to be: heat from environment --> Brownian motion of liquid particles (Kinetic Energy) --> energy lost to membrane barrier The energy lost to the barrier must be small compared to the increased input from the environment, otherwise you wouldn't be able to do work like elevate the solution against gravity. I would therefore speculate that the rate of heat intake in the system is greater on the solute-solvent side, because for the Brownian motion to remain constant, one needs an increased amount of energy to compensate for the energy lost at the membrane. So that's my hypothesis about osmotic mechanism. Any thoughts? The next question I have is: if this description is correct, does it imply that the total osmotic pressure is linked (proportional to) to the surface area of the membrane, or that the surface area of the membrane merely affects the rate of osmosis overall? Intuition at first tells me that the increased surface area of a membrane should increase the osmotic pressure overall, however as far as I know, the osmotic pressure is directly proportional to the solute concentration only, not the membrane surface area. This may imply that the surface area of the membrane only affects the rate of exchange, but not the overall osmotic pressure. This could be tested empirically by simply having two separate identical systems in terms of water mass, solute concentration on one side, and varying only the surface area of the membrane, and then measuring (1) what the rate of water movement is, and (2) what the overall end result is at equilibrium. If the rate varies but the end result is the same, then the membrane surface area doesn't affect the osmotic pressure. If the end result varies, then the osmotic pressure is proportional to the surface area of the membrane. As a secondary experiment, you could measure the temperature of the fluids and the rate of heat exchange on both sides of the membrane.
hi i got a little bit confuse but may u answer he some of the questions? All about osmosis this is what i understand from this video , inside the molecule have sugar (salute).So the water outside the cell will go into the molecule so the water outside the molecule and inside the molecule become the same. Am I right? pls answer me, I am going to have a exam.pls thx
um i dont quite understand ur question.. but osmosis is when "water moves from an area of lower solute concentration to area of higher solute concentration" which in plain english means the water will go towards and across the cell membrane to basically get to the solute if that helps:)
u should probably start w/ the topic. some may think attacking your bloodstream for dietry has something to do w/ hormones, which is produced in the!!!!
This didn't explain hydrostatic or osmotic pressure to me at all which is the part I'm having trouble understanding. Osmosis by itself isn't that much of a hard concept to understand.
When the solute molecule, which can't cross the membrane, bumps into the wall, it reverses its motion. When it reverses its motion, it "bumps" other water molecules back into the direction it starts traveling after bumping into the wall. In other words, when the solute particle bumps into the wall, it reverses its motion, but then it also reverses the motion of the water molecules that were behind it traveling toward the membrane. Because this happens in greater quantity on the side with a higher concentration, the overall tendency is for water to go to the side with the higher solute concentration.
Is probability of molecules passing through really the explanation of the mechanism of osmosis? He kept saying it was just a theory, so what's the actual scientific reason for it?
Any membrane that can block solute (like salt) molecules and pass solvent (like water) molecules will do. For example, the membrane could have holes with sizes between the size of water and salt molecules.
is it also the case that, on the side of the higher water concentration, then a higher proportion of the molecules which hit the semi-permeable membrane are molecules which can pass through it (so water), so this side has a higher rate of molecule interactions which could cause the molecule to pass through and therefore on average more molecules will pass through, causing net movement from high water concentration to low concentration?
You do see this explanation in many places. But the problem with explaining osmosis using the concentration of water is that osmosis happens even when you have a solute that *increases* the concentration of water. (This sounds weird, but some compounds break up the interactions between water molecules in such a way that the water molecules move closer together than they would in a mass of pure water.) I've spent the last few days scouring the internet for a conclusive explanation for why osmosis happens, and so far it actually seems like there is no agreement on what the ultimate driving mechanism is! But I'm not a physicist, so take that with a grain of salt.
I found this a little bit tedious. For the information I was trying to learn, the presentation went about too long and redundant almost telling a story to small kids. Not something i would want to see if I'm pressed for time and at the level of my inquiry (Graduate school). Maybe, just clear and concise terms and going direct to the point. But still thankful for the video to educate people.I still will continue to use your channel :)
The video isn't meant for students in graduate school, probably because graduates shouldn't need this video anyway. Maybe you should work harder instead of complaining about this video.
