• The important role of lipid bilayer helps in stabilizing the membrane structure, but leaves it flexible, not rigid.
• At the same time, the fatty acids of the phospholipids make the hydrophobic interior of the membrane somewhat fluid.
• Overall shape changes, as occur in during locomotion or cell division fusion budding, fertilization.
• Another important role of lipid bilayer is self assembly.

Some of the characteristic features of membrane lipids as follows

Table of Contents

• Asymmetrical distribution of Lipid Bilayer
• Types of movements of lipid molecules
  • Flexion
  • Flip Flop
  • Lateral diffusion
• Membrane Fluidity
  • Temperature
  • Fatty Acid Composition
  • Cholesterol (Sterol)
• Maintaining Membrane Fluidity

Asymmetrical distribution of Lipid Bilayer

• Most important characteristic feature or role of lipid bilayer of all membranes is an asymmetry in lipid composition across the bilayer.
• Although most phospholipids are present in both of the leaflets of membrane, but they are commonly more abundant in one or the other leaflet an thus can give rise to different curvature.

• For example, in cell membranes of human erythrocytes and certain kidney cells grown in culture, it was observed that almost all the sphingomyelin and phosphatidylcholine, both of which form less fluid bilayers, are found in the exoplasmic leaflet. (figure below).
• On the other hand, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol, which form more fluid bilayers, are preferentially located in the cytosolic leaflet. (figure below).

• This segregation of lipids found across the lipid bilayer may influence membrane curvature (figure below).

• Lipid molecule spontaneously form one or other structure in water, depending on their shape.
• A bilayer made in form of spherical vesicle called liposomes (25 nm), PL assemble spontaneously to form wall  of fluid filled spherical vesicle, develop as vesicle to deliver drugs or DNA molecule with in body.

• Unlike phospholipids, cholesterol is relatively evenly distributed in both leaflets of cellular membranes.
• The relative abundance of a particular phospholipid in both the leaflets of a plasma membrane can be determined on the basis of its susceptibility to hydrolysis by phospholipases (enzymes that cleave various bonds in the hydrophilic ends of phospholipids).
• Phospholipids in the cytosolic leaflet are resistant to hydrolysis by phospholipases added to the external medium because the enzymes cannot penetrate to the cytosolic face of the plasma membrane.

Types of movements of lipid molecules

• Flexion
• Rotation
• Lateral Diffusion (Fast)
• Flip- Flop Trans bilayer / transverse diffusion/ very slow
• Flippase (P type ATPase) – Outer to Inner/ ATP require
• Floppase (ABC transporter) Inner to Outer/ ATP require
• Scramble (PL translocator/ No ATP require)

Flexion

• When a lipid molecule rotates at its place; or flexes its hydrophobic tail.
• These movements are too rapid to be recorded.

Flip Flop

• When movement takes place between two monolayers.
• Energetically, such flip-flopping is extremely unfavorable because it entails movement of the polar phospholipid head group through the hydrophobic interior of the membrane.
• However, in membranes where lipids are actively synthesized, such as smooth ER, there is a rapid flip-flop of specific lipid molecules across the bilayer and there are present certain membrane-bound enzymes, called phospholipid translocators like flippases to catalyze this activity.

Lateral diffusion

• When lipid molecules move or exchange places in the same monolayer.
• It is the most common type of movement with a diffusion coefficient of 10^-8cm² sec^-1 and occurs approximately 10⁷ times in a second.

Membrane Fluidity

• Adequate amount of membrane fluidity is required and is essential role of lipid bilayer for many membrane functions.
• Physical state of lipid membrane is described by its fluidity (ease of flow) or viscosity (resistance to flow) = inversely related.

Factors responsible for membrane fluidity

Temperature

• Temperature increases fluidity increases and vice versa.
• The temperature at which this change occur called transition temperature. This change due to saturated FA straight shape flexible rod.
• Liquid Solution—>  Gel (Frozen).
• The lipid is converted from liquid crystalline phase to a frozen crystalline gel in which the movement of PL fatty acid chain greatly restricted.
• Membrane phase transitions in membrane depend on more variables like pressure, pH, and chemical potential of ions Ca⁺² than just temp.

• Importantly, the temperature at which this phase transition occurs becomes lower if the hydrocarbon chains are short or have double bonds.
• This is so because, double bonds in cis unsaturated hydrocarbon chains tend to increase the fluidity of a phospholipid bilayer by making it more difficult to pack the chains together.
• Thus, to maintain fluidity of the membrane, cells of organisms that are living at low temperatures (polar environment) have high proportions of unsaturated fatty acids in their membranes, than do cells at higher temperatures.

Fatty Acid Composition

• The lipid melting transition depend on the chain length & degree of saturation.
• More longer the FA tail is without double bond, the higher melting temperature.
• The more double bond in fatty acid tail lower the melting temperature & degrade quicker. eg: Stearic acid (no double bond mp 70^oC) add one double bond lowering melting point 60^oC.
• Lipid with short chain/unsaturated FA due to transition at lower temp thereby increase fluidity at reduced temperature and kink as adopt more fluid state (less Vander waal forces).
• PC + PE with FA largely unsaturated, kept warm (37^oC) exist fluid state.
• In general membrane fluidity is decrease by sphingolipid &  cholesterol and increase by phosphogycerides.

Cholesterol (Sterol)

• Hydrophobic in nature 27 C atom molecule 4 fused ring = 6 C – 3 ring; 5C – 1 ring.
• Hydrophobic ring of cholesterol flat & rigid they interfere with the movement of FA tail of PL.
• Mycoplama & bacterial membrane lack cholesterol.

Physical state also affected by cholesterol

• Disrupt close packing of fatty acyl chain & interfere with their mobility.
• In the presence of cholesterol tends to eliminate sharp transition temp &  create a condition of intermediate fluidity.
• In physiological term cholesterol tends to increases durability while decreasing permeability of membrane.

Cholesterol act as buffer for fluidity

• At high temp, cholesterol interfere with phospholipid fatty acid movement so that making the membrane less fluid.
• Another hand low temperature interfere with interaction between FA chain & prevent membrane from freezing.
• At 37^o C temp, cholesterol content increase in eukaryotic cell that tend to make membrane less fluid at growth stage of eukaryotic.

Maintaining Membrane Fluidity

• Maintenance of membrane fluidity is an example of homeostasis at the cellular level and can be demonstrated in various ways.
• Suppose, if the temperature of a cultured cells is lowered, the initial “emergency” response is mediated by enzymes that remodel membranes, making the cell more cold resistant.
• Remodeling is achieved by:-
  • Desaturating single bonds in fatty acyl chains to form double bonds, which is catalyzed by enzymes called desaturases
  • Reshuffling the chains between different phospholipid molecules to produce ones that contain two unsaturated fatty acids, which greatly lowers the melting temperature of the bilayer.
  • Reshuffling is accomplished by phospholipases, which split the fatty acid from the glycerol backbone, and acyltransferases, which transfer fatty acids between phospholipids.

In addition, the cell changes the types of phospholipids being synthesized in favor of ones containing more unsaturated fatty acids. Maintenance of fluid membranes by adjustments in fatty acyl composition has been demonstrated in a variety of organisms, including hibernating mammals, pond-dwelling fish whose body temperature changes markedly from day to night, cold-resistant plants, and bacteria living in hot springs.