describe the fluid mosaic model of membrane structure with reference to the hydrophobic and hydrophilic interactions that account for the formation of the phospholipid bilayer and the arrangement of proteins

Fluid Mosaic Membranes

Learning Objective

Describe the fluid mosaic model of membrane structure with reference to the hydrophobic and hydrophilic interactions that account for the formation of the phospholipid bilayer and the arrangement of proteins.

1. Basic Components of the Membrane

• Phospholipids – amphipathic molecules with a hydrophilic (polar) head group and two hydrophobic (non‑polar) fatty‑acid tails.
• Proteins – integral (spanning) and peripheral (surface‑associated) proteins that perform transport, signalling, enzymatic and structural roles.
• Carbohydrates – covalently attached to lipids (glycolipids) or proteins (glycoproteins) on the extracellular surface.

2. Formation of the Phospholipid Bilayer

The spontaneous arrangement of phospholipids in aqueous environments is driven by the opposing affinities of their regions:

1. Hydrophilic heads interact favourably with water molecules through hydrogen bonding and ionic interactions.
2. Hydrophobic tails avoid water and interact with each other via van der Waals forces, forming a low‑energy interior.

When many phospholipids are present, they align such that the heads face the aqueous compartments (extracellular fluid and cytosol) and the tails face each other, creating a stable bilayer.

\$\$

\text{Phospholipid} \; \rightarrow \; \underbrace{\text{Head}{\text{hydrophilic}}}{\text{exposed to water}} \; \big| \; \underbrace{\text{Tail}{\text{hydrophobic}}}{\text{shielded}} \; \big| \; \underbrace{\text{Head}{\text{hydrophilic}}}{\text{exposed to water}}

\$\$

3. The Fluid Mosaic Model

Proposed by Singer and Nicolson (1972), the model visualises the membrane as a dynamic, semi‑fluid structure:

• Fluid – phospholipid molecules can laterally diffuse, rotate, and occasionally flip‑flop, giving the membrane flexibility.
• Mosaic – proteins are interspersed like tiles, each with distinct functions and varying degrees of mobility.

4. Arrangement of Membrane Proteins

5. Interactions Governing Structure

Several non‑covalent forces maintain membrane integrity:

• Hydrogen bonds between polar head groups and surrounding water.
• Electrostatic attractions between charged head groups and ions in the surrounding medium.
• Van der Waals forces among the fatty‑acid tails, stabilising the hydrophobic core.
• Hydrophobic effect – the entropic drive for water molecules to minimise contact with non‑polar tails, effectively pushing the tails together.

6. Factors Influencing Membrane Fluidity

1. Fatty‑acid composition – unsaturated (cis‑double bonds) create kinks, preventing tight packing and increasing fluidity; saturated tails pack tightly, reducing fluidity.
2. Cholesterol content – at low temperatures, cholesterol prevents phospholipids from packing too closely; at high temperatures, it restrains excessive movement.
3. Temperature – higher temperatures increase kinetic energy, enhancing lateral diffusion.
4. Presence of specific lipids – sphingolipids and glycolipids can form more ordered microdomains (lipid rafts).

7. Summary

The fluid mosaic model explains how the phospholipid bilayer forms through hydrophilic–hydrophobic interactions, creating a semi‑permeable barrier. Embedded within this dynamic lipid matrix are proteins that are arranged according to their affinity for the hydrophobic core or the aqueous surfaces, giving the membrane its functional versatility. Understanding these principles is essential for explaining processes such as selective transport, signal transduction, and cell‑cell communication in A‑Level Biology.