B3.2.7 Transport of water from roots to leaves during transpiration Notes

The Role of Transpiration in Water Movement

1. Water not only provides hydration but also aids in nutrient transport and photosynthesis.
2. One of the critical processes involved in the movement of water is transpiration, which plays a key role in pulling water from the roots to the leaves.
3. The evaporation of water creates a pulling force (tension) that draws water upward through the plant’s xylem vessels.

Key Processes in Transpiration

1. Water Evaporation in Leaves

1. Water in the plant is constantly moving.
2. The water that is absorbed from the soil travels up through the roots, into the xylem, and into the leaf cells.
3. Once inside the leaf, water evaporates from the surface of mesophyll cells into the stomata (tiny pores on the leaf surface) and eventually into the atmosphere.
4. This is known as transpiration.

2. Creation of Tension in the Xylem (Cohesion-tension theory)

1. When water evaporates from the cell walls in the leaf, a negative pressure (tension) is created inside the plant.
2. This tension pulls more water from the xylem vessels up to the leaves.
3. The water molecules are pulled up because of cohesion, the force that holds water molecules together.
4. Water molecules are also adhesive, meaning they stick to the walls of the xylem vessels, aiding in the movement of water upward through the plant.

3. Capillary Action

1. Capillary action occurs in the small diameter xylem vessels, where adhesion to the walls of the xylem and cohesion between water molecules causes the water to move upward through the narrow tubes.
2. This phenomenon helps water travel from the roots to the leaves, against the force of gravity.
3. The smaller the xylem tube, the more efficient the capillary action.

4. Continuity of Water Column

1. As long as there is a continuous column of water in the xylem, the water can move upward.
2. This column is maintained by the cohesive forces between water molecules, which prevent the column from breaking.
3. If the water column is broken, for example, by air entering the xylem, it can disrupt the flow of water.
4. This is why plants need to continuously ensure the integrity of the water column through processes like root pressure and water absorption from the soil.

Generating Tension: Negative Pressure in the Xylem

1. Process: Water loss due to transpiration creates negative pressure (tension) within the xylem.
2. Driving Force: This negative pressure acts like suction, pulling water upward from the roots.
3. Structural Necessity: Xylem vessels are adapted (thickened walls and continuous tubes) to withstand this negative pressure without collapsing.

Why Transpiration Matters

1. Nutrient Transport: Water carries dissolved minerals from the soil to the leaves for photosynthesis and other metabolic processes.
2. Cooling Effect: Evaporation of water from leaf surfaces cools the plant, much like sweating cools the human body.
3. Maintaining Turgor Pressure: The tension created by transpiration maintains turgor pressure, which is vital for cell rigidity and plant structure.
4. Water Cycle Contribution: Transpiration releases water vapor into the atmosphere, contributing to the global water cycle and influencing weather patterns.