Water Transport Question (7 Marks)
Describe the pathways AND mechanisms by which water passes from the soil to the xylem vessels in the root. (7 marks)
As there is a higher water potential in the soil, and there is a lower water potential in the root hair cells, the water molecules move from the area of high potential to the area of low potential. From the soil into the roots. The process by which the molecules of water move into the root hair cells is 'osmosis'. This is a passive process, meaning that no energy (in the form of ATP) is used.
There are two main ways in which water makes its way to the xylem (cells). These are: the apoplast pathway, and the symplast pathway. With the apoplast pathway, the water enters the cell wall and then moves through it. The water molecules are able to move from cell wall to cell wall, across the intercellular spaces. After this, the water may move directly from cell wall to cell wall.
With the symplast pathway, the water enters the cytoplasm across the partially permeable plasma membrane. Water then can move into the sap in the vacuole, through the tonoplast. Water may then move from cell to cell through the plasmodesmata. Water may also move from cell to cell through adjacent plasma membranes and cell walls.
Once the water reaches the endodermis, it comes across the Casparian Strip made from a waxy, waterproof substance called Suberin. This strip is impenetrable. The water moves into the cytoplasm and continues to move down the water potential gradient until it reaches a pit. The water then moves through the pit and into the xylem vessel and up towards the leaves.
The mechanisms 'in charge' of the movement of water within the xylem is transpiration pull. This is where the water which is evaporating (as water vapour) through the stomata in the leaves is pulling the water at the bottom of the plant (roots and bottom of xylem) up towards the leaves. This is a passive process (as described above). As the water evaporates through the leaves at the top
As there is a higher water potential in the soil, and there is a lower water potential in the root hair cells, the water molecules move from the area of high potential to the area of low potential. From the soil into the roots. The process by which the molecules of water move into the root hair cells is 'osmosis'. This is a passive process, meaning that no energy (in the form of ATP) is used.
There are two main ways in which water makes its way to the xylem (cells). These are: the apoplast pathway, and the symplast pathway. With the apoplast pathway, the water enters the cell wall and then moves through it. The water molecules are able to move from cell wall to cell wall, across the intercellular spaces. After this, the water may move directly from cell wall to cell wall.
With the symplast pathway, the water enters the cytoplasm across the partially permeable plasma membrane. Water then can move into the sap in the vacuole, through the tonoplast. Water may then move from cell to cell through the plasmodesmata. Water may also move from cell to cell through adjacent plasma membranes and cell walls.
Once the water reaches the endodermis, it comes across the Casparian Strip made from a waxy, waterproof substance called Suberin. This strip is impenetrable. The water moves into the cytoplasm and continues to move down the water potential gradient until it reaches a pit. The water then moves through the pit and into the xylem vessel and up towards the leaves.
The mechanisms 'in charge' of the movement of water within the xylem is transpiration pull. This is where the water which is evaporating (as water vapour) through the stomata in the leaves is pulling the water at the bottom of the plant (roots and bottom of xylem) up towards the leaves. This is a passive process (as described above). As the water evaporates through the leaves at the top