Question about Membrane Permeability

[dmission]

My book says, when talking about diffusion across cell membranes, that water is larger than a sodium ion, but water is polar, while the sodium ion has a complete charge. It then says that for this reason, a natural membrane is more likely to let the water through.

My intuition is telling me that since nonpolar molecules, as well as smaller molecules, have an easier time getting across the membrane, the sodium ion should go through easier (unless the ion means it's super polar??).

Would you say that generally, cells are permeable to water?

Would appreciate any clarification, thanks!

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[PingPongPro]

Yes ions are as you put it "super polar".

Remember "polar" means it has a slight charge. So water is considered polar because the electronegativity difference between the hydrogen and oxygen makes oxygen slightly negative and H slightly positive. Na+ Ions are full positive, therefore super!

 

[dmission]

Yes ions are as you put it "super polar".

Remember "polar" means it has a slight charge. So water is considered polar because the electronegativity difference between the hydrogen and oxygen makes oxygen slightly negative and H slightly positive. Na+ Ions are full positive, therefore super!

Thanks -- for some reason I'd always thought of polar being part + and part -, but I can see why an ion behave similarly. Thanks

 

[MT Headed]

Thanks -- for some reason I'd always thought of polar being part + and part -

I think of polar as being "unbalanced", and it doesn't get more unbalanced than having all +.

Also, here's another memory trick. Think about the classic osmosis example with salt water and fresh water. Or think about how they purify salt water through reverse-osmosis. Apparently it's not too hard to manufacture a membrane that lets big water through, but not little Na+ ions.

 

[MD Odyssey]

Thanks -- for some reason I'd always thought of polar being part + and part -

You would probably be well-served to go back and review the causes for polarity. Specifically, the fact that polar bonds are caused by differences in the electronegativity of the atoms involved in a bond. You should be able to recognize why something like CO2 is non-polar, but NH3 is.

The textbooks are only telling half truths when it comes to water. This is largely because, up until several years ago, no one really knew how water was able to diffuse so easily across the membrane. Since water is a polar molecule, it shouldn't be able to get across the membrane very easily, yet osmosis is a clear indication that it does. Biologists had to come up with a lot of hand-waving arguments about how water got across the membrane, but I don't think anyone really knew for sure until about 10 years ago.

Around 2000, biochemists began to discover a family of proteins referred to as aquaporins that are essentially water channels that allow water to move either depending upon its concentration gradient. These are fascinating little devices - they allow water molecules, which are polar, to cross through them, but not protons. Anyway, these are the real reason that water is able to get across the membrane.

Most passages and questions I've seen at the MCAT level seem to pretend that water is able to make its way through the lipid bi-layer. The textbooks notwithstanding, this is probably not accurate, but for the MCAT, it may be the answer they're looking for.

 

[whiteshadodw]

You would probably be well-served to go back and review the causes for polarity. Specifically, the fact that polar bonds are caused by differences in the electronegativity of the atoms involved in a bond. You should be able to recognize why something like CO2 is non-polar, but NH3 is.

The textbooks are only telling half truths when it comes to water. This is largely because, up until several years ago, no one really knew how water was able to diffuse so easily across the membrane. Since water is a polar molecule, it shouldn't be able to get across the membrane very easily, yet osmosis is a clear indication that it does. Biologists had to come up with a lot of hand-waving arguments about how water got across the membrane, but I don't think anyone really knew for sure until about 10 years ago.

Around 2000, biochemists began to discover a family of proteins referred to as aquaporins that are essentially water channels that allow water to move either depending upon its concentration gradient. These are fascinating little devices - they allow water molecules, which are polar, to cross through them, but not protons. Anyway, these are the real reason that water is able to get across the membrane.

Most passages and questions I've seen at the MCAT level seem to pretend that water is able to make its way through the lipid bi-layer. The textbooks notwithstanding, this is probably not accurate, but for the MCAT, it may be the answer they're looking for.

Its accurate enough. There are 'semipermeable membranes' that are permeable to somethings and not others. Osmotic pressure can cause water to diffuse across a membrane, it might not happen fast on a macro or micro timescale, but it happens. In fact, it happens in your capillaries all the time as filtration on the capillary end and reabsorption on the venule end, both driven by hydrostatic pressure and osmotic pressure, respectively. Here, osmosis is driven by several factors including pressure and osmotic/oncotic pressure.

There are some instances in which you want to control water reabsorption in tissues. This is where aquaporins come into play, because you can control their expression (collecting duct in kidney nephron). Ion channels are pretty darn similar, and the general principal is that aquaporins and ion channels can select based on charge and size. Na+ ion is equalled charged as K+, so how do you distinguish then? Size. Amino acids are amazing, and while they can speed things up, it doesn't necessarily mean things won't happen without them.

Also, as another testament to "osmosis" think about how reverse osmosis is accomplished. You have two comparments of water separated by a plasma membrane, the salt water has an osmotic pressure (sucking force, drawing water into the compartment with more water). But if you can overcome this osmotic pressure with an applied pressure, you get reverse osmosis and you can purify water.

 

[dmission]

You would probably be well-served to go back and review the causes for polarity. Specifically, the fact that polar bonds are caused by differences in the electronegativity of the atoms involved in a bond. You should be able to recognize why something like CO2 is non-polar, but NH3 is..

It seems that thinking like that is what messed me up in the Na+ case, though 😛

 

[MD Odyssey]

]In fact, it happens in your capillaries all the time as filtration on the capillary end and reabsorption on the venule end, both driven by hydrostatic pressure and osmotic pressure, respectively. Here, osmosis is driven by several factors including pressure and osmotic/oncotic pressure.

True. But I'm pretty sure that capillaries are a lot more permeable than a cell membrane. The capillaries have a bunch of cells with gaps in between them, so I'm not really sure they're the same thing. The OP is mostly asking about how water gets through a plasma membrane, not tissues like capillaries or collecting ducts.

Amino acids are amazing, and while they can speed things up, it doesn't necessarily mean things won't happen without them.

True, but the speed that cells respond to changes in free water gradients is pretty fast and I'm pretty sure you need channels for that sort of thing.

Also, as another testament to "osmosis" think about how reverse osmosis is accomplished. You have two comparments of water separated by a plasma membrane, the salt water has an osmotic pressure (sucking force, drawing water into the compartment with more water). But if you can overcome this osmotic pressure with an applied pressure, you get reverse osmosis and you can purify water.

I don't deny that you can get water to move through a membrane, but it doesn't move through the plasma membrane at the speeds we see happen without channels. This is what drove the search for aquaporins - researchers knew that there had to be a channel because thermodynamically, you can't get water through a lipid bi-layer fast enough to explain what we actually see.