cell membrane article notes

Lipid Bilayer as the Basis for Cell Membrane Structure

  • The lipid bilayer is universally established as the foundational structure of cell membranes.

    • Its presence is easily observable using electron microscopy.

    • Revealing details of its organization requires specialized techniques like:

    • X-ray diffraction.

    • Freeze-fracture electron microscopy.

    • The spontaneous formation of bilayers is due to the unique properties of lipid molecules under simple artificial conditions.

Membrane Lipids Are Amphipathic Molecules

  • Lipid molecules, primarily fatty molecules, constitute about 50% of the mass of most animal cell membranes.

    • Nearly all remaining mass is accounted for by proteins.

    • Approximately 5 × 10^6 lipid molecules can be found in a 1 μm × 1 μm area of a lipid bilayer.

    • This translates to nearly 10^9 lipid molecules in the plasma membrane of a small animal cell.

  • All lipid molecules in cell membranes are described as amphipathic (or amphiphilic):

    • Hydrophilic ("water-loving") or polar end.

    • Hydrophobic ("water-fearing") or nonpolar end.

Phospholipids as the Most Abundant Membrane Lipids

  • The most prevalent type of membrane lipid is phospholipids:

    • Composed of a polar head group and two hydrophobic hydrocarbon tails.

    • Tails usually consist of fatty acids, ranging in length from 14 to 24 carbon atoms.

    • One tail typically possesses one or more cis-double bonds (unsaturated), while the other is generally saturated.

    • Kinks caused by cis-double bonds affect packing and fluidity of the membrane.

Amphipathic Nature and Spontaneous Bilayer Formation

  • The shape and amphipathic property of lipid molecules lead to spontaneous bilayer formation in aqueous environments:

    • Hydrophilic molecules dissolve in water due to favorable electrostatic interactions or hydrogen bonds.

    • Hydrophobic molecules are insoluble; they reorganize surrounding water into less favorable structures, increasing free energy.

    • Burying hydrophobic tails reduces water interaction:

    • Aggregation can form spherical micelles or bimolecular sheets (bilayers).

    • Phospholipid bilayers are energetically favorable due to head group-water interaction.

Self-Healing Properties of Bilayers

  • Phospholipid bilayers exhibit self-healing properties:

    • A small tear leads to an energetically unfavorable free edge with water.

    • Lipids spontaneously rearrange to eliminate the free edge, forming a sealed compartment.

    • Larger tears are repaired via intracellular vesicle fusion.

Fluidity of Lipid Bilayers

  • The lipid bilayer functions as a two-dimensional fluid:

    • Individual lipid molecules can diffuse freely within bilayers.

    • Important studies conducted in the 1970s demonstrated lipid mobility within synthetic bilayers.

Experimental Setups

  • Two prominent preparations for studying lipid bilayers include:

    1. Spherical vesicles (liposomes): Diameter ranging from 25 nm to 1 μm.

    2. Planar bilayers (black membranes): Formed across holes between aqueous compartments.

Lipid Mobility Studies

  • Various techniques such as electron spin resonance (ESR) spectroscopy track lipid molecule motion:

    • Flip-flop (migration between monolayers) occurs infrequently (~< once per month).

    • Lateral diffusion occurs rapidly (~10^7 times per second); diffusion coefficient approximately 10^-8 cm²/sec.

Composition Impacting Fluidity

  • The fluidity of a lipid bilayer is significantly influenced by its composition and temperature:

    • Phase transition is noted when a bilayer changes from liquid to crystalline state.

    • The transition point is lower when hydrocarbon chains are shorter or unsaturated.

Cholesterol's Role in Membrane Fluidity

  • Many eukaryotic membranes consist of phospholipids and cholesterol:

    • Cholesterol molecules can reach ratios of 1:1 with phospholipids and enhance permeability barrier properties.

    • They position themselves near polar head groups and interact with regions of hydrocarbon chains to reduce mobility and permeability to small water-soluble molecules.

    • Cholesterol's presence inhibits crystallization of hydrocarbon chains, affecting phase transitions.

Lipid Composition Variations

  • Biological membranes vary in lipid composition:

    • Eukaryotic membranes: Large amounts of cholesterol plus various different phospholipids.

    • Bacterial membranes: Usually no cholesterol; stability is provided by an exterior cell wall.

Major Phospholipids in Mammalian Cells

  • Key phospholipids in many mammalian plasma membranes include:

    • Phosphatidylcholine

    • Phosphatidylethanolamine

    • Phosphatidylserine (carries a negative charge)

    • Sphingomyelin (electrically neutral at physiological pH).

  • Together, these phospholipids account for over 50% of the total lipid mass in membranes.

Functional Asymmetry of the Lipid Bilayer

  • The lipid bilayer exhibits asymmetry, which is functionally significant:

    • Composition differs between inner and outer monolayers of membranes.

    • For instance, in human red blood cells:

    • Choline-containing lipids predominantly occupy the outer monolayer.

    • Phosphati-dylethanolamine and phosphatidylserine exist mostly in the inner monolayer, leading to a charge difference.

Role of Lipid Asymmetry in Membrane Function

  • Asymmetry is crucial for cytosolic proteins that bind to specific lipid head groups.

    • For example, protein kinase C (PKC) binds to the cytosolic face of the plasma membrane and is activated by phosphatidylserine.

  • Phosphatidylinositol is also significant in cell signaling:

    • It can be phosphorylated to create recruiting sites for signaling proteins.

Glycolipids

  • Glycolipids are found exclusively in the noncytosolic monolayer of plasma membranes:

    • Generally represent about 5% of lipid molecules in the outer monolayer, primarily in lipid rafts.

    • They exhibit hydrogen bond formation between their sugar groups and van der Waals forces among hydrocarbon chains.

    • Types include gangliosides, which possess one or more negatively charged sialic acid residues, crucial in nerve cell membranes.

Functions of Glycolipids

  • Provide protection against harsh conditions in epithelial cell membranes.

  • Alter electrical fields and ion concentrations across membranes.

  • Involved in cell-recognition processes via carbohydrate-binding proteins (lectins).

  • Certain glycolipids, such as gangliosides, serve as entry points for toxins (e.g., cholera toxin).

Summary

  • Biological membranes consist of a double layer of lipid molecules, with membrane proteins embedded within.

  • Lipid bilayers are fluid, allowing rapid diffusion of lipid molecules within their monolayers.

  • Major classes of membrane lipids include phospholipids, cholesterol, and glycolipids, each contributing uniquely to membrane functionality and integrity.

  • The distinct lipid compositions between the inner and outer monolayers reflect their specific roles and interactions within cellular processes.