Cohesion and Adhesion

Introduction

Key Concepts

Cohesion

Cohesion is the intermolecular force that binds water molecules together through hydrogen bonds. Each water molecule can form up to four hydrogen bonds with neighboring molecules, creating a strong network of interactions. This cohesive property results in high surface tension, allowing water to form droplets and enabling certain organisms to walk on water surfaces.

The cohesive force of water is crucial in biological systems, particularly in the vascular transport systems of plants. When water evaporates from the leaves through a process called transpiration, it creates a negative pressure that pulls more water upward from the roots through the xylem. This continuous movement is possible due to the cohesive forces between water molecules, which counteract gravity.

Mathematically, the cohesive force can be described by the surface tension (γ) of water, which is the force per unit length operating at the interface between liquid and air. The surface tension of water at room temperature is approximately 72.8 mN/m. This high surface tension is a direct consequence of the strong hydrogen bonding between water molecules.

Adhesion

Adhesion refers to the attraction between water molecules and different substances, such as the walls of plant vessels or the surfaces of soil particles. This property enables water to climb against gravity in narrow tubes, a phenomenon known as capillary action. Adhesion is equally important as cohesion in facilitating the movement of water within plants.

In the xylem of plant stems, water molecules adhere to the walls of the vessels. Combined with the cohesive forces between water molecules, this adhesion allows water to form a continuous column that can be transported from the roots to the leaves. Without sufficient adhesive forces, the upward movement of water would be inefficient, disrupting vital processes like photosynthesis.

The effectiveness of adhesion can be influenced by the diameter of the vessel through which water moves. Narrow vessels enhance capillary action because the relative surface area for adhesive interactions increases. This principle explains why plants with thinner xylem vessels can transport water more efficiently.

Capillary Action

Capillary action is the combined result of cohesion and adhesion that allows water to move through narrow spaces without the assistance of external forces. In plants, capillary action is responsible for the ascent of sap, ensuring that nutrients and water reach all parts of the plant.

The height to which water can rise in a capillary tube is determined by the balance between cohesive forces, adhesive forces, and gravitational forces. The Jurin's Law equation describes this relationship:

Where:

• h = height of the liquid column
• γ = surface tension of the liquid
• θ = contact angle
• ρ = density of the liquid
• g = acceleration due to gravity
• r = radius of the capillary tube

This equation demonstrates that narrower tubes (smaller r) and higher surface tension (γ) result in greater capillary rise. Thus, cohesion and adhesion are pivotal in enabling plants to thrive by ensuring efficient water transport.

Applications in Biology

The properties of cohesion and adhesion are integral to numerous biological functions. For instance, in insects like water striders, surface tension due to cohesion allows them to walk on water surfaces without sinking. Similarly, adhesion helps in the formation of water films on leaves, facilitating gas exchange and nutrient absorption.

Moreover, in human physiology, although not as prominently as in plants, cohesion and adhesion play roles in blood circulation. The cohesive properties of blood help maintain its integrity as it flows through the circulatory system, while adhesion assists in the interaction between blood cells and vessel walls.

Environmental Implications

Cohesion and adhesion also have environmental significance. They influence the hydrological cycle by affecting how water moves through ecosystems. For example, the transport of water through soil is governed by these intermolecular forces, impacting soil moisture levels and plant growth.

Understanding cohesion and adhesion is also essential in designing artificial systems that mimic biological water transport, such as in biomimetic materials and engineering applications where efficient fluid movement is desired.

Limitations and Challenges

While cohesion and adhesion are highly beneficial, they also present certain limitations. High surface tension can make it difficult for water to penetrate hydrophobic surfaces, posing challenges in both natural and artificial systems. Additionally, the reliance on narrow pathways for capillary action means that blockages or damage to these vessels can impede water transport.

In agriculture, managing soil moisture involves balancing these forces to ensure optimal water availability for crops. Excessive adhesion can lead to waterlogging, while insufficient cohesion may result in inadequate water retention, affecting plant health.

Comparison Table

Summary and Key Takeaways