Goldfarb D. Biophysics DeMYSTiFied

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272 Biophysics De mystifieD

somehow has a very large kinetic energy, then it may be able to penetrate into

the bilayer, at which point moving out of the bilayer to either side is highly

favorable.) Large molecules are also unable to pass through the bilayer, unless

they have enough energy to disrupt the forces that hold the bilayer together.

Being large, they have to push aside more lipid molecules in order to pass

through than a smaller molecule would have to do. So, they also have to have

enough energy to disrupt more of the forces holding the lipid molecules

together. On the other hand, small, uncharged, nonpolar (or only slightly polar)

molecules can pass through the bilayer with relative ease.

The forces that hold the bilayer together are dispersion forces, the hydropho-

bic effect, and, if cations are present, an additional stabilization by the cations

acting as counterions to reduce the repulsive force between the phosphate head

groups. Obviously since cations stabilize the bilayer, their presence has a

strengthening effect on the bilayer. This makes it more difficult for large mol-

ecules to enter, since large molecules have to somehow push the lipids aside.

Pushing the lipids aside requires disrupting the dispersion forces that attract the

hydrocarbon tails to one another. This takes energy, and the larger the molecule,

the more lipid molecules it has to push aside (and the more energy required).

On the other hand, at lower concentrations of cations, the repulsive force of the

head groups may assist the large molecules getting through, by making it easier

to push the lipids aside.

The strength of the dispersion forces depends largely on the amount of close

contact between the hydrocarbon chains. This in turn depends on how close

together the chains can pack and how long the chains are. Longer chains have

more contact over which the dispersion forces can attract adjacent molecules. So,

longer chains make stronger dispersion forces and more stable bilayers. (Longer

chains also mean a thicker bilayer and thus a greater distance over which a mol-

ecule passing through the bilayer has to travel.) Another significant factor is

whether the hydrocarbon chains are saturated, unsaturated, or polyunsaturated.

Unsaturated chains (recall from Chap. 7) have one or more double bonds between

some of the carbon atoms. Double bonds restrict free rotation, so each chain will

be stiff at the location of the double bond, with a particular angle or kink in the

chain at that location. The end result, due to these kinks, is that unsaturated and

polyunsaturated chains prevent the lipid molecules from packing as closely

together as saturated lipids can. This is illustrated in Fig. 11-6. Dispersion forces

are proportional to 1/r

;

that is, they decrease in proportion to the sixth power of

the distance. Less tightly packed chains mean that on average the chains are fur-

ther apart from each other; this leads to weaker dispersion forces.

Chapter 11 MEMBRANE BIOPHySICS 273

Fluid Mosaic Model

Our current working model of biological membranes is the fluid mosaic model,

which was first proposed by Singer and Nicolson in 1972. The model states that

biological membranes are composed primarily of a phospholipid bilayer. Other

Figure 11-6 • Lipid bilayers that contain only saturated lipids are

able to pack the lipids much closer together than bilayers that

contain unsaturated lipids or a mixture of saturated and

unsaturated lipids.

still struggling

The main forces that hold the bilayer together are the hydrophobic effect and dis-

persion forces. The hydrophobic effect is driven by entropy: water has more freedom

of movement when it stays away from hydrophobic portions of the lipid molecules.

Bilayer formation is favorable because it keeps the hydrophobic lipid tails away from

the water, and the polar lipid head groups facing the water. Dispersion forces are

due to synchronous fluctuations in the electron density of the atoms within the

hydrophobic tails. The electron densities fluctuate rapidly and in synch with each

other. At any given moment, in a place where one tail has a slightly positive charge

an adjacent tail has a slightly negative charge. This causes the hydrophobic tails to

attract one another giving the membrane strength and stability.

274 Biophysics DemystifieD

molecules that are part of the membrane structure, such as proteins, glycopro-

teins, cholesterol, and carbohydrates, form a mosaic with the lipid molecules. See

Fig. 11-7. This is in contrast to earlier biomembrane models in which the lipids

were thought to form a monolayer or in which proteins were also thought to

form one or more layers within the membrane. In the fluid mosaic model, the

lipids and other molecules are free to move about within the two-dimensional

mosaic.

