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The Pathway of Water through a Plant Objectives To be able to describe with the aid of diagrams the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and stomata. 1 Minerals ions either move down their diffusion gradient into the root hair cell or are actively transported across the membrane into the root hair cell using energy from ATP. 2 The minerals lower the water potential in the cytoplasm of the root hair cell so water crosses the cell membrane by osmosis moving down the water potential gradient 3 Mineral ions are actively pumped into the xylem and water follows by osmosis Water with dissolved mineral ions (such as nitrate) moves through the cortex down the water potential gradient until it reaches the xylem. Transporter proteins actively pump nitrate ions into the xylem, reducing the water potential of the xylem so water moves from the cortex through the endodermis by osmosis. The pathway taken by the water through the cortex cells could be the APOPLASTIC, the SYMPLASTIC or the VACUOLAR pathway. A) The apoplast pathway – Water travels in between the cellulose strands of the cell wall and through spaces between cells. The water does not pass through any membranes so the mineral ions are carried with the water. B) The symplast pathway Water enters the cytoplasm through the cell membrane of the root hair cell and then travels from cell to cell through plasmodesmata. C) The vacuolar pathway Similar to the symplast pathway but the water can enter the vacuoles as well. The Casparian Strip • The endodermis (or starch sheath because starch granules are present) is a layer of cells surrounding the xylem in the root • The endodermal cells have a band of waterproof suberin in their cell wall forming the Casparian Strip • This blocks the apoplast pathway between the cortex and the xylem forcing water into the symplast pathway just before the endodermis The Casparian strip ensures the plant controls substances entering the xylem. Movement of Water up the Stem Three processes enable water to move up the stem: 1. Root Pressure 2. Capillary Action 3. Transpiration Pull Root Pressure • Active transport of minerals from cortex to xylem forces water into the root Capillary Action • Water molecules are charged so are attracted to the sides of the xylem vessels by ADHESION. This pulls the water up the sides of the narrow vessels. Transpiration Stream • Water molecules are attracted to each other by forces of COHESION which hold the water molecules together in a continuous column. • As water is lost from the top of the column (evaporation from leaves), the whole column is pulled up – the transpiration stream. • It creates tension in the lignified xylem vessels •This is the COHESION- TENSION theory of water movement and it relies on an unbroken column of water. If a xylem vessel is damaged the column can be maintained through another vessel via the pits. • Most water exits the plant through stomata (pores in the epidermis) • It evaporates from the cells immediately below the guard cells and diffuses out of the leaf. • This lowers the water potential in those cells, and so water enters them by osmosis from surrounding cells and so on, until water leaves the xylem and enters the leaf cells.