Membrane Structure & Function — Lipid Bilayer Fundamentals

Membrane Overview

• All cells are enclosed by a plasma membrane; eukaryotes additionally compartmentalize their interiors with membranes.
• Basic architecture: lipid bilayer + embedded proteins.
  • Thickness ≈ $5\ \text{nm} \;(\text{~}50\ \text{atoms wide})$.
• Functions of the plasma membrane (PM):
  • Selective barrier: permits some molecules, excludes others.
  • Nutrient uptake & waste removal via protein channels and transporters (highly specific).
  • Signal transduction: converts external information into intracellular responses.
  • Dynamic shape changes enable motility and growth.
• Despite a common core design, each membrane’s protein composition confers unique properties.

Lipid Chemistry & Amphipathicity

• Membrane lipids are amphipathic: hydrophilic (polar) region + hydrophobic (non-polar) region.
• Key lipid classes in membranes:
  • Phospholipids (e.g.
  • Two fatty-acid tails (hydrophobic)
  • Glycerol backbone (3 carbons)
  • Phosphate + polar “head group” (hydrophilic); example: phosphatidylserine.)
  • Steroids (cholesterol):
  • Four fused rings + short hydrocarbon tail (hydrophobic)
  • Single hydroxyl $\,(\text{–OH})$ as polar head.
  • Glycolipids:
  • Sugar moiety (polar) attached to lipid with two hydrocarbon tails.

Hydrophilic vs. Hydrophobic Interactions with Water

• Hydrophilic molecules (e.g. acetone) form H-bonds/electrostatic interactions with water → energetically favorable.
• Hydrophobic molecules force water into “cage-like” order → energetically unfavorable.

Fat (Triacylglycerol) vs. Phospholipid

• Triacylglycerol (TAG): glycerol + three fatty-acid tails ⇒ entirely non-polar → aggregates as oil droplets.
• Phospholipid (e.g. phosphatidylethanolamine): glycerol + two fatty-acid tails + phosphate-based head ⇒ amphipathic → forms bilayers.

Bilayer Formation & Vesicle Sealing

• In water, amphipathic phospholipids self-assemble into bilayers:
  • Heads face aqueous environment; tails face inward, excluding water.
• Flat bilayer “sheets” are unstable at edges; they spontaneously curve into sealed spheres (liposomes/vesicles) eliminating free hydrophobic edges.
• Small tears reseal immediately; large tears prompt vesicle formation.

Synthetic Bilayers (Liposomes)

• Pure phospholipids in water → liposomes; experimental model to study membrane properties.
• Cross-section shows bilayer with aqueous interior & exterior.

Membrane Fluidity – Types of Lipid Motion

• Within one leaflet (side):
  • Lateral diffusion (side-to-side shuffling)
  • Rotation (spinning around long axis)
  • Tail flexion (wagging)
• Flip-flop (one leaflet → other) is rare; needs specific enzyme.

Factors Governing Fluidity

1. Tail Length

   • Range: $14\text{–}24$ carbons (common $18\text{–}20$).
   • Short tails → fewer van-der-Waals interactions → ↑ fluidity.

2. Degree of Saturation

   • Saturated (no C=C) → straight, pack tightly (think butter) → ↓ fluidity.
   • Unsaturated (≥1 C=C) → kinked, hinder packing (think oil) → ↑ fluidity.
   • Typical phospholipid: one saturated + one unsaturated tail.

3. Cholesterol Content

   • Short rigid molecule inserts between kinked tails.
   • Fills gaps → decreases fluidity & permeability.

Biological Reasons for Fluidity

• Allows proteins to diffuse for cell signaling interactions.
• Ensures equal partitioning of proteins/lipids to daughter cells during division.
• Lets newly synthesized proteins/lipids redistribute from insertion point.

Lipid Synthesis & Initial Distribution

• Site: cytosolic leaflet of endoplasmic reticulum (ER).
• Result: asymmetric growth—new lipids concentrated on cytosolic side.
• Scramblase (ER enzyme) randomly flips phospholipids → symmetric ER bilayer.

Generating Membrane Asymmetry

• Other membranes (PM, Golgi) are asymmetric—specific lipids enriched on particular leaflet.
• Flippase/Floppase enzymes (in Golgi & PM):
  • Use ATP to selectively move defined phospholipids to one leaflet.
  • Establish & maintain asymmetry.

“Topology Rule” – Orientation Preservation

• Once established in ER/Golgi, leaflet identity remains:
  • Cytosolic leaflet always faces the cytosol.
  • Luminal leaflet (facing ER/Golgi lumen) becomes the exoplasmic side when vesicle fuses with the PM.
• Consequence: proteins/lipids retain orientation during vesicle traffic and exocytosis.

Plasma Membrane Asymmetry – Specific Lipid Localizations

• Cytoplasmic leaflet:
  • Phosphatidylserine (PS) – negative charge.
  • Phosphatidylethanolamine (PE).
  • Phosphatidylinositol (PI) – crucial in signaling.
• Extracellular leaflet:
  • Glycolipids – sugar-decorated lipids.
  • Function: protective carbohydrate coat (glycocalyx), cell recognition.
  • Sphingomyelin, phosphatidylcholine, etc.
• Glycolipid synthesis: addition of sugars occurs in Golgi lumen; vesicular traffic sends sugar-bearing head to extracellular side.

Enzymatic Summary

• Scramblase: non-selective, equilibrates both leaflets in ER.
• Flippase/Floppase: selective, ATP-dependent, create asymmetric distribution post-Golgi.

Practical / Real-World Connections

• Drug delivery: liposomes exploit bilayer self-assembly to carry therapeutics.
• Dietary fat composition (saturated vs. unsaturated) can influence membrane properties in organisms.
• Apoptosis exposes PS on outer leaflet as an “eat-me” signal for macrophages—relies on loss of flippase activity.

Key Numbers & Equations Recap

• Bilayer thickness: $\approx 5\ \text{nm}$ (50 atoms).
• Hydrocarbon tail length: $14\text{–}24\ \text{C}$ (commonly $18\text{–}20\ \text{C}$).

Conceptual Take-Home Messages

• Amphipathicity drives self-assembly and underpins all membrane behavior.
• Membrane fluidity is a delicate balance among tail length, saturation, and cholesterol.
• Asymmetry is functionally critical and enzymatically maintained, with orientation preserved throughout vesicular traffic.