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
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.
Mechanism 1: Root Pressure Model
The root pressure model suggests that xylem sap is pushed upwards from the roots to the leaves. This mechanism relies on osmosis and active transport.
1. Active Transport: The root cells actively pump mineral ions from the soil into the root cells, creating a higher concentration of solutes inside the root compared to the surrounding soil. This creates a lower water potential inside the root cells.
2. Osmosis: The lower water potential in the root cells leads to water entering the root cells from the surrounding soil through osmosis. As a result, the root cells become turgid, exerting pressure on the xylem vessels.
3. Root Pressure: The turgor pressure created by the influx of water pushes the water upwards in the xylem vessels, a process known as root pressure. This pressure can lead to the rise of xylem sap for a short distance, particularly in certain plant species or under specific environmental conditions.
Mechanism 2: Cohesion-Tension Model
The cohesion-tension model explains that xylem sap is pulled upwards from the roots to the leaves, primarily driven by transpiration and the cohesion and tension of water molecules.
1. Transpiration: The process of transpiration occurs when water evaporates from the stomata on leaf surfaces. This creates a negative pressure or tension in the leaf cells, causing water molecules to be pulled towards the leaf.
2. Cohesion: Water molecules have adhesive forces that allow them to stick to the inner walls of the xylem vessels. Moreover, they also exhibit cohesive forces, attracting each other and forming a continuous water column.
3. Tension: As water molecules evaporate from the leaf, they create a continuous pull or tension throughout the water column. This tension extends all the way to the xylem vessels in the roots, causing the xylem sap to be pulled upwards.
4. Capillary Action: Capillary action, which occurs due to the adhesion of water molecules to the xylem vessel walls, helps facilitate the upward movement of xylem sap. Additionally, the small diameter of the xylem vessels aids in this process.
In summary, the root pressure mechanism relies on active transport and osmosis to create pressure that pushes xylem sap upwards, while the cohesion-tension model depends on transpiration and the cohesive and adhesive properties of water to create tension that pulls xylem sap upward. These two mechanisms can both contribute to the upwards movement of xylem sap in plants, with the cohesion-tension model playing a dominant role in long-distance transport.