Peanut247
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Why can't polar molecules pass through the lipid bilayer?
Why can't water soluble molecules pass through the bilayer but water molecules itself can?

Why can lipid soluble molecules pass through the bilyer and what does lipid soluble actually mean?


Super confused. Thanks for the help in advance!
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Tillybop
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(Original post by Peanut247)
Why can't polar molecules pass through the lipid bilayer?
Why can't water soluble molecules pass through the bilayer but water molecules itself can?

Why can lipid soluble molecules pass through the bilyer and what does lipid soluble actually mean?


Super confused. Thanks for the help in advance!
I don't know if all of this is correct, but hopefully someone will correct me where I'm wrong.

Why can't polar molecules pass through the lipid bilayer?

Polar molecules cannot dissolve into lipids, so they cannot diffuse through the bilayer.

Why can't water soluble molecules pass through the bilayer but water molecules itself can?

Water molecules cannot pass through the phospholipid bilayer, because it is a polar molecule. And the hydrophobic heads prevent it from moving through.

Water soluble molecules cannot pass through because they are hydrophilic but the hydrophobic heads prevent them moving through. And they cannot dissolve in the lipid because they are not lipid soluble.

Why can lipid soluble molecules pass through the bilyer and what does lipid soluble actually mean?

Lipid soluble means that the substance can dissolve in lipids (which are fats basically).

Because the membrane is a phospholipid bilayer, this means that it a lipid. Therefore the molecules will need to be able to dissolve into the lipid to pass through. Lipid soluble molecules are hydrophobic, so they hydrophobic heads let them pass through too.

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A good way of thinking of lipids and water is to imagine oil and water. They do not mix, they just sit on top of one another. In quite the same way, if you imagine the oil layer as a the bilayer, then you'll see that the water does not pass through it. The oil will just sit on top.
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Hype en Ecosse
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(Original post by Tilly-Elizabeth)
I don't know if all of this is correct, but hopefully someone will correct me where I'm wrong.

Why can't polar molecules pass through the lipid bilayer?

Polar molecules cannot dissolve into lipids, so they cannot diffuse through the bilayer.
Why can't polar molecules dissolve into lipids?

Why can't water soluble molecules pass through the bilayer but water molecules itself can?

Water molecules cannot pass through the phospholipid bilayer, because it is a polar molecule. And the hydrophobic heads prevent it from moving through.
Tilly is right. Water can't pass through the phospholipid bilayer, but the plasma membrane expresses lots of water channels called aquaporins, which allow water to passively diffuse.


Why can lipid soluble molecules pass through the bilyer and what does lipid soluble actually mean?

Lipid soluble means that the substance can dissolve in lipids (which are fats basically).

Because the membrane is a phospholipid bilayer, this means that it a lipid. Therefore the molecules will need to be able to dissolve into the lipid to pass through. Lipid soluble molecules are hydrophobic, so they hydrophobic heads let them pass through too.
Fats and oils - most people imagine solids when they hear the word "fat". Best to add the qualifier to make full sense!

Heads are hydrophilic, not hydrophobic. Hydrophilic substances just don't repel uncharged molecules, like lipids. And a lot of lipid hormones (like steroids) are small enough to diffuse through. But a lot of lipids are too large to diffuse through passively.
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Tillybop
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(Original post by Hype en Ecosse)
Why can't polar molecules dissolve into lipids?
Lipids are non-polar right? And non-polar substances cannot dissolve in a polar solvent. I can't remember why because I haven't studied chemistry in so long.
(Original post by Hype en Ecosse)
Tilly is right. Water can't pass through the phospholipid bilayer, but the plasma membrane expresses lots of water channels called aquaporins, which allow water to passively diffuse.
Got one right!
(Original post by Hype en Ecosse)
Fats and oils - most people imagine solids when they hear the word "fat". Best to add the qualifier to make full sense!

Heads are hydrophilic, not hydrophobic. Hydrophilic substances just don't repel uncharged molecules, like lipids. And a lot of lipid hormones (like steroids) are small enough to diffuse through. But a lot of lipids are too large to diffuse through passively.
I forget which one is hydrophobic and which one is hydrophilic. Since the tails are hydrophobic that kind of acts as a barrier to the water molecules? Which is why they need the channels to let them through? I might be wrong again. You wouldn't think I got an A in my mock.
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Hype en Ecosse
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(Original post by Tilly-Elizabeth)
Lipids are non-polar right? And non-polar substances cannot dissolve in a polar solvent. I can't remember why because I haven't studied chemistry in so long.Got one right!
I forget which one is hydrophobic and which one is hydrophilic. Since the tails are hydrophobic that kind of acts as a barrier to the water molecules? Which is why they need the channels to let them through? I might be wrong again. You wouldn't think I got an A in my mock.
Bang on.
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Tillybop
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(Original post by Hype en Ecosse)
Bang on.
Cheers
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