AP_Biology_Lab_9_Transpiration - AP Biology Lab#9...

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Chapter 34 / Exercise 09
Biology
Martin/Solomon
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AP Biology Lab #9Transpiration LabINTRODUCTIONThe amount of water needed daily by plants for growth and maintenance of tissues is small in comparison to the amount that is lost through the process of transpiration and guttation. If this water is not replaced, the plant will wilt and die.The transport of water up from the roots in the xylem is governed by differences in water potential. These differencesaccount for water movement from cell to cell and over long distances in the plant. Gravity, pressure and solute concentration all contribute to water potential, and water always moves from an area of high water potential to an area of low water potential. The movement itself is facilitated by osmosis, root pressure, and adhesion and cohesion of water molecules.The Overall Process:Minerals actively transported into the root accumulates in the xylem, increasing solute concentration and decreasing water potential. Water moves in by osmosis. As water enters the xylem, it forces fluid up the xylem due to hydrostatic root pressure. But this pressure can only move fluid a short distance. The most significant force moving the water and dissolved minerals in the xylem is upward pull as a result of transpiration, which creates tension. The “pull” on the water from transpiration results from the cohesion and adhesion of water molecules.The Details: Transpiration begins with evaporation of water through the stomates (stomata). The moist air in these spaces has a higher water potential than the outside air, and water tends to evaporate from the leaf surface. The moisture in the air spaces is replaced by water from the adjacent mesophyll cells, lowering their water potential. Water will then move into the mesophyll cells by osmosis from surrounding cells with higher water potentials, including the xylem. As each water molecule
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Chapter 34 / Exercise 09
Biology
Martin/Solomon
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