MY MAN KHAN! although you give lessons for free and shit 2 million subscribes and views getting you MULLA and you be hustling and horsing around the city with your eyes closed doing wheelie on yo Harley Davidson. keep it up.
Thank you! I am a junior in college, and I have always a hard time grasping this concept until I watched this video!! After 14 years of school, I can say I have learned more from youtube than most of my teachers.
Overview of the entire thing (for bio students) High h2O concentration goes down the concentration gradient and where the solutes go the water ultimately will follow by probability and then remain there due to them being attracted to their full change. Probability: Water molecules have a higher chance of crossing a semi-permeable membrane than larger molecules I.E. sodium, due to there small size. Thank me later
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I'm sorry but this is not correct. It is not because of ionic charge. It is because the solute particles bump up against the semi-permable membrane and then _reverse_ their motion. When they reverse their motion, they bump into water molecules on their own side, and then bump more water molecules _back_ into the same side as the solute. The tendency, therefore, is for a net movement of water onto the side with more concentration of solute.
Can i ask you something Khan ? The table salt or the sodium chloride ( NaCl ) is neutral because when sodium ion ( Na- ) bonds with chloride ion ( Cl+ ) it becomes neutral because the positive charge cancels the negative charge then how are the water molecules attracted to the salt the salt is neutral how come it will attract ?????????? Please answer 😊😀
I know this is a while later but the answer to your question is because water is highly polar, which means that both sides have a different charge. The oxygen side is negative (-) and the hydrogen side is positive (+). This polarity is strong enough to break the bonds between the Na+ and the Cl-. Basically, Cl- thinks that the Hydrogens on water look more positively handsome so she leaves Na+. Na+ wants to make Cl- jealous so he goes to hang out with oxygen, who is more negative than Cl- ever was! Since Na- and Na+ are no longer together, they no longer cancel each other out. The fact that Cl- and Na+ break up is also why salt disappears in water. Together they are a beautiful crystal! But when water cuts in-between, they fall apart and you can't see anything of what they used to be! At least that's how I think of it. Hope that helps!
The first model shown in the video, which ignores any charge-mediated interactions between the solvent and solute, doesn't seem to provide a reason why solvent molecules should be more likely to move from left to right than from right to left. A solvent molecule in the right-hand compartment could be prevented from crossing because it'd meet solute molecules heading away from the the membrane, having just "bounced off" the membrane - sure. But isn't the same true of solvent molecules in the left compartment heading towards the membrane? They could just as well be blocked by a solute molecule headed leftwards, TOWARDS the membrane, and at any given moment, there should be just as many solute molecules headed towards the membrane as away from it.
Hi. Most of industrial RO membrane pore size is around 0.0001-0.001mcm. As you know the dimension of water molecule is around 0.275nm. How can water pass through the RO membrane?
I just enjoy listening to Sal’s voice.
I love how he says the same thing twice, I really need that
You should have 7 billion subscribers. Your organization helps so many people!
This is a beautiful example of Osmosis, I came out learning a new concept.
You can demonstrate osmosis at home using Umibudo (sea grapes.) The umibudo are packed in super-salty water, which makes them shrivel by osmosis. When you rehydrate them in fresh water, you can see them swell up as the water rushes in by osmosis. Osmosis before your eyes!
Water model have moved beyond "molecules bouncing of each other" and "water molecules that (supposedly while flying by Na+) MIGHT stick to Na+". Why is this "billiard ball" model still taught?
THANK YOU.THE VIDEO CLEAR EVERY CONFUSION I HAD :_:
i am here from 2020 to say thank u
I have doubts about the mechanical blocking argument. The big molecules (of the high solute concentration) would equally block input and output which would balance out it's presence.
I was happy to hear the charge argument, that made more sense to me.
What about capillary action? Is it not possible that the water flows to the high solute concentration caused by adhesion towards the solute molecules? Or even initially adhesion towards the cell membrane, and after that greater adhesion to the high solute molecules?
Last but not least, from Tesla's perspective: think of the universe in terms of vibration. Does vibration have a say in this story whatsoever?
