Cohesion: Water molecules stick to each other due to hydrogen bonding.
Adhesion: Water molecules stick to other substances, such as the xylem wall, due to polarity.
Polarity of Water:
Water is a dipole, meaning it has a partially positive charge on the hydrogen atoms and a partially negative charge on the oxygen atom, allowing it to bond with other polar molecules.
Universal Solvent:
Water can dissolve polar substances, like minerals (e.g., nitrates, phosphates), because of its polarity.
Structure of the Xylem
Xylem vessels are dead cells without end walls, forming continuous tubes.
Xylem has pits (unlignified cell walls) which are permeable to water and minerals.
The xylem wall is made of lignin and cellulose, which have some polarity, allowing interaction with water.
Process of Water and Mineral Movement
Interaction with Xylem Wall:
Water molecules adhere to the xylem wall due to its polarity.
This adhesion and cohesion form a continuous chain of water molecules.
Transpiration Pull:
Water evaporates from the leaf through transpiration, creating a pulling effect on the water in the leaf.
Pressure Gradient:
Loss of water at the leaf creates a lower pressure at the top of the xylem compared to the bottom.
This pressure difference is known as a pressure gradient.
Mass Flow:
Water and dissolved minerals move up the xylem from high to low pressure (mass flow driven by the pressure gradient).
Summary of Process
Transpiration in the leaves drives the movement of water and minerals up the xylem.
The combination of cohesion and adhesion keeps the water and minerals in a continuous column, facilitating mass flow.
Water and minerals move from higher pressure at the bottom of the xylem to lower pressure at the top, following the pressure gradient.
Exam Focus
Understand and explain cohesion and adhesion.
Describe how the pressure gradient is formed and drives mass flow.
Mention the universal solvent property of water and its role in dissolving minerals.