Understanding Cohesion in Plant Water Transport
One of the most remarkable features of water is its ability to move upward in plants—sometimes over 100 meters—without pumps or external energy. This upward movement, known as transpiration pull, relies heavily on the cohesive properties of water. Cohesion is the attraction between water molecules caused by hydrogen bonding. Because each water molecule can form multiple hydrogen bonds, the entire column of water inside a plant’s xylem behaves as a continuous, interconnected chain.
When water evaporates from a leaf’s surface during transpiration, it creates a negative pressure, or tension, at the top of the xylem. This tension pulls the next molecules upward, and because cohesion holds water molecules tightly together, the entire column moves as one unit. Without cohesion, the water column would break apart under tension, preventing water from traveling from the roots to the leaves. Cohesion is therefore essential to maintaining the integrity of this column, ensuring that plants can transport water and dissolved minerals efficiently.
The structure of the xylem complements cohesion. Xylem vessels are narrow and rigid, helping to minimize the risk of the water column collapsing under tension. The smaller the diameter of the tube, the stronger the cohesive forces within the water column. This allows trees to transport water to great heights, supporting photosynthesis, nutrient transport, and cooling through transpiration. The cohesion–tension model explains how water movement happens passively yet reliably, even in large, tall plants.
Cohesion also works hand-in-hand with adhesion, another important property of water. Adhesion allows water molecules to interact with the walls of xylem vessels, helping counteract gravity and maintain continuous flow. While cohesion holds the water molecules together, adhesion anchors the water film to the vessel surfaces, allowing the process to proceed smoothly. Together, these two forces form the foundation of long-distance transport in plants.
For IB Biology students, understanding cohesion is more than just recalling a definition—it’s recognizing how molecular interactions scale up to support whole-organism physiology. The cohesive properties of water make life possible for plants, and by extension, for the countless organisms that depend on them.
FAQs
Why doesn’t the water column break under tension?
The water column remains intact because hydrogen bonds create strong cohesive forces between molecules. Even though each hydrogen bond is weak individually, their combined effect forms a resilient column capable of withstanding substantial tension. Xylem vessels support this by being narrow and rigid, which reduces the chance of the column breaking. The continuous evaporation of water from the leaves maintains negative pressure that keeps the water moving upward. This balance makes the system stable under typical environmental conditions.
How does cohesion relate to transpiration?
Transpiration creates the driving force that pulls water upward, but cohesion is what allows that pulling force to affect the entire water column. When water evaporates from the stomata, the resulting tension transmits down through the xylem. Because water molecules stick together, the pull at the top moves every molecule below it. This relationship ensures a steady supply of water to leaves, supporting photosynthesis and maintaining turgor pressure. Without cohesion, transpiration pull would not work effectively.
What happens if cohesion is disrupted?
When cohesion is interrupted, such as by air bubbles forming in the xylem, the water column can break. This process, called cavitation, interrupts water transport and can severely stress the plant. Some plants have adaptations to limit cavitation, including smaller xylem diameters or the ability to reroute water through adjacent vessels. If cohesion fails extensively, the plant cannot transport water to its leaves, leading to wilting or death. Maintaining cohesion is therefore critical for plant survival.
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