You are partially right when the solute particles block the approach to the semipermeable membrane openings.it would equally block particles moving in or out. So there will be no net movement at that moment.but what you have to know is that theses solute particles keep moving around so when the move out of place the water molecules get the chance to diffuse into the membrane from left to right but the particles on the other side do not get the chance because when these solute particles move the y ricochet and Bounce off the water particles pushing them away from the openings so because of this solute hinderance it impedes the movement from right to left.but keep in mind that still some particles will be able to move but if u look at the greater picture there is a net movement from left to right.hope this helps.
@@nazihaibrahim6537 this is exactly the explanation I was looking for, thanks!
thanx brother. you solved my confusion.Your video is very helpful.thanx bro.keep it going.
You are the only explainer which I don’t know how the minutes are passing, awesome 👏
Thanks. This helped a lot. 😄
Im from India
And you're the reason that I'm capable to complete my graduation...
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Thanks khan academy
Are these all the possible explanations?
Great! Saw this video after final, goddammit, TH-cam stop sending me Tic Tok videos, please!
In general these are great videos, however, you are mistaken in the forces producing osmosis. First, water molecules are not randomly moving in the same way that the solute particles are. The polarity of water dictates that there will be hydrogen bonds formed between adjacent molecules. In the liquid form, the water molecules "line up nose to tail". A such, it is not the random movement of individual molecules that dictates the probability of crossing from one side to the other. In addition, the solute molecules do not "get in the way" preventing water from moving toward the membrane on the hypertonic side.
The force causing diffusion of solutes is the repulsion of solute by other solute molecules increasing the probability that solute will collide and be "pushed" from the hypertonic side to the hypotonic side. As water molecules do not repel each other, the same force is not present for osmosis as diffusion.
The forces causing osmosis are not from the water molecules, but from the solute. Solute molecules will be surrounded by water molecules which are hydrogen-bonded in a "shell" around the solute. The solute, along with the shell, are moving with Brownian motion, but can't cross the membrane, so there is a high probability that solute will concentrate at the membrane (they are moving randomly in 3 dimensions, but prevented from progressing across the membrane, so they concentrate there. This relatively high concentration of solute results in repulsion of solute, along with the shell of water, away from the membrane. The movement of solute and shell of water away from the membrane exerts inertia on other water molecule, and the solute, the shell of bound water, and the water molecules in the same general area all move away from the membrane. This creates a small area where water is absent on the hypertonic side of the membrane. As water on the hypotonic side can cross the membrane, water molecules are drawn across the membrane to fill the vacant space. In a simple form, diffusion is a "pushing" force produced by solute molecules repelling other solute molecules. Osmosis is a "pulling" force produced by solute molecules moving away from the membrane.
I don't think this explanation really makes comprehensive sense.
Osmosis is said to be colligative, i.e. it occurs regardless of the properties of the solute. Your explanation seems to depend on the properties of the solute.
Secondly, why would the solute molecules concentrate near the membrane? I see no reason they should concentrate there any more than any other point.
Thirdly, for every solute molecule moving away from the membrane, you should have a solute molecule moving towards the membrane, cancelling the effect.
The idea of the formation of the "shell" being the driving force sounds more like the entropy explanation we hear for the hydrophobic-hydrophilic grouping effect.
@@superdog797 do you think Sal's explanation is correct? Because if the solute physically blocked the water molecules from moving to the hypotonic side, it would pretty much do the same to the other side and block water trying to go from the hypotonic to the hypertonic side
@@Pala4765467141 Except that on the side with the solute the solute reflects against the membrane, whereas the membrane, being semipermeable, does not, in effect, reflect the water. In the instant in which the solute particle reverses against the membrane it pushes solution back into the solute side and in that moment its motion is reversed and allows for other water to come in.
@@superdog797 I think the idea is that solute molecules would concentrate near the membrane because they lose momentum after colliding/interacting with the membrane.
But I also agree with JV Victor that the charge-less model described in the video suggests that water is just as likely to be blocked by the solute from either direction. Okay, so when some solute has been reflected by the membrane and is now moving right, water can enter the space left between this solute and the membrane. But so what? Moments later, solute will be moving left towards the membrane again, pushing the water back out.
