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Why don't phospholipid bilayers dissolve?

Why don't phospholipid bilayers dissolve?


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I just started learning about the structure and composition of cell's membrane and there is something that I fail to understand.

The membrane is composed of a phospholipid bilayer. The phospholipid has a polar, hydrophilic head and a nonpolar, hydrophobic tail. My question: Why doesn't the hydrophilic head dissolve in the extracellular fluid outside the cell? I suppose that the extracellular fluid is watery, so how come that the water-loving head doesn't interact with the water from the extracellular fluid?


We should first understand what happens when a substance dissolves. During dissolution water interacts with the solute molecule; if the strength of interaction between the molecule and water is higher than the strength of interaction among the solute molecules then the solute dissolves.
(Also have a look at this post).

Phospholipid is an amphipathic molecule - it has both hydrophobic and hydrophilic parts (this you understand very well). The hydrophobic part is a long chain of hydrocarbon (hence tail) whereas the hydrophilic part is a small but highly polar region (hence head). You should note that these parts are not disjoint but are covalently bonded.

Even though the hydrophilic part interacts with water (which helps the cell survive in an aqueous environment), the strength of covalent bond (between hydrophilic and hydrophobic regions) is very strong and cannot be broken by physical interaction with water. The lipid bilayer (even a monolayer) stays together because the water cannot interact with hydrophobic parts but they interact with each other via a pseudo-interaction called hydrophobic interaction.

I call it a pseudo-interaction (not a standard usage) because there is not actual molecular interaction that stabilizes the system. Hydrophobic interaction happens via increase in entropy of the system. Read the wikipedia page for details.


The answer to your question is basically: It is a bilayer. There are two layers of phospholipids, thus tucking the hydrophilic ends safely away from any extracellular and intracellular fluids. The whole surface of the bilayer is hydrophilic.

(Picture from here.)


The hydrophilic region of phospholipids is actually interacting with water or other hydrophilic molecules. At the same time hydrophobic region of phospholipids would be repelled from water because water is forming hydrogen-bond networks, which is a more stable state. You could imagine hydrophobic molecules in water would disturb the network. As a result the hydrophobic parts of phospholipids aggregate together and phospholidis would not come out into water not to expose the hydrophobic parts.


Phospholipid bilayer question

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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(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.

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.


Why can't ions pass through the lipid bilayer?

The lipid bilayer layer is actually a phospholipid bilayer made up of a lot of phospholipid molecules. Each phospholipid molecule has following parts:

Polar & hydrophilic #("water-loving")# head :
It is made up of negatively-charged phosphate group #(PO_4^(3-))# and glycerol #(C_3H_8O_3)# molecule. One of the oxygen of phosphate group is attached to a variant, i.e. #"R"# . Thus the nature of phospholipid can vary with the nature of #"R"# . Glycerol is a bridge between the phosphate group and hydrophobic tails. That's why it's known as glycerol backbone.

Non-polar & hydrophobic #("water-fearing")# tails : Phospholipid molecule consists of 2 hydrophobic tails made up of fatty acids. The fatty acids chain consist of carbon atoms bonded to hydrogen atoms. One tail is made up of saturated fatty acids, and the other containing double bond is made up of unsaturated fatty acids. Both these tails get attached to the glycerol molecule of the hydrophilic head.

Thus the overall structure of phospholipid is constructed as:

#"Fatty acids + glycerol = lipid"#

#"Lipid + phosphate = phospholipid"#

Now consider the rule: Like dissolve like.

So the ions being polar in nature can easily cross the polar and hydrophilic head. But still, they can't enter the cell because their entry gets restricted by the presence of hydrophobic tails. The fatty acid tails being non-polar in nature repel any polar or charged particle and hence don't allow them to enter the cell or escape out of it.


Why don't phospholipid bilayers dissolve? - Biology

As we just learned, the main fabric of the membrane is composed of two layers of phospholipid molecules. The hydrophilic or “water-loving” areas of these molecules (which looks like a collection of balls in an artist’s rendition of the model) (Figure 1) are in contact with the aqueous fluid both inside and outside the cell. Thus, both surfaces of the plasma membrane are hydrophilic. In contrast, the interior of the membrane, between its two surfaces, is a hydrophobic or nonpolar region because of the fatty acid tails. This region has no attraction for water or other polar molecules (we will discuss this further in the next page).

Figure 1. The fluid mosaic model of the plasma membrane structure describes the plasma membrane as a fluid combination of phospholipids, cholesterol, proteins, and carbohydrates.

Hydrophobic , or water-hating molecules, tend to be non-polar. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the interior of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell.

A phospholipid molecule (Figure 2) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon.

This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot. A molecule with this arrangement of a positively or negatively charged area and an uncharged, or non-polar, area is referred to as amphiphilic or “dual-loving.”