Phase Transitions in Phospholipid Bilayers

Phospholipids in bilayers and micelles can exist in a solid or fluid phase. We will

focus our discussion on bilayers, since that is what we find in cell membranes,

but the basic concepts apply also to micelles. The solid phase is called the gel

phase. In the gel phase the bilayer is still somewhat flexible and permeable, but

lipid molecules (and other molecules within the mosaic) are fixed to a particu-

lar position within the two-dimensional mosaic. In simple terms, they don’t

move around within the membrane.

The fluid phase is called the liquid crystalline phase. In the liquid crystalline

phase phospholipids next to each other are able to change positions, sometimes

as often as millions of times per second. In this way, any given lipid molecule is

able to gradually diffuse and move about in two dimensions within the plane

Figure 11-7 • Fluid mosaic model of biological Membranes. The phospholipids in the bilayer form a mosaic with other

molecules making up the membrane. These molecules and the phospholipids are free to move about in two dimensions,

that is, in parallel with the bilayer.

chapter 11 MeMBrane Biophysics 275

of the bilayer. However, in both the gel and the liquid crystalline phases, lipid

molecules generally do not move from one layer to the other, since this would

involve a large, positive Gibbs energy change. Proteins and other molecules in

the mosaic also move about within the plane of the bilayer.

The Melting Transition

Phase transitions in lipid bilayers can be temperature induced. Therefore (as we

saw with DNA unwinding) the transition is sometimes referred to as melting.

The melting transition in phospholipid bilayers is highly cooperative, meaning

that it tends to happen in an all-or-none, two-state manner. Figure 11-8 shows

a typical melting curve for a sample of liposomes. The melting temperature T

is the halfway point of the transition. Below the melting temperature the lipids

are in the gel state; above the melting temperature they are in the liquid crystal-

line state. The transition results from the disruption of the dispersion forces

among the hydrocarbon tails. The dispersion forces otherwise hold adjacent

phospholipids together. The lipid bilayer itself remains intact due to hydropho-

bic forces, which can be further strengthened by counterions along the phos-

phate surfaces of the bilayer.

Biological Significance

Biological membranes are most commonly found to be in the fluid (liquid

crystalline) state; this allows the free movement of molecules within the two-

dimensional mosaic. However, the melting temperature is usually close to

0.1

0.2

15 25

Temperature (°C)

Heat capacity

35 45 55

Figure 11-8 • Typical heat capacity melting profile for a highly

cooperative gel to liquid crystal phase transition in a sample of

liposomes (phospholipid bilayer vesicles).

276 Biophysics DemystifieD

physiological temperatures. This means that the cell can somewhat regulate the

extent of gel versus liquid crystalline state of the membrane. In the liquid crys-

talline state, membranes are more permeable; this makes it easier for various

molecules to pass into or out of the cell. Cells can decrease their overall perme-

ability and increase the rigidity of their boundary by having some of the cell

membrane in the gel state. However, if the entire cell membrane is in the gel

state for a significant period of time, this typically results in the cell “freezing”

to death. The ability of organisms to regulate the fluidity of their own mem-

branes is called homeoviscous adaptation.

In the fluid state, the mobility of molecules within the mosaic plane enhances

a cell’s ability to move membrane proteins and receptors around to where they

are needed. It also gives the cell the ability to reseal small holes in the bilayer,

by shifting molecules around to fill in the hole. This important feature is lacking

in gel phase bilayers. In some cases it may be necessary to keep some molecules

on one particular side of the cell, for example, cilia (hairlike filaments) that

extend from the surface of cells in the lung. These cilia could not do their job

of sweeping dust out of the lungs if they were allowed to freely diffuse to the

opposite side of the cell. On the other hand, keeping the cell membrane in the

gel state would reduce permeability too much and prevent important mole-

cules (e.g., oxygen) from getting through the cell membrane. The cell solves this

problem by maintaining the bilayer in a liquid crystalline state, while anchoring

some molecules in place with the cytoskeleton.

Factors Affecting Membrane Fluidity

The same factors that affect membrane permeability also affect membrane

fluidity. Longer hydrocarbon tails increase dispersion forces making it more

difficult to melt the bilayer. Experimentally we observe a higher melting tem-

perature for bilayers made from lipids with longer hydrocarbon tails. Con-

versely, unsaturated lipids and lipids with shorter tails have lower melting

temperatures due to weaker dispersion forces. In general, anything that weak-

ens dispersion forces will reduce the melting temperature, increase fluidity, and

increase permeability. Anything that strengthens dispersion forces will increase

the melting temperature, decrease fluidity, and decrease permeability.