And yet, as you say, there must be an explanation here that doesn't depend on the charge of the particles, because osmosis is supposed to be able to occur with uncharged solute as well.
@@OlleLindestad The membrane is in a fixed position and being a solid means its molecules are in a much more cohesive state than the solvent and solute ones, so any momentum these lose from colliding against the membrane is elastically restored.
Also, to block a water molecule coming from the other side of the membrane, the solute needs to be completely covering the opening, whereas a water molecule from the other side can be blocked by the solvent at any point as long as it's between it and the opening. Another factor to take into consideration is pressure, as in the force water molecules exert on the solute molecules by bouncing on them. This causes the solute molecules to occasionally act in a similar way to valve checks, where pressure from one side causes a ball bearing to get pushed against an opening, sealing it, and pressure from the other side causes it to move away, allowing water to flow but only in a single direction. It's no coincidence that in this case the direction matches the concentration gradient.
THIS HELP A LOT THANKS
I feel like nobody really knows why osmosis occurs and it's best to just remember that adding solute to solution lowers the hydrostatic pressure. The amount of pressure drop is the "osmotic pressure" on the solvent. That's it.
Diffusion really doesn't explain it because diffusion is just a statistical property - it can't work against any force that acts on every molecule (such as the force of gravity). The interference explanation doesn't make sense to me because if a solute molecule blocks a solvent molecule from crossing the membrane from high to low concentration solution, then you should equally have a blocking motion on the opposite side of the solute molecule occurring at the same moment. I read somewhere that what is "actually" happening is that the solute molecules are actually preventing the solvent from interacting with themselves and thus preventing them from exerting pressure on themselves, thus decreasing the hydrostatic pressure, thus accounting for the fluid pressure difference that we call the osmotic pressure. That doesn't really make intuitive sense to me though - why doesn't the solute molecule exert pressure also?
If anybody knows how it really works mechanistically I'd like to know. Please cite your source, by the way, because I've heard a lot of people try to explain it thinking they understand it and I point out something to them and they're wrong or incomplete. Even people who are knowledgable...
Can u explain the principal of osmosis
THANK YOU. I've been driving myself nuts researching osmosis for the past few days. It's such a bizarre subject. On the one hand, it's described in nearly all lower-level sources (school textbooks, youtube explainers, etc.) as a simple and settled issue, usually with a reference to water diffusing towards lower water concentration.
On the other hand, there appears to be conclusive evidence that osmosis is NOT simple diffusion (experiments with isotope-labeled water, for example, show that osmosis can move water into a compartment faster than the water can diffuse passively), and that it is NOT driven by differences in water concentration (some solutes actually increase the number of water molecules per volume, but additional water will apparently still osmose into such a solution).
The explanation involving interactions between molecules based on electric charge, which is also mentioned in the second half of the video, also can't seem to cover every case, because uncharged solutes can also drive osmosis - and the myth-debunking articles I've read point this out, too.
Then the myth-debunking articles typically go on to describe a mechanism similar to the one outlined in the first half of the video, where solute molecules block the way of solvent molecules as they bounce off the membrane - but fail to explain, as you point out, why they don't block the way of water molecules headed into the hypertonic solution as well.
Usually when there's a physical phenomenon that is poorly understood, there is at least widespread acknowledgement that it IS poorly understood. And I would be fine with that! There's much in the universe we don't understand yet But osmosis isn't talked about in this way. It's either described as a settled issue, or it's described as "okay, biology textbooks get it wrong, there are widespread misconceptions, but the true explanation is known and here it is", and then another seemingly flawed explanation is provided. So yeah, I am losing my mind over here. :P
@@OlleLindestad I agree with you and think you are correct. The phenomenon is a complex set of thermodynamic phenomenon at the end of the day. However, the notion I mentioned about the solute molecules being reflected _backwards_ and thereby creating the flow tendency seems to be the primary driving mechanism. One question, and the reason thermodynamics must be invoked to give a complete explanation, is _why_ the solute molecules are able to _keep_ on bouncing around without losing energy and thereby create the energy input in the system to generate the osmotic flow. The short answer is that a complex system of equations can be used to model the energy input into the fluid system. Part of this involves terms such as brownian motion, enthalpy, entropy, gibbs energy, etc. One simplified way of thinking bout it is that, basically, the fluid takes in heat from the surrounding environment, which keeps it in motion, and thus ultimately you are able to get the energy needed to drive osmosis. I think that if you comprehend, however, the idea that the solute molecules are changed in direction by the membrane, and then they create the flow tendency of the water, you should have a pretty good intuitive understanding of why osmosis works. It makes sense to me at least and I find it unfortunate many biologists and teachers of that subject don't discuss some of the simple physics involved. It leads to confusion and frustration and even misunderstanding. It sounds to me like you are on the right track, however, so I wish you luck.
