Membrane Fluidity

Sections

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MEMBRANE LIPID MOVEMENT

• Rotate
• Drift laterally
• Do NOT flip

MEMBRANE FLUIDITY DETERMINANTS

1. Lipid structure

• Degree of overlap
• Tail length
• Double bonds

2. Temperature

• Transition temperature (Tm) determined by membrane composition
• Membrane phase changes as physiologic temp. fluctuate about Tm

3. Cholesterol

• Temperature buffer that resists changed in fluidity

MEMBRANE PHASES

1. Gel-like

• Little overlap of phospholipid tails
• Longer/saturated tails
• Higher Tm
• No cholesterol

2. Liquid ordered

• Physiologic temperature ~ membrane Tm
• Cholesterol present
• An intermediate phase and most common physiologically

3. Liquid disordered

• Tails overlap
• Shorter/unsaturated tails
• Lower Tm
• No cholesterol

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• Here we will learn about membrane fluidity and the varying phases of the lipid bilayer.

• To begin, let's list the ways in which membrane lipids move within the bilayer.

• Write that membrane lipids can:
  • Rotate.
  • Drift laterally.
  • They do not, however, spontaneously flip between bilayer leaflets. Enzymes must facilitate flipping, we will see why shortly.

• Now that we know how lipids move within the membrane, start a table to learn the factors that influence their fluidity.

• Denote the key components of membrane fluidity:
  • Lipid structure; certain membrane lipids are more fluid than others.
  • Temperature, the effects of which we will describe in detail shortly.
  • Cholesterol, which inserts into bilayers and stabilizes them.

• All of these factors interact to maintain membrane dynamics under various physiologic conditions.

Now, let's illustrate lipid movement within a bilayer.

• First, draw two rows of lipids to represent our bilayer.

• For review, label the heads as hydrophilic.

• Label the tails as hydrophobic.

• Now, use a circular arrow to indicate that lipids can rotate within the bilayer.

• Use another arrow to illustrate that phospholipids can move laterally within the bilayer.
  • This is due to weak hydrophobic interactions between phospholipid tails.

• Now, let's extend our bilayer to remind ourselves that it is in fact a three-dimensional structure.

• Show that lipids fill this extra space.

• Next, use an arrow to illustrate that phospholipids can move laterally in all three dimensions.
  • Note that in order to flip from one leaflet to the other, a lipid's hydrophilic head must cross the hydrophobic interior of the membrane.
  • This is energetically unfavorable and therefore requires enzymatic assistance.

Before we move on, we must first illustrate the three phases of the membrane.

• First, draw a horizontal line and show that the membrane exists along a continuum.

• Show that it tanges from gel to liquid disordered.

• Illustrate that the liquid ordered state represents an intermediate state of fluidity.

Draw these phases as follows:

• Gel: draw a phospholipid bilayer in which the lipid tails are stiff and parallel to each other.
  • Here, the membrane is solid-like.

• Liquid disordered: draw a lipid bilayer in which the tails are "disordered" and the phospholipids are not so tightly packed.
  • Here, the membrane is fluid; a phase often referred to as "liquid-crystalline."

In between these phases, draw the liquid ordered phase:

• Draw a lipid bilayer with cholesterol molecules wedged between loosely packed phospholipid tails.
  • Here, the membrane is in an intermediate state; it is more fluid than the gel-phase, but "ordered" in comparison to the liquid disordered phase.

• In general, naturally occurring membranes are fluid at physiologic temperatures.

Now, let's illustrate how lipid structure, temperature and cholesterol determine which of these phases dominate.

First, let's note some key points about lipid structure and fluidity.

• Denote the three features of lipids that influence their fluidity:
  • Degree of overlap of lipid tails
  • Tail length
  • Number of double bonds (degree of saturation)

• Let's start with lipid overlap; nonpolar lipid tails are most mobile in the middle of the membrane, where weak hydrophobic interactions dominate.

• Indicate that there is little overlap between nonpolar lipid tails in the gel phase.
  • Tail-to-tail interactions are minimized, and the lipids are less mobile.

• Now, indicate in the liquid disordered phase there is more overlap of nonpolar tails.
  • Lipids are more mobile and this phase is more fluid.

Now, tail length.

• Write that lipids with long fatty acid tails tend to assemble in an ordered gel-phase where there is minimal tail overlap.

Finally, double bonds.

• Write that lipids with double bonds in their tails (unsaturated lipids) tend to assemble in the liquid disordered phase.
  • Their kinks prevent them from packing closely together.

• Again, the liquid ordered phase represents an intermediate between the gel and liquid disordered phases.

Now, let's introduce temperature.

• Draw the x-axis of a graph; we will add the y-values shortly.

• Label it temperature.

• Draw an S-curve; it describes the relationship between temperature and membrane fluidity.

Now, let's add the y-values.

• Label the beginning of the curve 'gel.'
  • Membranes are more gel-like at lower physiologic temperatures.

• Label the end of the curve 'liquid disordered.'
  • As temperature increases, the membrane becomes more disordered and fluid.

• Draw a dashed line through the center of the curve and label it transition temperature.
  • A particular membrane changes phases if physiologic temperatures fluctuate about the membrane's transition temperature.

• A membrane's composition affects its transition temperature.

How do lipids fit into this equation?

• Indicate that, lipids with long fatty acid tails increase the transition temperature.
  • They make membranes more gel-like.

• Next, indicate that unsaturated lipids lower the transition temperature.
  • They make membranes more disordered and fluid.

This brings us to the role of cholesterol.

• Write that cholesterol acts as a "temperature buffer"; it stabilizes the membrane by resisting changes in membrane fluidity.

Let's learn how.

• At moderate temperature, cholesterol decreases fluidity (lessens lateral movement); it stiffens the bilayer.

• At low temperature, cholesterol increases fluidity; it prevents the close packing of fatty acid tails, and thus prevents solidification.

• Thus, the liquid ordered phase, which includes cholesterol, represents an intermediate in fluidity.

• Illustrate that the addition of cholesterol can increase the fluidity of a gel-like membrane and decrease the fluidity of a liquid-disordered membrane.