plasma membrane what can and can't pass through

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grace10101
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I'm currently doing a-level biology but I don't understand why charged ions and large polar molecules can't pass through the plasma membrane and also why lipid-soluble molecules can
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Reality Check
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(Original post by grace10101)
I'm currently doing a-level biology but I don't understand why charged ions and large polar molecules can't pass through the plasma membrane and also why lipid-soluble molecules can
Questions for you, to help you answer your question:

1) What is the plasma membrane made up of? What is its structure?
2) Given your answer to 1) above, can you see why a charged ion or polar molecule wouldn't be able to cross the membrane directly, and need a channel through the membrane to cross?
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grace10101
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from my knowledge, the plasma membrane is made up of a phospholipid bilayer where the hydrophilic phosphate heads position themselves outwards (towards the water) and the hydrophilic fatty acid tails position themselves inwards away from the water but I still don't understand why they require a channel to pass-through
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(Original post by grace10101)
from my knowledge, the plasma membrane is made up of a phospholipid bilayer where the hydrophilic phosphate heads position themselves outwards (towards the water) and the hydrophilic fatty acid tails position themselves inwards away from the water but I still don't understand why they require a channel to pass-through
Please quote me using the 'reply' button if you want me to reply - I get a notification that way (I just happened to see this reply in the list).




This is good (you meant to say '...and the hydrophobic fatty acid tails position themselves inwards away from the water).

So, having established that the membrane comprises a phospholipid bilayer, can you now answer your question as to why can lipid-soluble molecules cross?

And, having also established that the hydrophillic heads point outwards towards the aqueous environment, can you work out why a charged or polar molecule cannot cross the bilayer? What is is about the hydrophilic heads which make them hydrophilic?
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grace10101
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(Original post by Reality Check)
Please quote me using the 'reply' button if you want me to reply - I get a notification that way (I just happened to see this reply in the list).




This is good (you meant to say '...and the hydrophobic fatty acid tails position themselves inwards away from the water).

So, having established that the membrane comprises a phospholipid bilayer, can you now answer your question as to why can lipid-soluble molecules cross?

And, having also established that the hydrophillic heads point outwards towards the aqueous environment, can you work out why a charged or polar molecule cannot cross the bilayer? What is is about the hydrophilic heads which make them hydrophilic?
aha sorry about that I thought I did

the heads are hydrophilic as they are polar molecules therefore because water molecules are also polar they are attracted to them
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Reality Check
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(Original post by grace10101)
aha sorry about that I thought I did

the heads are hydrophilic as they are polar molecules therefore because water molecules are also polar they are attracted to them
Yes, that's great So, polar molecules are going to be attracted by the polar, hydrophilic heads of the lipid bilayer. But what's going to happen to them when they hit the hydrophobic, fatty acid tails in the middle of the bilayer?
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grace10101
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(Original post by Reality Check)
Yes, that's great So, polar molecules are going to be attracted by the polar, hydrophilic heads of the lipid bilayer. But what's going to happen to them when they hit the hydrophobic, fatty acid tails in the middle of the bilayer?
I'm gonna assume that the hydrophobic tails repel them?
so when it says that hydrophobic molecules repel water does that mean that they repel all water-soluble substances like ions and polar molecules?
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(Original post by grace10101)
I'm gonna assume that the hydrophobic tails repel them?
Correct And this is the answer as to why large polar molecules and anything charged (including simple cations and anions) cannot cross the lipid bilayer via simple diffusion


so when it says that hydrophobic molecules repel water does that mean that they repel all water-soluble substances like ions and polar molecules?
It's a bit more complicated than that. Water is a small, highly polar molecule but importantly not charged. This can 'slip' through the membrane easily, as can other polar, uncharged molecules like ethanol. Gases, too, can easily diffuse across the membrane (CO2, O2 etc). Larger polar molecules such as glucose are too big to 'squeeze through' and thus need carrier proteins.

Does this help? You've answered both of your questions now (I presume you can now see why a lipid-soluble molecule can cross a phospholipid bilayer)?
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grace10101
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(Original post by Reality Check)
Correct And this is the answer as to why large polar molecules and anything charged (including simple cations and anions) cannot cross the lipid bilayer via simple diffusion




It's a bit more complicated than that. Water is a small, highly polar molecule but importantly not charged. This can 'slip' through the membrane easily, as can other polar, uncharged molecules like ethanol. Gases, too, can easily diffuse across the membrane (CO2, O2 etc). Larger polar molecules such as glucose are too big to 'squeeze through' and thus need carrier proteins.

Does this help? You've answered both of your questions now (I presume you can now see why a lipid-soluble molecule can cross a phospholipid bilayer)?
ok I think I get it now thanks
are you also able to explain why it's self-sealing?
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Jpw1097
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(Original post by grace10101)
ok I think I get it now thanks
are you also able to explain why it's self-sealing?
You've already said what the plasma membrane is made up of. If I make a small hole in the plasma membrane, how do you think the hole is patched up?
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grace10101
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(Original post by Jpw1097)
You've already said what the plasma membrane is made up of. If I make a small hole in the plasma membrane, how do you think the hole is patched up?
If i had to guess do the phospholipids move closer together?
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Jpw1097
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(Original post by grace10101)
If i had to guess do the phospholipids move closer together?
Exactly, the plasma membrane is very fluid, phospholipids are constantly moving around - this is known as the fluid mosaic model.
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Anholm
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(Original post by grace10101)
I'm currently doing a-level biology but I don't understand why charged ions and large polar molecules can't pass through the plasma membrane and also why lipid-soluble molecules can
The Phospholipid bilayer (cell membrane) is non polar (not charged) inside so repels anything with a charge. Very small molecules like H2O, CO2 and O2 can slip through the membrane as they are not big enough for their charge to have an effect. Lipid soluble molecules are molecules without a charge so are not repelled by the non polar (hydrophobic) 'fatty acid tails' of the phospholipid membrane. Large molecules can't pass through the membrane without the assistance of channel and carrier proteins for the same reason you can't push a beech ball through a cheesecloth - too big to fit.
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