What is exactly meant by the net movement of water molecules in osmosis?
Water molecules pass in both directions randomly not down a concentration gradient as well, so is the net movement the movement of water molecules that do move down a concentration gradient?
Michael Jackson Fan I know it's a bit late but it's the overall movement
Thanks
i still don't understand what osmosis is after watching this vid.
It is way better then average tutorials throughout the world
water moves from lower Conc. to Higher conc. and membrane (boundary or Filter) only allow water molecule to neutralize to pass blocking salt
i cant understand osmoses
Mousehead2000 it’s the diffusion of water through a semi permeable membrane. There are three different variants hypertonic, hypotonic and isotonic. Think of the semi permeable membrane as a filter for water.
Omar Khattab it’s the diffusion of water through a semi permeable membrane. There are three different variants hypertonic, hypotonic and isotonic. Think of the semi permeable membrane as a filter for water.
Wow it's help me a lot thank u so much I understand better
See [2003 Philip Nelson: Biological Physics: Energy, Information, Life, Chapter 7.3 Osmotic Flow].
"Now I can see the actual mechanism of force generation. When a membrane is impermeable to solute particles, then those particles bounce off the membrane when they approach it. Because of viscous friction, the particles entrain some water as they move, and so water too, is pulled away from the membrane. But water can pass through pores in the membrane, so some is also swept through it [note: what is meant is some water is pulled through the membrane to replace the other water that was pulled away from the membrane]. That's osmotic flow; a backward pressure is needed to stop it."
"But wait. Even when the particles are free (no membrane), their Brownian motion disturbs the surrounding fluid! [...] That's true, but the effect you mention is random and averages to zero. In contrast, the membrane [...] create a net motion in one direction."
"[...] where the net momentum flow into the fluid comes from. Particles constantly impinge on the membrane in Figure 7.6a from the right, never from the left. Each time the membrane is obliged to supply a kick to the right. Each kick delivers some momentum; these kicks don't average to zero. Instead, they pull fluid through the channel until equilibrium is reached."
"Our discussion makes clear how misleading it can be to refer to 'the osmotic pressure'. Suppose we throw a lump of sugar into a beaker. Soon we have a very nonuniform concentration c of sugar. Yet the pressure is everywhere constant, not equal to kTc as we might have expected from a naive application of the van 't Hoff relation. After all, we know that osmotic pressures can be huge; the fluid would be thrown in to violent motion if it suddenly developed such big pressure variations. Instead it sits there quietly, and the concentration spreads by diffusion. The flaw in the naive reasoning is the assumption that concentration gradients themselves somehow cause pressure gradients. But pressure can only change if a force acts. Thus osmotic pressure can only arise if there is a physical object (the semipermeable membrane) present to apply force to the solute particles. In the absence of such an object (for instance, if we just throw a lump of sugar into the water) there is no force and no pressure gradient. [...] Only when solute molecules have had a chance to diffuse from the initial lump of sugar to the membrane will the latter begin to rectify their Brownian motion [note: by rectify the Brownian motion Nelson means to create a net motion in one direction] and so transmit force to them, and thence to the fluid."
