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Animation: Transport of Xylem Sap

by Pearson
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As a plant lives and grows, it absorbs carbon dioxide and oxygen from the air, and water and minerals from the soil. How does the plant move these substances to where they are needed for chemical reactions such as photosynthesis? Water and minerals are transported upward within the xylem as xylem sap. First, however, water and minerals are absorbed by the roots, so we will start there. The surface area of a root is enormously expanded by thousands of root hairs and symbiotic mycorrhizae. The root hairs are in direct contact with water and dissolved inorganic ions held in tiny spaces between soil particles. Water and ions can travel into the root by the apoplastic route-- moving between cells along cell walls-- or by the symplastic route-- moving from cell to cell through plasmodesmata. A molecule or ion may also move through the root via a combination of these two routes, or by moving slowly from cell to cell across plasma membranes and cell walls. All water and ions eventually have to pass through cell membranes and this is how the plant controls their uptake. Even water and ions that are traveling by the apoplastic route must eventually pass through cells, because of a waxy waterproof band called the Casparian strip. This forces all water and solutes to pass through the plasma membranes of endodermis cells, and thus these cells are able to exert ultimate control over the kinds and amounts of substances that enter the plant. Once the selected solutes and water are in the endodermis cells, they can be discharged into the xylem for transport to the rest of the plant. How do water and minerals get to the leaves? Although roots can exert a slight upward pressure, it is evaporation of water from leaves, a process called transpiration, that moves water molecules and ions up from the roots. Transpiration exerts a pull that is relayed downward along a string of water molecules from leaf to root. Hydrogen bonds cause water molecules to stick together-- a phenomenon called cohesion. As each water molecule evaporates, it pulls on the next water molecule, and it pulls on the next. This relays the pull of evaporating water molecules all the way down to the roots. The adhesion of water to walls of the xylem cells helps to keep gravity from pulling the water molecules back down. As each water molecule escapes from the leaf, it pulls a column of water molecules upward. Solutes are transported along with the water. Thus, the plant's xylem uses the movement of evaporating water molecules, cohesion, and adhesion to draw water and dissolved minerals from the soil into its roots and upward to its leaves. Hot, dry, windy conditions increase transpiration. If not enough water moves up from the soil to replace the water lost to evaporation, the plant will wilt and it could die. How does a plant prevent excessive water loss? An opening in a leaf is called a stoma. Stomata are generally open during the day to allow carbon dioxide into the leaf, where it is used in photosynthesis. While the stomata are open, water moves out of the leaf by transpiration. This allows the plant to move water and minerals to the leaf, and the evaporation of water also functions to cool the plant. If hot, dry conditions require the plant to conserve water, guard cells close the stomata. This is a tradeoff. Although closing the stomata reduces water loss, this also slows down photosynthesis, and may cause the plant to overheat.
As a plant lives and grows, it absorbs carbon dioxide and oxygen from the air, and water and minerals from the soil. How does the plant move these substances to where they are needed for chemical reactions such as photosynthesis? Water and minerals are transported upward within the xylem as xylem sap. First, however, water and minerals are absorbed by the roots, so we will start there. The surface area of a root is enormously expanded by thousands of root hairs and symbiotic mycorrhizae. The root hairs are in direct contact with water and dissolved inorganic ions held in tiny spaces between soil particles. Water and ions can travel into the root by the apoplastic route-- moving between cells along cell walls-- or by the symplastic route-- moving from cell to cell through plasmodesmata. A molecule or ion may also move through the root via a combination of these two routes, or by moving slowly from cell to cell across plasma membranes and cell walls. All water and ions eventually have to pass through cell membranes and this is how the plant controls their uptake. Even water and ions that are traveling by the apoplastic route must eventually pass through cells, because of a waxy waterproof band called the Casparian strip. This forces all water and solutes to pass through the plasma membranes of endodermis cells, and thus these cells are able to exert ultimate control over the kinds and amounts of substances that enter the plant. Once the selected solutes and water are in the endodermis cells, they can be discharged into the xylem for transport to the rest of the plant. How do water and minerals get to the leaves? Although roots can exert a slight upward pressure, it is evaporation of water from leaves, a process called transpiration, that moves water molecules and ions up from the roots. Transpiration exerts a pull that is relayed downward along a string of water molecules from leaf to root. Hydrogen bonds cause water molecules to stick together-- a phenomenon called cohesion. As each water molecule evaporates, it pulls on the next water molecule, and it pulls on the next. This relays the pull of evaporating water molecules all the way down to the roots. The adhesion of water to walls of the xylem cells helps to keep gravity from pulling the water molecules back down. As each water molecule escapes from the leaf, it pulls a column of water molecules upward. Solutes are transported along with the water. Thus, the plant's xylem uses the movement of evaporating water molecules, cohesion, and adhesion to draw water and dissolved minerals from the soil into its roots and upward to its leaves. Hot, dry, windy conditions increase transpiration. If not enough water moves up from the soil to replace the water lost to evaporation, the plant will wilt and it could die. How does a plant prevent excessive water loss? An opening in a leaf is called a stoma. Stomata are generally open during the day to allow carbon dioxide into the leaf, where it is used in photosynthesis. While the stomata are open, water moves out of the leaf by transpiration. This allows the plant to move water and minerals to the leaf, and the evaporation of water also functions to cool the plant. If hot, dry conditions require the plant to conserve water, guard cells close the stomata. This is a tradeoff. Although closing the stomata reduces water loss, this also slows down photosynthesis, and may cause the plant to overheat.