Why cannot a lipid bilayer in cells exist as a 3-D structure?????

Answer 1

Phospholipids **DO** form 3D structures in the cell.

Phospholipids have negatively charged phosphate on one end (water soluble) and fatty tails (water insoluble), thus they must form either 2D sheets (bilayer) or miscelles (tiny droplets with all tails pointed inward and no water inside).

To convince yourself of this, simply play with some pencils and imagine the erasers are the phosphates. If the air is water, how can you arrange the pencils so that the pencil wood contacts only each other but not air or the erasers? The erasers like air and other erasers but don't like to be next to pencil wood.

There are only two solutions! At least until you bring other molecules into the equation. Soap or detergent also have hydrophobic and hydrophilic ends on the same molecule. By coating oil they make it water soluble.

Also imagine that the air is oil, instead of water. How would the phospholipids behave in this hydrophobic environment? This is what it's like for proteins fully contained within the cell membrane.

However the 2D sheets do form 3D structures in eukaryotes, namely the endomembrane system -- nuclear envelope, ER, and Golgi (btw - the nuc. membrane and ER are contiguous with each other).

Polypeptides destined for secretion are translated at the surface of the RER, fed into the ER, pass through the Golgi, and ultimately carried to the plasma membrane by transport vesciles. Its because of the 3D structure that the endomembrane system is able to accomplish this function (yet another example of structure-function relationships!).

Answer 2

It can. There would just be no point.

Think about how a bilayer works: You've got water bordered by a hydrophilic side, a hydrophobic interior, and then another hydrophilic side and more water. Because the layer is so thin, small molecules can often wander across and not get lost along the way, even if they're not too partial to the hydrophobic part. And because the layer is so thin, you can have some protein structures that span the whole gap, allowing you to move large things across and also provide more rapid gated access to any number of other things.

Now, there's no reason why you couldn't pack all kinds of hydrophobic stuff in the middle and swell the thickness of the bilayer all you want. But then it becomes a wall instead of a membrane. You won't have much of anything getting across... which isn't great for any cell.

So the biggest reason you can't have a bilayer that isn't pretty two-dimensional is that it's pretty lethal!

Answer 3

Are there no spherical structures with lipid bilayers?