Also [2013 Kramer, Myers: Osmosis is not driven by water dilution]
"The key interactions take place in the small region of space adjacent to a pore aperture that allows water molecules to pass but repels solute [...]. Each time a solute molecule enters this region, it is repelled. That is, the aperture gives to the solute molecule a small amount of momentum directed away from the membrane. Due to viscous interactions between solute and water, this momentum is rapidly shared among all nearby molecules, including both solute and water [...]. Thus, although the pore aperture repels only the solute, the net effect is a force directed away from the membrane acting on the solution as a whole. This is the counterintuitive idea at the center of osmotic theory: a pore that lets water molecules pass freely will effectively repel the water if solute is present."
Thanks a lot for this extremely useful video! :)
Thank you!
OK I think I figured out a mechanism for osmosis. Sal's explanation is kind of correct but doesn't quite express it right.
The gist of it is that there is a net momentum vector for all the matter in the system that sits on the solute-solvent mixture side of the membrane. If you break the system down into two masses, the mass of water, and the mass of solute, we see that the mass of water's (solvent's) center of momentum movement is directly in the middle of the system over the membrane. However, when we look at the mass of solute's center of momentum, we see that it's in the middle of only the solute-solvent side. When you take the average of these two momentum vectors you get a net momentum vector that has a center somewhere between the two in physical space, so the tendency overall is for the water to move in the direction of the solute-solvent side toward the center of mass of the system.
Another way to think of it is that the barrier imparts energy to the system only on the side in which it is capable of deflecting matter (solute side). The Brownian motion of the molecules is the driving energy of the movement of molecules in the system. Where does the energy come from from the Brownian motion? Well, perhaps there is some internal energy at the subatomic/nuclear level, but I suspect it's more driven by the addition of heat from the environment and the transfer of kinetic energy to the particles from the barrier and walls. If a molecule hits the membrane, it is accelerated in the opposite direction. Energy is imparted to the molecule from the wall, and the wall gains energy from the particle. With each exchange, some kinetic energy is lost due to friction. Because the membrane is, on net, only interacting with the solute particles, any kinetic energy that the solute particles lose to the membrane barrier is lost only in that side of the system, but not the other half. This would imply the overall kinetic energy of the solute-solvent system is less than the pure-solvent side, which would obviously lower the water pressure and thus move water, on net, into the solute-solvent mixture side.
But, you might ask, osmosis is powerful enough, apparently, to work against gravity. This requires work, so energy LOSS doesn't seem to really explain how it can do work. Well, like I said, the Brownian motion of the particles is constant overall, so whatever inputs to the Brownian motion of the particles are, it must be the energy into these inputs that osmotic energy is driven by. It must be the case that the heat of the environment is going into one side of the system at a higher right than the other. I suppose that the solution must have the same temperature throughout on both sides of the membrane (does it? I suppose this could be measured). The order of energy seems to be:
heat from environment --> Brownian motion of liquid particles (Kinetic Energy) --> energy lost to membrane barrier
The energy lost to the barrier must be small compared to the increased input from the environment, otherwise you wouldn't be able to do work like elevate the solution against gravity. I would therefore speculate that the rate of heat intake in the system is greater on the solute-solvent side, because for the Brownian motion to remain constant, one needs an increased amount of energy to compensate for the energy lost at the membrane.
So that's my hypothesis about osmotic mechanism. Any thoughts?
The next question I have is: if this description is correct, does it imply that the total osmotic pressure is linked (proportional to) to the surface area of the membrane, or that the surface area of the membrane merely affects the rate of osmosis overall? Intuition at first tells me that the increased surface area of a membrane should increase the osmotic pressure overall, however as far as I know, the osmotic pressure is directly proportional to the solute concentration only, not the membrane surface area. This may imply that the surface area of the membrane only affects the rate of exchange, but not the overall osmotic pressure. This could be tested empirically by simply having two separate identical systems in terms of water mass, solute concentration on one side, and varying only the surface area of the membrane, and then measuring (1) what the rate of water movement is, and (2) what the overall end result is at equilibrium. If the rate varies but the end result is the same, then the membrane surface area doesn't affect the osmotic pressure. If the end result varies, then the osmotic pressure is proportional to the surface area of the membrane. As a secondary experiment, you could measure the temperature of the fluids and the rate of heat exchange on both sides of the membrane.
thanks it helped alot
hi i got a little bit confuse but may u answer he some of the questions? All about osmosis
this is what i understand from this video , inside the molecule have sugar (salute).So the water outside the cell will go into the molecule so the water outside the molecule and inside the molecule become the same.
