Discuss the two possible mechanism (Xylem sap being pushed upwards from the roots or pulled downwards by leaves)by referring to the root pressure model and transportation pull/cohesion-tension model of xylem transport

Answer 1

The two possible mechanisms for the upward movement of xylem sap are the root pressure model and the transpiration pull/cohesion-tension model. Both mechanisms play a role in the movement of water and nutrients through the xylem vessels.

1. Root pressure model:
The root pressure model proposes that xylem sap is pushed upward from the roots due to the active transport of ions into the xylem vessels. This active transport creates a higher solute concentration in the xylem sap compared to the surrounding soil solution. As a result, water from the soil flows into the xylem vessels by osmosis, increasing the hydrostatic pressure within the roots.

This increased pressure pushes the water and dissolved nutrients upward through the roots and into the stem. Root pressure is often observed during periods of high water availability or when plants are actively taking up water, such as after rainfall or in the morning when transpiration rates are low. It is particularly important in herbaceous plants and young seedlings.

However, the root pressure mechanism alone is insufficient to account for the upward movement of water in tall trees. This is where the transpiration pull/cohesion-tension model comes into play.

2. Transpiration pull/cohesion-tension model:
The transpiration pull/cohesion-tension model is the predominant mechanism driving the upward movement of xylem sap in tall plants and trees. It involves the combined forces of transpiration, water cohesion, and tension.

Transpiration is the process by which water vapor escapes from the stomata on the surface of leaves. As water evaporates, it creates a negative pressure potential, or tension, within the leaf. This tension is transmitted through the water column in the xylem vessels, pulling water from the roots upwards.

Water molecules in the xylem column are held together by cohesive forces. As water is pulled upwards and transpired from the leaves, the cohesive forces between water molecules pull neighboring water molecules along, creating a continuous column of water within the xylem vessels.

The strong cohesion between water molecules and the tensile strength of water allow the water column to be pulled up to great heights, even against the force of gravity. This cohesion-tension mechanism helps maintain the continuity and integrity of the water column from the roots to the leaves.

Overall, while the root pressure model can contribute to the upward movement of xylem sap in some circumstances, the transpiration pull/cohesion-tension model is the primary mechanism responsible for long-distance water movement in plants.