Surface Tension

Surface tension is a physical phenomenon that occurs at the interface between a liquid and a gas (or another liquid). It is caused by the cohesive forces between the molecules in a liquid, which are stronger at the surface due to the lack of neighbouring molecules in the air. This results in a “skin” or “film” that resists external forces.

Definition:

Surface tension is the force per unit length exerted along the surface of a liquid, acting to minimize the surface area. It is quantified as

Surface Tension   (γ) = F/L

Where

F is the force acting along the surface, and

L is the length over which the force is applied.

Molecular Explanation:

• In a liquid, molecules experience cohesive forces from all directions. However, molecules at the surface experience a net inward force because other molecules on the air side do not surround them. This results in a contraction of the surface area and creates tension.

Measurement:

• Surface tension is commonly measured in units of force per unit length (e.g., dynes per centimetre or newtons per meter).

Significance in Biochemistry:

1. Biological Membranes:

   • Cell Membranes: The fluid mosaic model of cell membranes illustrates how surface tension helps maintain the integrity of the lipid bilayer. Surface tension impacts how proteins and lipids interact at the membrane interface, affecting cellular functions such as signal transduction and membrane fluidity.
   • Pulmonary Surfactant: In the lungs, surfactant reduces surface tension in the alveoli (air sacs) to prevent their collapse during exhalation. This is crucial for efficient gas exchange and respiratory function.

2. Protein Folding:

   • Hydrophobic Interactions: Surface tension influences the folding and stability of proteins. Hydrophobic regions of proteins tend to cluster away from the aqueous environment, driven by surface tension and reducing the system’s overall free energy.

3. Enzyme Function:

   • Substrate Binding: Surface tension affects how enzymes interact with substrates, especially in aqueous environments. The interaction between enzymes and substrates can be influenced by the surface tension of the solution, impacting reaction rates and efficiency.

4. Formation of Biological Structures:

   • Micelles and Liposomes: Surface tension forms micelles and liposomes, which are essential for various biochemical processes, including drug delivery and cellular transport. These structures form due to the self-assembly of surfactants in aqueous environments, driven by minimizing surface tension.

5. Drug Delivery Systems:

   • Nanoparticles and Vesicles: In pharmaceutical applications, surface tension affects the stability and release of drug-loaded nanoparticles and vesicles. Controlling surface tension helps in designing effective delivery systems with optimal release profiles.

6. Biochemical Assays:

   • Drop Shape Analysis: Surface tension is used in various biochemical assays to study interactions at interfaces, such as the adsorption of proteins or lipids to surfaces, which can provide insights into molecular interactions and surface properties.

7. Cellular Mechanics:

   • Cell Shape and Movement: Surface tension influences cell shape and movement. For instance, the tension at the cell surface affects processes like cell division, migration, and the formation of cellular structures.