Am I right?
pls answer me, I am going to have a exam.pls thx
um i dont quite understand ur question.. but osmosis is when "water moves from an area of lower solute concentration to area of higher solute concentration" which in plain english means the water will go towards and across the cell membrane to basically get to the solute if that helps:)
u should probably start w/ the topic. some may think attacking your bloodstream for dietry has something to do w/ hormones, which is produced in the!!!!
This didn't explain hydrostatic or osmotic pressure to me at all which is the part I'm having trouble understanding. Osmosis by itself isn't that much of a hard concept to understand.
Osmotic pressure is the amount of pressure it takes to stop osmosis
When the solute molecule, which can't cross the membrane, bumps into the wall, it reverses its motion. When it reverses its motion, it "bumps" other water molecules back into the direction it starts traveling after bumping into the wall. In other words, when the solute particle bumps into the wall, it reverses its motion, but then it also reverses the motion of the water molecules that were behind it traveling toward the membrane. Because this happens in greater quantity on the side with a higher concentration, the overall tendency is for water to go to the side with the higher solute concentration.
I solute you for clearing my doubt from root
Is probability of molecules passing through really the explanation of the mechanism of osmosis? He kept saying it was just a theory, so what's the actual scientific reason for it?
What micron size membrane is needed for that to happen?
Any membrane that can block solute (like salt) molecules and pass solvent (like water) molecules will do. For example, the membrane could have holes with sizes between the size of water and salt molecules.
like a foot sock that is a good example of that membrane
very useful. thanks.
so it from High concentration to low?
Thank you thank you thank you!!
Some water molecules can't pass from right to left because of hydrogen bonds with solute molecules
Sir, you are awesome!😎
? I can'can't understand
is it also the case that, on the side of the higher water concentration, then a higher proportion of the molecules which hit the semi-permeable membrane are molecules which can pass through it (so water), so this side has a higher rate of molecule interactions which could cause the molecule to pass through and therefore on average more molecules will pass through, causing net movement from high water concentration to low concentration?
You do see this explanation in many places. But the problem with explaining osmosis using the concentration of water is that osmosis happens even when you have a solute that *increases* the concentration of water. (This sounds weird, but some compounds break up the interactions between water molecules in such a way that the water molecules move closer together than they would in a mass of pure water.)
I've spent the last few days scouring the internet for a conclusive explanation for why osmosis happens, and so far it actually seems like there is no agreement on what the ultimate driving mechanism is! But I'm not a physicist, so take that with a grain of salt.
Why do the water molecules go form right to left and vice versa
Equilibrium
Why passing lower concentratio to higher
nice
Wouldn't it block the other side too
CLEARED*
hello
it me austin class
Please make sure that what are you speaking is must be written, it is easy to understand...
Turn on the captions. You will understand every word he is saying
IN MY HEAD IT IS STILL LIKE WHAT!
A god. You saved my grade and therefore I call you a GOD
Don't say that u dumbas*
do all cells have semi permeable membrane/?/
yes i think
All cells have cell membrane so it's mean that all cells have semi-permeable membrane
Yes
Yes
My teacher sent me here
I don't have patience. You never get to the point it could have been explained in just one minute.
I found this a little bit tedious. For the information I was trying to learn, the presentation went about too long and redundant almost telling a story to small kids. Not something i would want to see if I'm pressed for time and at the level of my inquiry (Graduate school). Maybe, just clear and concise terms and going direct to the point. But still thankful for the video to educate people.I still will continue to use your channel :)
+Sarah Davis What a retarded comment.
+Nameless One Please find someone your kind to bully around. Have a good one!
+Sarah Davis If you really are in graduate school as you claim then maybe you shouldn't be watching this video
He was actually very helpful in my opinion. It cannot be that hard to dedicate 8 minutes to something important.
The video isn't meant for students in graduate school, probably because graduates shouldn't need this video anyway. Maybe you should work harder instead of complaining about this video.