Figure 2. A hydrophilic head and two hydrophobic tails comprise this phospholipid molecule. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.

This characteristic is vital to the structure of a plasma membrane because, in water, phospholipids tend to become arranged with their hydrophobic tails facing each other and their hydrophilic heads facing out. In this way, they form a lipid bilayer—a barrier composed of a double layer of phospholipids that separates the water and other materials on one side of the barrier from the water and other materials on the other side. In fact, phospholipids heated in an aqueous solution tend to spontaneously form small spheres or droplets (called micelles or liposomes), with their hydrophilic heads forming the exterior and their hydrophobic tails on the inside (Figure 3).

Figure 3. In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward. (credit: modification of work by Mariana Ruiz Villareal)


[Biochemistry] How do small nonpolar molecules just ɽissolve' through the phospholipid bilayer like that?

So, phospholipid bilayers have a hydrophobic "tail" and a hydrophillic "head" which conveniently creates the phospholipid bilayer. That part and the fluidity associated with this makes perfect sense.

However, am I wrong to assume that the cytoplasm and the extracellular ɿluid' of a cell composes of mostly water? Because this raises a few questions.

First, I understand that nonpolar molecules can ɽissolve' inbetween the tails of the phospholipid bilayers. That makes sense. However, nonpolar molecules are not water-soluable because water is a polar molecule. How is it, then, that they can merely ɽiffuse' through the heads and into the water-containing cytoplasm? How do they even float around in the extracellular ɿluid' in the first place? Wouldn't small nonpolar molecules, like Carbon Dioxide, be repelled from the cytoplasm and would 'stay' dissolved between the hydrophobic interior of the phospholipid bilayer? Wouldn't they be repelled from entering the extracellular fluid in the first place?

am I wrong to assume that the cytoplasm and the extracellular ɿluid' of a cell composes of mostly water?

I understand that nonpolar molecules can ɽissolve' in between the tails of the phospholipid bilayers.

Before I go further, remember that the act of dissolving is when the molecules of the substance quit interacting with each other and interact with the surrounding solvent (think of different ethnic groups who tend to cluster and talk to each other and not really mingle with others)

How is it, then, that they can merely ɽiffuse' through the heads and into the water-containing cytoplasm?

A quick answer is that a small percentage of these molecules are actually dissolved already [remember that polar and non polar are not black and white, and that non-polar forces can cause transient dipoles and therefore transient attraction], it's just that the vast majority of these nonpolar molecules are separate from the aqueous phase

How do they even float around in the extracellular ɿluid' in the first place? Wouldn't small nonpolar molecules, like Carbon Dioxide, be repelled from the cytoplasm and would 'stay' dissolved between the hydrophobic interior of the phospholipid bilayer? Wouldn't they be repelled from entering the extracellular fluid in the first place?

Consider Brownian motion, which is essentially erratic random movement of microscopic particles in a fluid (copied from wiki). this motion will tend to cause molecules to constantly bounce/collide against the plasma membrane. If a bunch of glommed up nonpolar molecules collides with this plasma membrane, it will then receive the necessary contact to then dissolve into the membrane.

Another mechanism by which the nonpolar substance can gain access is through the substance that is already dissolved in the aqueous phase, and this will also conduct molecules across the plasma membrane


Phospholipid bilayer

Phospholipid bilayers form the plasma membrane that surrounds all cells. The molecules that make up the cell membrane are called phospholipids.

Phosholipid Structure

Phospholipids are amphipathic molecules. This means that they have a hydrophilic, polar phosphate head and two hydrophobic fatty acid tails. These components of the phospholipids cause them to orientate themselves, so the phosphate head can interact with water and the fatty acid tails can't, hence forming a bilayer. This arrangement can also be called a bio-molecular sheet, as the hydrophobic tails from each individual lipid sheet interact with one another forming a hydrophobic interior that acts as a permeability barrier [1] . The hydrophilic head is made up of Gycerol and a phosphate group - it is the phosphate group which makes the head hydrophilic. The hydrophobic tail is made up of 2 fatty acid chains, one of which usually contains a Cis double bond (C=C). This double bind causes the tail to 'kink' which affects the packing structure and fluidity of the bilayer. In mammalian plasma membranes 4 main structures of phospholipids can be found [2] :

