Explain how water moves from the soil to the leaves of a plant. At first water enters the root by osmosis because the soil water has a lower solute concentration of minerals than the epidermal cell cytoplasm (there is a water potential gradient). Water movement across the cortex cell is by two pathways both involving a water potential gradient. The cortex cell cytoplasm has a solute concentration gradient. This moves water simplistically from cell to cell by osmosis. The Apoplectic pathway moves water by capillary action of mass flow through the connecting cellulose cell wall.

The endoderm marks the beginning of the central stele of vascular tissue. Both minerals and water must pass through the plasma membrane of the endoderm. Water enters epidermal cell cytoplasm by osmosis. The solute concentration is lower than that of soil water due to the active transport of minerals from the soil water to the cytoplasm. Apoplectic: water moves by capillarity through the cellulose cell walls. Hydrogen bonding maintains a cohesion between water molecules which also adhere to the cellulose fibers. The endoderm is the outer tissue of the vascular root tissue.

The Caspian strip of the endoderm is a barrier to the movement of water of minerals by the apoplectic pathway. All solute and water must move through the plasma membrane of the endoderm cells before entering the stele. The cellulose cell wall contains a strip (Caspian strip) of a waxy water repellent substance called Siberian. The Siberian prevents water and dissolved minerals from passing into the xylem by the apoplectic pathway. Therefore water solution must pass through the plasma membrane of the endoderm. The endoderm plasma membrane can then selectively control mineral uptake and rate of uptake.

Minerals are actively loaded into the xylem which in turn causes water to enter the xylem vessel. Pressure within the xylem increases forcing water upward (Root Pressure). This is probably not a major factor in transpiration of large plants. To get the water into the xylem, minerals are actively loaded into the xylem which in turn causes water to enter the xylem vessel. This creates a water potential gradient that moves water passively into the xylem. Pressure within the xylem increases forcing water upward (Root Pressure).

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This is probably not a major factor in remonstration Rather the pressure potential gradient (hydrostatic pressure) based on evaporation (tension) form the leaf is responsible for the upward movement of water in the xylem. However, some plants live in very humid environments where evaporation rates may not be that great. Xylem vessels form a continuous pipe from the root up through the stem. Along petioles to the leaf. Xylem cells are produced from the division of the cambium and then differentiation into xylem The cytoplasm full breaks down and the end wall break down to form the pipeline To support the cell wall extra thickening take place.

This often has characteristic patterns. Some spiral some annular (as here). This extra thickening resists the ‘tension’ created by the rate tot evaporation Water molecules are attracted to each other by Cohesion. This action extends down the xylem creating a ‘suction’ effect. There is also adhesion between water molecules and the xylem vessels The cohesion and adhesion act together to maintain the water column all the way up from the root to the stomata. The rapid loss of water from the leaf pulls the water column stressing the cohesion and adhesion between water molecules.

The movement of water is an example of mass flow due to a negative pressure potential. Water movement through the leaf: The heat raises the temperature of the leaf and water in the spongy mesosphere tissue is changed into water vapor. With the stomata pore open this gradient operates only over one cell thickness. Water evaporates into the air The water loss from the leaf draws new water vapor from the spongy mesosphere (simplistic & apoplectic movement) into stomata air space. In turn the water molecules of the mesosphere space draw water molecules from the end of the xylem


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