Lipids plasma membrane

[MedPR]

I'm pretty sure I knew this in freshman bio, but now I have no idea how or why this is true.

Phospholipid bilayer is extremely permeable to lipids, but not water. Is it because the polar heads hold onto the water so the water can't reach the non-polar tails? And once the non-polar lipids make it past the polar heads of the bilayer, wouldn't the nonpolar tails bind them, thus making it harder for them to continue moving through the bilayer?

I know this is really simple, but explain it to me like I've never taken biology before please!

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[CaptainScience]

In short, phospholipids and lipids are essentially the same thing. Both contains a polar head and hydrophobic tail, which explains why lipids can readily pass through a membrane without any problems. Water however is polar and because of that, it cant cross the membrane due to the polar head group of the phospholipid. Think of it as some sort of repulsion. Because of this, water can only pass through the membrane with the help of a transporter or carrier.

 

[SLC]

Just look at an artists depiction of phospholipid, how much of it is lipid, and how much is polar hydrophilic head? Regular lipids get through because they're so similar to phospholipid (like dissolves like), while water has virtually nothing in common with phospholipid (water and oil (lipid) don't mix). The vast majority of a cell membrane is lipid, with only a small portion composed of the polar head of the phospholipids, and the membrane proteins (some of which are hydrophilic, some of which are hydrophobic, and some of which are amphipathic, I'll let you think about the implications there). There are also some carbohydrates etc involved in the membrane but don't worry much about that now.

That's a fairly simplified answer, but it's good enough for the MCAT.

 

[MedPR]

In short, phospholipids and lipids are essentially the same thing. Both contains a polar head and hydrophobic tail, which explains why lipids can readily pass through a membrane without any problems. Water however is polar and because of that, it cant cross the membrane due to the polar head group of the phospholipid. Think of it as some sort of repulsion. Because of this, water can only pass through the membrane with the help of a transporter or carrier.

Just look at an artists depiction of phospholipid, how much of it is lipid, and how much is polar hydrophilic head? Regular lipids get through because they're so similar to phospholipid (like dissolves like), while water has virtually nothing in common with phospholipid (water and oil (lipid) don't mix). The vast majority of a cell membrane is lipid, with only a small portion composed of the polar head of the phospholipids, and the membrane proteins (some of which are hydrophilic, some of which are hydrophobic, and some of which are amphipathic, I'll let you think about the implications there). There are also some carbohydrates etc involved in the membrane but don't worry much about that now.

That's a fairly simplified answer, but it's good enough for the MCAT.

The whole like dissolves like is why I get confused. If you try to filter a polar solute through a polar column, you won't get any of the solute out at the end (like dissolves like) which is exactly the same as putting a lipid through another lipid. If you pass a polar solute through a nonpolar column, all of the polar solute will come out the other end, since it is insoluble in the nonpolar column, which is exactly the same as passing water through a lipid bilayer.

 

[Pisiform]

http://forums.studentdoctor.net/showthread.php?p=11748402

look at that link ... EPO did a good job explaining

 

[MedPR]

http://forums.studentdoctor.net/showthread.php?p=11748402

look at that link ... EPO did a good job explaining

Thanks.

I understand what EPO and the others in that thread are saying, but some things are still over my head.

Is it correct to say that polar molecules and charged ions are less permeable because they are trapped by the polar heads of the lipid membrane? In essence, the polar heads are like bouncers to a club, and unless you are nonpolar enough to overcome the bouncer's strength (ionic attraction forces) you aren't getting in?

I'm also still not really clear on how lipids and other mostly nonpolar substances are so permeable. Isn't the cytosol mostly water, and therefore polar? And don't nonpolar substances aggregate in polar solvents? So wouldn't the water in the cytosol push an incoming lipid back into the lipid membrane where it can be with the other nonpolar tails of the bilayer?

 

[MrNeuro]

Thanks.

I understand what EPO and the others in that thread are saying, but some things are still over my head.

Is it correct to say that polar molecules and charged ions are less permeable because they are trapped by the polar heads of the lipid membrane? In essence, the polar heads are like bouncers to a club, and unless you are nonpolar enough to overcome the bouncer's strength (ionic attraction forces) you aren't getting in?

I'm also still not really clear on how lipids and other mostly nonpolar substances are so permeable. Isn't the cytosol mostly water, and therefore polar? And don't nonpolar substances aggregate in polar solvents? So wouldn't the water in the cytosol push an incoming lipid back into the lipid membrane where it can be with the other nonpolar tails of the bilayer?

In order to traverse the plasma membrane there still needs to be some type of force. In the case of lipids either a diffusion force such as moving down its concentration gradient or some type of active transport into the cell.

 

[Pisiform]

As you said MedPR, I think polar/charged particles cannot cross the cell-membrane simply because a greater proportion of the membrane is hydrophobic and so for any hydrophilic molecule to be transported by simple diffusion, it needs to be soluble in the membrane. But charges/polar molecules cannot because they have non-uniform electron density which cause them to experience forces by polar heads ...

whereas, non-polar molecules are hydrophobic, same type found in the greater proportion of the membrane tails. And since they are hydrophobic they have uniform electron density and so does not experience any obstructive force by polar heads of the plasma membrane and can simply pass through.

As far as cell is concerned, most lipids are transported via protein attachement (apoprtotein) so they are pretty much soluble in cytoplasm. Only very very tiny particles of lipids just simply diffuse mainly because of their small surface area to volume ratio ....like chylomicrons but still I think they have some sort of protein attached.

And yes water inspite being a hydrophilic molecule can still easily diffuse through the cell membrane (osmosis) not because it is an exception to being hydrophilic or anything .... but because cell membrane have large amount of aquaporins - protein that allow water movement - so water basically travels via facilitated osmosis you can say (through protein). It still won't simply diffuse if no aquaporins present.

and yea i don't know if there is anything complex going on but I like to keep it simple, just enough to pass the mcat lol

 

[MedPR]

In order to traverse the plasma membrane there still needs to be some type of force. In the case of lipids either a diffusion force such as moving down its concentration gradient or some type of active transport into the cell.

I completely forgot about this. Thanks.

As you said MedPR, I think polar/charged particles cannot cross the cell-membrane simply because a greater proportion of the membrane is hydrophobic and so for any hydrophilic molecule to be transported by simple diffusion, it needs to be soluble in the membrane. But charges/polar molecules cannot because they have non-uniform electron density which cause them to experience forces by polar heads ...

whereas, non-polar molecules are hydrophobic, same type found in the greater proportion of the membrane tails. And since they are hydrophobic they have uniform electron density and so does not experience any obstructive force by polar heads of the plasma membrane and can simply pass through.

As far as cell is concerned, most lipids are transported via protein attachement (apoprtotein) so they are pretty much soluble in cytoplasm. Only very very tiny particles of lipids just simply diffuse mainly because of their small surface area to volume ratio ....like chylomicrons but still I think they have some sort of protein attached.

And yes water inspite being a hydrophilic molecule can still easily diffuse through the cell membrane (osmosis) not because it is an exception to being hydrophilic or anything .... but because cell membrane have large amount of aquaporins - protein that allow water movement - so water basically travels via facilitated osmosis you can say (through protein). It still won't simply diffuse if no aquaporins present.

and yea i don't know if there is anything complex going on but I like to keep it simple, just enough to pass the mcat lol

Thank you, that makes it really clear!