As well as these, the membrane also contains various other types of lipid such as cholesterol and proteins. These molecules contribute significantly to the mass of the membrane. Some of the fatty acids in the phospholipid molecules are unsaturated, with one or more carbon-carbon double bonds in its hydrocarbon chain. These double bonds create a kink in the hydrophobic tails. These kinks prevent adjacent phospholipid molecules from packing too close together, which causes an increase in the fluidity of the bilayer. The length of the fatty acid tails also has an effect on the fluidity of the bilayer. If the bilayer has shorter fatty acid chains they are less likely to 'stick' together and they'll be less tightly packed together increasing the fluidity of the membrane. The bilayer is arranged so that the phospholipid heads face outwards and the fatty acid chains face inwards, with cholesterol and proteins scattered throughout the membrane. This structure is described as fluid because the phospholipids can diffuse along the membrane [3] [4] . The bilayer can form spontaneously when in an aqueous environment which means it is also self-sealing. This is due to how the hydrophobic tail and hydrophilic head react when they come in contact with water. The hydrophilic head is soluble in water due to it being charged or polar. This allows it to form electrostatic forces or hydrogen bonds with the water molecules. However, the hydrophobic tail is insoluble in water due to it being uncharged and non-polar meaning it cannot form any interactions with water molecules. Therefore as the bilayer forms, the phospholipids are arranged so that the tails are in the middle of the bilayer and the heads are on the outside [5] [6] .


Why do the phospholipids surrounding the cell form a bilayer?

It is the shape and amphipathic nature of the lipid molecules that cause them to form bilayers spontaneously in aqueous environments.

Explanation:

The most abundant membrane lipids are the phospholipids . These have a polar head group and two hydrophobic hydrocarbon tails. The tails are usually fatty acids and they can differ in length.

Hydrophilic molecules dissolve readily in water because they contain charged groups or uncharged polar groups, that can form either favourable electrostatic interactions or hydrogen bonds with water molecules
Hydrophobic molecules are insoluble in water because all or most of their atoms are uncharged and non polar. They cannot form energetically favourable interactions with water molecules.
If dispersed in water , they force the adjacent water molecules to reorganise into ice like cages that surround the hydrophobic molecules.

For the above reason, lipid molecules spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophobic heads to water.

Being cylindrical phospholipid molecules spontaneously form bilayer in aqueous environments. In this energetically most favourable arrangement, the hydrophilic heads , face the water at each surface of the bilayer and the hydrophobic tails are shielded from the water in the interior.


A level Bio, phospholipid bilayer

I&rsquom a bit confused as to why the bilayer doesn&rsquot allow water soluble substances to pass through, I get that the centre is hydrophobic because of the fatty acid tails, but water is small enough to diffuse straight through it anyway even though it&rsquos polar and the centre is hydrophobic - so why can&rsquot molecules dissolve in this water and also pass straight through by simple diffusion?

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Very small polar molecules, such as water, can cross via simple diffusion due to their small size.
Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer.

So to answer your question, water can pass through, eventhough it's polarised (a wea, because it is so small.
The image in this post gives a good comparison of the sizes of molecules that can be dissolved in water - the don't have to fit inside a water molecule-, compared to the size of water itself.
The molecules that cannot pass through the phospholipid membrane cannot do so because of their large size or charge.


Why are cell membranes selectively permeable?

The hydrophobic center to a cell membrane (also known as a phospholipid bilayer) gives the membrane selective permeability.

Cell membranes are primarily composed of lipid molecules called phospholipids. (Membranes also have many embedded proteins.) Each phospholipid has a hydrophilic head that is attracted to water these are the white circles in the image below. Each phospholipid also has two hydrophobic fatty acid tails that are repelled by water these are yellow in the image below. When many phospholipids are put in a watery solution, they therefore spontaneously form spheres called liposomes that point all the water-loving heads toward the water and shield all of the water-fearing tails from it.

The result of the hydrophobic center of the membrane is that molecules that dissolve in water are not capable of passing through the membrane. Charged atoms (ions) and polar molecules such as glucose are repelled by the hydrophobic center of the membrane (these molecules can, however, pass through with the help of membrane protein channels). On the other hand, hydrophobic molecules such as lipids can pass through the membrane, as can small non-polar molecules (such as oxygen gas or carbon dioxide).


Do all cells have a phospholipid bilayer?

The plasma membrane is primarily composed of phospholipids arranged in a bilayer, with the hydrophobic tails on the interior of the membrane, and the hydrophilic heads pointing outwards.

Subsequently, question is, what is the importance of a phospholipid bilayer to living cells? Phospholipid bilayers are critical components of cell membranes. The lipid bilayer acts as a barrier to the passage of molecules and ions into and out of the cell. However, an important function of the cell membrane is to allow selective passage of certain substances into and out of cells.

Hereof, what can cross the phospholipid bilayer?

Small nonpolar molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes. Small uncharged polar molecules, such as H2O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot.

How does the phospholipid bilayer work?

The lipid bilayer is arranged in two layers of phospholipids with the hydrophilic heads forming the outer edges and the tails forming the interior. In this arrangement, the bilayer has a hydrophobic core that prevents the passage of polar molecules while allowing the relatively free diffusion of non-polar molecules.



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