It's not semai it's semi
“Sal-loot”
MY MAN KHAN! although you give lessons for free and shit 2 million subscribes and views getting you MULLA and you be hustling and horsing around the city with your eyes closed doing wheelie on yo Harley Davidson. keep it up.
"SAH-LOOT" He just kept saying it and I couldn't focus on anything else!
me too
I'm watching this the day before my finals haha
same bruh
Same😂
same
not same
Same
Thank you! I am a junior in college, and I have always a hard time grasping this concept until I watched this video!! After 14 years of school, I can say I have learned more from youtube than most of my teachers.
We are learning about osmosis and osmotic pressure in school... all of this is floating right by me 🙃
This video helped me a lot in my exams ,thanks a lot Khan academy for publishing this video and .I love your teaching style
Thanks so much for your time and efforts Sal!
Overview of the entire thing (for bio students)
High h2O concentration goes down the concentration gradient and where the solutes go the water ultimately will follow by probability and then remain there due to them being attracted to their full change.
Probability: Water molecules have a higher chance of crossing a semi-permeable membrane than larger molecules I.E. sodium, due to there small size.
Thank me later
Thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you thank you-
I'm sorry but this is not correct. It is not because of ionic charge. It is because the solute particles bump up against the semi-permable membrane and then _reverse_ their motion. When they reverse their motion, they bump into water molecules on their own side, and then bump more water molecules _back_ into the same side as the solute. The tendency, therefore, is for a net movement of water onto the side with more concentration of solute.
الشرح يجنن ، مع اني جبت 100 بالاحياء لكن بعمري مجنت فاهمة الماده بهالدرجه ابداع والله❤️❤️❤️❤️
Great video
this is such a good resource. i'd rejoice if it were merely one, let alone thousands of videos
Nice work done there you have really helped me learning osmosis,good work for your organisation and God bless you
Is this good for as level curriculum?
But salman what about they molecules that are gonna enter, aren't they gonna get attracted to the water molecule that's close to the entrance pores
Page 76
Osmolority = 278 mOsm/L
Biology finals?
I got you
sal at it again (y)
i thought that with osmosis water would balance the solute
i'm gonna pass A and P thanks to this guy
Can i ask you something Khan ? The table salt or the sodium chloride ( NaCl ) is neutral because when sodium ion ( Na- ) bonds with chloride ion ( Cl+ ) it becomes neutral because the positive charge cancels the negative charge then how are the water molecules attracted to the salt the salt is neutral how come it will attract ??????????
Please answer 😊😀
I know this is a while later but the answer to your question is because water is highly polar, which means that both sides have a different charge. The oxygen side is negative (-) and the hydrogen side is positive (+). This polarity is strong enough to break the bonds between the Na+ and the Cl-.
Basically, Cl- thinks that the Hydrogens on water look more positively handsome so she leaves Na+. Na+ wants to make Cl- jealous so he goes to hang out with oxygen, who is more negative than Cl- ever was! Since Na- and Na+ are no longer together, they no longer cancel each other out.
The fact that Cl- and Na+ break up is also why salt disappears in water. Together they are a beautiful crystal! But when water cuts in-between, they fall apart and you can't see anything of what they used to be!
At least that's how I think of it. Hope that helps!
@@TheCommaA that's some next level explaination 😆
The first model shown in the video, which ignores any charge-mediated interactions between the solvent and solute, doesn't seem to provide a reason why solvent molecules should be more likely to move from left to right than from right to left.
A solvent molecule in the right-hand compartment could be prevented from crossing because it'd meet solute molecules heading away from the the membrane, having just "bounced off" the membrane - sure. But isn't the same true of solvent molecules in the left compartment heading towards the membrane? They could just as well be blocked by a solute molecule headed leftwards, TOWARDS the membrane, and at any given moment, there should be just as many solute molecules headed towards the membrane as away from it.
I love your channel
Hi. Most of industrial RO membrane pore size is around 0.0001-0.001mcm. As you know the dimension of water molecule is around 0.275nm. How can water pass through the RO membrane?
lecithine is good transporter though the cellular membrane
What's osmotic pressure?
Do a video on reverse osmosis, please :)
Why does the smoke spread out
osmosis,exchange,synergia
Soo confusing