How does a plant transport water from roots to leaves?

Plants transport water from their roots to their leaves through a process known as the transpiration-cohesion-tension mechanism. Here’s how it works step by step:

1. Water Absorption by Roots

• Water from the soil is absorbed by the root hairs through osmosis, as the water concentration in the soil is higher than in the root cells.
• The absorbed water moves from cell to cell in the root cortex via the symplast (through cytoplasm) or apoplast (through cell walls) pathway until it reaches the xylem vessels.

2. Transport Through Xylem

• Water is carried upward through the xylem vessels, which are long, hollow tubes made of dead cells.
• The movement is driven by three key forces:
  • Root Pressure: A small push of water upward caused by osmotic pressure in the roots.
  • Capillary Action: The adhesive property of water helps it climb up narrow xylem tubes.
  • Cohesion and Adhesion: Water molecules stick together (cohesion) and to the walls of the xylem (adhesion), forming a continuous column of water.

3. Transpiration Pull

• Water evaporates from the stomata (tiny pores) on the surface of leaves during transpiration.
• This creates a negative pressure (suction) in the leaf, pulling water upward through the xylem from the roots to replace the lost water.

4. Water Distribution in Leaves

• Once in the leaves, water moves into the mesophyll cells where it is used for photosynthesis and maintaining turgidity.
• Excess water evaporates into the air through the stomata in a process called transpiration.

This system is highly efficient and driven by physical forces, requiring no energy expenditure by the plant.