19. Predominant interactions between phospholipids that stabilize a biological membrane include
    (1) hydrogen bonds and covalent interactions.
    (2) van der Waal and ionic interactions.
    (3) hydrophobic interactions and hydrogen bonding.
    (4) covalent and hydrophobic interactions.

Key Interactions Responsible for Stabilizing Biological Membranes

Understanding Biological Membranes
Biological membranes are dynamic structure consisting primarily of phospholipids, proteins, and cholesterol. Their stability is essential to cell function, compartmentalization, and signal transduction.

Structure of Phospholipids
Phospholipids consist of:

Hydrophilic (polar) head – Is attracted to water.

Hydrophobic (nonpolar) tails – Are repelled by water and associate with one another.

Molecular interactions will dictate membrane stability.

Dominant Interactions that Contribute to Membrane Stability

1. Hydrophobic Interactions (Primary Force) ✅
Hydrophobic tails repel water and cluster, creating a bilayer.

Self-assembly is the primary force for membrane construction.

Prevents fluidity and flexibility of the membrane.

2. Hydrogen Bonding (Secondary Force) ✅
Polar heads of phospholipids bond with water molecules through hydrogen bonding.

This keeps the membrane surface stable and ensures its structure.

Why Other Interaction Types Are Less Important?
Covalent Bonds ❌
Membranes are not stabilized by covalent bonds, as they must be flexible for transport and signaling.

Covalent bonds are too stable and would make membranes stiff.

Ionic Interactions ❌
Though some charged molecules get attracted to the membrane, ionic interactions do not dictate membrane stability.

The lipid bilayer is neutral in charge and does not have strong interactions based on charge.

Correct Answer: (3) Hydrophobic Interactions and Hydrogen Bonding ✅

Significance of These Interactions

1. Membrane Fluidity & Function
Dynamic movement of lipids and proteins is facilitated by hydrophobic interactions.

2. Selective Permeability
The bilayer structure allows regulated transport of molecules.

3. Cellular Signaling & Stability
Membrane proteins depend on these for function and communication.

Conclusion
The stability of biological membranes is mainly ensured by hydrophobic interactions and hydrogen bonding. These allow for flexibility, compartmentalization, and selective permeability, which are critical for cellular function.