Transport in plants

Transport in Plants

·       Movement of Xylem Sap:

o   Xylem sap can move at speeds of up to 15 meters per hour in vessels, and this movement can even occur against gravity in tall trees.

o   Some examples of impressive heights for water transport in trees include redwood trees (110 meters), Douglas fir (100 meters), and Sitka spruce (90 meters).

o   The primary forces behind xylem sap movement are root pressure and transpiration pull.

o   Root pressure contributes to the "push" force, creating pressure in the roots and pushing water up.

o   The "pull" force is primarily driven by transpiration, which is the loss of water from leaves.

·       Transpiration Pull:

o   Leaves play a significant role in water loss through transpiration.

o   Mature maple trees, for instance, can lose up to 200 liters of water per hour on a sunny day.

o   The cohesion and adhesion properties of water are essential in this process. Cohesion refers to water molecules sticking together due to hydrogen bonding.

o   Xylem cells, including tracheids and vessel elements, are hollow and joined end to end to facilitate water movement.

o   The cell walls of these xylem cells are composed of lignin, which is hydrophobic.

o   The Transpiration-Cohesion-Tension Theory explains the process by which water is pulled up the plant through xylem due to transpiration in the leaves.

·       Movement of Phloem Sap:

o   Phloem sap moves from sources (where it's made or stored, like leaves) to sinks (where it's used or stored, like roots, terminal buds, flowers, fruits, or seeds).

o   Some organs can act as both sources and sinks depending on the plant's needs.

·       Phloem Loading:

o   Phloem sap is propelled by hydrostatic pressure, which is created by the high sucrose concentration in the phloem.

o   This high solute concentration lowers water potential (y).

o   Water, obtained from the xylem, is drawn into the phloem at the site of phloem loading, creating pressure that pushes the sap away from the source.

·       Regulation of Stomata Opening and Closing:

o   Stomata are regulated by specialized epidermal cells called "guard cells" that flank the stomatal pore.

o   Guard cells have unique features, such as chloroplast development and reinforcement of their cell walls.

o   They can take up or release water to open or close the stomatal pore, which controls gas exchange.

·       Uptake of Water by Guard Cells:

o   Decreasing water potential (y) by taking up water from the surrounding tissue allows guard cells to become turgid and open the stomata.

o   This process is achieved by increasing the concentration of potassium ions (K+) in guard cells.

o   ATP-driven proton pumps move H+ ions out of the guard cells, favoring the uptake of K+ and Cl- ions.