Water Transport

• Water has to get from the soil, through the root and into the xylem to be transported around the plant.
• Water enters through root hair cells and then passes through the root cortex, including the endodermis, to reach the xylem.
• Water is drawn into the roots via osmosis. This means it travels down a water potential gradient.
• Water always moves from areas of higher water potential to areas of lower water potential - it goes down a water potential gradient.
• The soil around the roots generrally has a high water potential and leaves have a lower waters potential as water constantly evaporates from them. This creats a water potential gradient which keeps water moving.
• The symplast pathway - goes through the living parts of the cells. The cytoplasms of neighbouring cells connect through plasmodesmata. Water moves through the symplast pathway via osmosis.
• The apoplast pathway - goes through the non-living parts of the cells. The cell walls are very absorbent and water can simply diffuse through them, as well as pass through the spaces between them. The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure due to mass flow.
• When water in the apoplast pathway gets to the endodermis cells in the root, its path is blacked by a waxy strip in the cell walls, called the casparian strip. Now the water has to take the symplast pathway.
• Xylem vessels transport water all around the plant.
• At the leaves, water leaves the xylem and moves into the cells mainly by the apoplast pathway.
• Water evaporates from the cell walls into the spaces between the cells in the leaf.
• When the stomata open, the water diffuses out of the leaf down the water potential gradient into the surrounding air.
• The loss of water from a plant's surface is called transpiration.
• The movement of water from the roots to leaves is called the transpiration stream. The mechanisms that move the water include cohesion, tension and adhesion.
• Cohesion and tension help water move up plants, from roots to leaves against the force of gravity.
• Water evaporates from the leaves at the 'top' of the xylem which creates a tension, which pulls more water into the leaf.
• Water molecules are cohesive so when some are pulled into the leaf others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards.
• Water enters the stem through the root cortex cells.
• Adhesion is also partly responsiresponsible for the movement of water.
• As well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels. This helps water to rise up through the xylem vessels.