How do plants transport water from the roots to the leaves to replace losses from transpiration?

Sourav PanNovember 9, 2024

Answered

Plants transport water from the roots to the leaves primarily through a process known as the cohesion-tension theory, which relies on the principles of transpiration, cohesion, and adhesion. Here’s a detailed explanation of how this mechanism works to replace water lost from transpiration:

Mechanism of Water Transport

1. Transpiration:
   • Transpiration is the process by which water evaporates from the surfaces of leaves, primarily through small openings called stomata. This evaporation creates a negative pressure (tension) within the leaf’s air spaces, which is transmitted down through the xylem vessels.
2. Cohesion and Tension:
   • Water molecules exhibit strong cohesive properties due to hydrogen bonding. This cohesion allows water molecules to form a continuous column within the xylem vessels. As water evaporates from the leaf surface, it pulls on the adjacent water molecules in the xylem, creating tension that draws more water upward from the roots through capillary action.
3. Xylem Structure:
   • The xylem consists of specialized vessels (tracheids and vessel elements) that are designed to transport water efficiently. These vessels are long, hollow tubes with lignified walls that provide structural support and prevent collapse under negative pressure. The absence of end walls in xylem vessels allows for uninterrupted water flow.
4. Capillary Action:
   • In addition to cohesion, adhesion plays a role in water transport. Water molecules adhere to the hydrophilic walls of xylem vessels, helping to counteract gravity and facilitate upward movement. This combination of cohesion (water molecules sticking together) and adhesion (water molecules sticking to vessel walls) allows for efficient water transport against gravitational forces.
5. Root Uptake:
   • Water is absorbed by root hairs from the soil through osmosis, driven by differences in solute concentration between the soil solution and the root cells. Once inside the roots, water moves into the xylem via two pathways:
     • Apoplastic Pathway: Water moves through cell walls and intercellular spaces.
     • Symplastic Pathway: Water moves through the cytoplasm via plasmodesmata (small channels connecting plant cells).
6. Water Potential Gradient:
   • The combination of transpiration and root uptake creates a continuous gradient of water potential from the soil (high potential) to the atmosphere (low potential). This gradient drives water movement upward through the plant.