Introduction
Transport in the xylem of plants is a fundamental process that enables the movement of water and dissolved minerals from the roots to the leaves. This process is essential for maintaining plant structure, nutrient transport, and photosynthesis. In this study note, we will explore the mechanisms of water transport in the xylem, the structure and function of xylem vessels, and the role of cohesion and adhesion in the transpiration stream.
The Transpiration Stream
Xylem Vessels
Xylem vessels are specialized structures within plants responsible for the transport of water and dissolved minerals. They are formed from long lines of cells connected end-to-end. As these cells develop, their walls degrade and their contents break down, forming long, continuous, hollow tubes. Mature xylem vessels are non-living and have thickened walls reinforced with cellulose and lignin, making them extremely tough and capable of withstanding very low internal pressures without collapsing.
NoteMature xylem vessels are non-living cells, which means they do not participate in metabolic processes. Their primary function is structural and transport-related.
Cohesion and Adhesion
Cohesion
Cohesion refers to the attraction between water molecules. Water is a polar molecule, with a slight negative charge on the oxygen atom and a slight positive charge on the hydrogen atoms. This polarity allows hydrogen bonds to form between adjacent water molecules, creating a cohesive force that helps water molecules stick together.
Adhesion
Adhesion is the attraction between water molecules and the hydrophilic surfaces of the cell walls within xylem vessels. This property helps water to move upwards through the plant, counteracting the force of gravity.
TipRemember that cohesion involves attraction between like molecules (water to water), while adhesion involves attraction between unlike molecules (water to cell walls).
Water Transport Mechanisms
Transpiration
Transpiration is the process by which water evaporates from the leaves, creating a negative pressure that pulls water up through the xylem vessels from the roots. This process is driven by the evaporation of water from the stomata, small openings on the leaf surface.
ExampleFor instance, on a hot, dry day, transpiration rates can be very high, leading to increased water uptake from the soil.
Cohesion-Tension Theory
The cohesion-tension theory explains how water moves up through the xylem vessels. As water evaporates from the leaves (transpiration), it creates a negative pressure (tension) within the xylem. Due to the cohesive properties of water, this tension is transmitted down the column of water molecules, pulling water up from the roots to the leaves.
$$ \text{Transpiration} \rightarrow \text{Negative Pressure} \rightarrow \text{Cohesion} \rightarrow \text{Water Movement Upwards} $$
