Phospholipid Bilayers and Membrane Permeability

Amphipathic Lipids: Do not dissolve in water; hydrophilic heads interact with water, hydrophobic tails do not.
Structures formed in water:
- Micelles: Spherical aggregates with hydrophilic heads outward, hydrophobic tails inward. Form from simple amphipathic lipids with single hydrocarbon chains.
- Lipid Bilayers (Phospholipid Bilayers): Lipids align in paired sheets; hydrophilic heads face surrounding solution, hydrophobic tails face each other inside. Form from phospholipids with two hydrocarbon tails. They are the foundation of cellular membranes.
Spontaneous Formation: Micelles and phospholipid bilayers form spontaneously in water, requiring no energy input.
Entropy Explanation: Water molecules form highly organized cages around dispersed nonpolar tails. When tails aggregate into micelles/bilayers, these water cages melt, decreasing water organization and thus increasing the overall entropy of the system.

Purpose: Used by researchers to study membrane function.
Liposomes: Small, bubble-like structures made of lipid bilayers surrounding an aqueous solution. Provide a $3D$ model mimicking cell membranes.
Planar Bilayers: Lipid bilayer constructed across a hole in a wall separating two aqueous solutions. Used for controlled experiments.
Experimental Applications: Studying permeability, substance movement rate, effects of different phospholipids, and impact of proteins on membrane properties.

Definition: Some substances cross membranes more easily than others.
Factors Affecting Permeability (Figure 6.9 Pattern):
- Small, nonpolar molecules (e.g., O\text{_}2): Cross quickly.
- Small, polar, uncharged molecules (e.g., H\text{_}2O): Cross slower than nonpolar molecules.
- Larger polar molecules (e.g., glucose): Cross much slower.
- Charged solutes (e.g., Na $\text{+}$ ): Do not effectively cross without membrane proteins (billions of times slower than water).
Hypothesis: Charged substances and larger polar molecules are more stable dissolved in water (polar environment) than in the nonpolar interior of membranes.

Bond Saturation and Hydrocarbon Chain Length:
- Short, kinked, unsaturated hydrocarbon tails: Create spaces, reduce van der Waals interactions, result in higher permeability.
- Long, straight, saturated hydrocarbon tails: Fewer spaces, more van der Waals interactions, result in lower permeability (denser membrane).
Cholesterol Content:
- Effect: Dramatically reduces membrane permeability.
- Mechanism: Cholesterol's hydrophobic steroid rings force phospholipid tails closer, increasing packing density and making the membrane less permeable.

Fluidity: Phospholipids generally have an olive oil-like consistency, allowing lateral movement.
Permeability and Fluidity Relationship: Membrane permeability is closely related to its fluidity.
Effect of Temperature Drop:
- Molecules move slower, fluidity decreases.
- Hydrophobic tails pack more tightly. Bilayers can solidify at very low temperatures.
- Permeability decreases: Membranes become less permeable to molecules at low temperatures.