Cholesterol

Cholesterol is a significant component of many biological membranes, and it

has a mixed effect on membrane fluidity depending on whether the bilayer is

in the gel or liquid crystalline phase. Cholesterol tends to weaken dispersion

chapter 11 MeMBrane Biophysics 277

forces by positioning itself between the phospholipid molecules. It also disrupts

the orderly arrangement of the phospholipids in the gel phase, increasing

entropy. Both of these effects lower melting temperature and favor the fluid

phase of the bilayer.

However, in the fluid phase, the flat surface of cholesterol’s interconnected

rings (see Fig. 7-13) is significantly wider than the two chains of a single phos-

pholipid molecule. And the interconnected rings are rigid. So each cholesterol

molecule acts as an unbending barrier that limits some of the motion of the

phospholipids. So, while the cholesterol prevents the formation attractive dis-

persion forces, the steric restriction of phospholipid movement decreases mem-

brane fluidity and increases membrane rigidity. Many organisms are able to

regulate membrane fluidity and rigidity by regulating the amount of cholesterol

incorporated into their membranes. Organisms that live primarily in cold envi-

ronments are typically found to have less cholesterol in their membranes. This

prevents the organism from freezing to death.

The cholesterol molecule is almost entirely hydrophobic. Only the hydroxyl

group at one end of the cholesterol molecule is hydrophilic. The hydroxyl group

hydrogen bonds itself to the base of the phosphate head groups of the bilayer.

Since the hydroxyl group is smaller than the phosphate head groups, the cho-

lesterol is entirely submerged in the bilayer. The vast majority of the cholesterol

molecule is comfortably inside the hydrophobic portion of the bilayer.

Membrane growth

As cells grow, their membranes need to grow also. The cell somehow needs to

add lipid molecules to the bilayer. Lipids are synthesized in the cell interior

through a series of biochemical reactions mediated by enzymes. The resulting

lipids aggregate to form liposomes. The lipids are then added to the membrane

by fusing the liposomes with the existing membrane. See Fig. 11-9.

As their membranes grow, cells need to incorporate additional protein and

other molecules that are part of the membrane structure. This can be done in

a variety of ways. Some relatively hydrophobic proteins and glycolipids are able

to simply penetrate the bilayer due to a favorable Gibbs energy change. Once

inside the bilayer the hydrophobic effect helps to keep them in place. Some-

times proteins are inserted into the cell membrane while still in the form of a

polypeptide chain, before the protein folds into its native state. The fact that

protein synthesis typically takes place just below the surface of the cell mem-

brane facilitates this process. Once the protein is inside the bilayer, the Gibbs

278 Biophysics DemystifieD

energy change of protein folding becomes favorable. At the same time, the fold-

ing of the protein typically further stabilizes its position in the bilayer making

it unfavorable for the protein to exit the bilayer.

Membrane Permeability and Transport

Let’s revisit the idea of bilayer permeability and the need to transport mole-

cules across membranes. As we noted earlier in this chapter, the lipid bilayer is

semipermeable, meaning that some molecules can pass through the bilayer while

others can’t. Sometimes biological membranes are described as selectively

Unfused

Hemifused

Fully fused

Figure 11-9 • Fusion of two liposomes. The process is the same for

a liposome fusing with an existing larger membrane.

chapter 11 MeMBrane Biophysics 279

permeable, meaning that the cell somehow chooses what to let in, what to let

out, what to keep in, and what to keep out. The cell does this with a combina-

tion of active and passive transport. Active transport means that the cell expends

energy to move a molecule or ion across the membrane that would otherwise

not favorably move across the membrane. Passive transport means that the mol-

ecule or ion passes through the membrane by virtue of a favorable Gibbs energy

change without the need for the cell to expend any energy.

Passive Transport

In the case of passive transport there are two important points to keep in mind.

