How Is Sugar Transported In Phloem (Pressure-flow Hypothesis)

The movement of water through the xylem is a passive process (it does not require the plant to expend energy) and is best explained by the Cohesion-Tension Theory. It relies on the physical properties of water.

1. 1. Transpiration (The Pull): The process begins at the leaves. Water evaporates from the moist surfaces of cells within the leaf and exits the leaf through small pores called stomata. This evaporation, called transpiration, creates a negative pressure, or "sucking" force (tension), in the xylem of the leaf.
2. 2. Cohesion (The Chain): Water molecules are polar, meaning they have a slight positive and negative charge. This causes them to stick to each other strongly through E+E- hydrogen bonds. This property is called cohesion. Because they are cohesive, water molecules form an unbroken, continuous chain or column that extends all the way from the leaf, down the stem, and into the roots.
3. 3. Adhesion (The Grip): Water molecules are also attracted to other polar substances. This property, called adhesion, causes the water molecules to stick to the lignified walls of the xylem vessels. This adhesion helps to counteract the force of gravity and prevents the water column from breaking.
4. 4. Water Uptake (The Source): As the tension from transpiration pulls the entire "chain" of water up the plant, it creates a pull in the roots. This pull (a low water potential) draws water from the soil into the root hairs, through the root cortex, and into the xylem to replace the water that was lost.

In essence, the evaporation of water from the leaves acts like a "suck" on a straw, pulling a continuous, unbroken column of water up from the roots through the xylem.