The first is that sometimes passive transport is the result of some earlier active

transport. In other words, the cell regularly expends energy moving molecules

against an otherwise unfavorable Gibbs energy change. Later, when the needs

of the cell require it, the cell allows the molecules to flow in the direction of a

favorable Gibbs energy change via passive transport. This type of passive trans-

port may be for the purpose of simply adjusting the concentration of an ion or

molecule. But that is usually not the case. Rather this type of passive transport

is more commonly used as the driver of a secondary active transport. In second-

ary active transport, the energy released from a passive transport is used to drive

an active transport.

The second point to know regarding passive transport is that it is not always

a simple diffusion or movement through the lipid bilayer (combined with a

favorable Gibbs energy change). Instead, passive transport can occur as the

result of a tunnel, or channel, created through the membrane by the presence

of transport protein. An example is shown in Fig. 11-10. Transport proteins can

be involved in either active or passive transport. Transport proteins involved in

passive transport contain a combination of hydrophobic and hydrophilic amino

acid residues. The sequence of resides and the forces involved in protein folding

cause the protein to fold in a way that creates a tunnel or channel through the

center of the folded protein. The hydrophobic residues are in the outside of the

protein exposed to the hydrophobic portion of the bilayer. The hydrophilic

residues are on the inside of the protein, away from the hydrophobic lipid

chains. These hydrophilic residues line the walls of the channel through the

center of the protein. There are also usually some hydrophilic residues near the

openings of the channel at each end of the transport protein. These may form

hydrogen bonds with the phosphate head groups and help to keep the protein

oriented so that the transport channel remains perpendicular to the plane of

the bilayer (see Fig. 11-10).

280 Biophysics De mystifieD

Gated Ion Channels

Earlier in this chapter we pointed out that lipid bilayers are very much not

permeable to ions. In almost all cases, transport of ions across the membrane

requires the existence of an ion channel, which is a transport protein as

we described above with a hollow channel lined with hydrophilic residues.

(One exception is vesicle transport discussed later in this chapter.) In some

cases the native state of the membrane transport protein involves two or more

native conformations. In at least one of these conformations the transport chan-

nel is blocked, preventing the movement of ions into the channel through

either steric or hydrophobic forces. Such a channel is said to be gated because

the conformation of the protein acts as a gate that can open and close. Depend-

ing on the particular transport protein and its specific purpose, various factors

can act to trigger the conformational change to open or close the gate. For

example, binding of a protein or other molecule to a specific receptor on the

surface of the membrane may trigger the gate to open or close. Or the presence

of a certain voltage difference across the membrane may trigger the conforma-

tional change that opens or closes the ion channel. This latter situation is

referred to as a voltage gated ion channel. Voltage gated ion channels are com-

mon in nerve cells (neurons) and other excitable tissue (muscle cells). Typically

there is a very specific voltage at which the gated ion channels of a cell open in

a highly cooperative manner, so that once one channel is open, the other chan-

nels along the length of the neuron open rapidly in succession, creating an

impulse that travels down the length of the neuron.

Figure 11-10 • A channel is a type of

transport protein that folds in a way so as to

form a hydrophilic channel or tunnel through

the phospholipid bilayer.

chapter 11 MeMBrane Biophysics 281

Active Transport

Active transport always involves the use of energy to drive the transport of a

molecule or ion across the membrane. There are two reasons why energy may

be needed to drive the transport of a molecule or ion across the membrane. The

first reason is that there may be an otherwise unfavorable Gibbs energy change

just to get the molecule into the hydrophobic interior of the bilayer. This is the

case with ions and other charged or large and highly polar molecules. The over-

all Gibbs energy change to get the ion from one side of the membrane to the

other may be small or even negative (favorable), but there is an energy hump,

or barrier, to get over in order to get the ion into the bilayer. This energy barrier

is illustrated in Fig. 11-11. In most cases, if the overall Gibbs energy change is

Edge of bilayer Distance into bilayer

∆G

Figure 11-11 • Gibbs energy change for passing a positive ion into a lipid

bilayer, as a function of distance into the bilayer. The Gibbs energy change is

initially negative as the ion approaches the negatively charged phosphate

head groups.

if the concentration of the ion is smaller on the other side of the

bilayer, then the overall Gibbs energy change will be negative (favorable). But

as the positive ion passes the lipid head groups and into the hydrophobic

interior of the bilayer the Gibbs energy rises sharply.