Phloem (answers) - SI Leader: Anita Vijapura Spring 2010...

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SI Leader: Anita Vijapura Spring 2010 BSC2011 03/04 Transport through Xylem Water and minerals ascend from roots to shoots through the __xylem__. This is __long__-distance transport of __xylem__ sap. To move material upward, we can apply __positive__ pressure from below or __negative__ pressure from above. (as when sucking liquid through a straw) The sap flows upward from roots throughout the shoot system to veins that branch throughout each leaf. Leaves depend on this efficient delivery system for their supply of water. Plants lose a lot of water by __transpiration__ (the loss of water vapor from leaves and other aerial parts of the plant.) Unless this lost water is replaced by water transported up from __the roots__, the leaves will __wilt__ and the plants will eventually __die__. The upward flow of __xylem sap__ also brings __mineral nutrients__ to the shoot system. Is sap pushed upward from the roots, or is it pulled upward by the leaves? 1) Pushing xylem sap via __Root Pressure__ a. When? At night when transpiration is very low bc stomata is closed b. Why? Root cells continue pumping mineral ions into the xylem of the vascular cylinder. The resulting accumulation of minerals (more negative solute potential) lowers the water potential within the vascular cylinder. c. How? Thus, water flows into vascular cylinder (high water potential to low water potential) from the root cortex, generating positive root pressure (which is an upward push of xy sap). d. Result: root pressure sometimes causes more water to enter the leaves than is transpired resulting in Guttation (the exudation/discharge—forming beads of dew-- of water droplets on tips of grass blades or the leaf margins of some small, herbaceous eudicots). 2) Pulling Xylem Sap via __the Transpiration-Cohesion-Tension__ mechanism a. Transpirational Pull i. Why? During photosynthesis, __CO2__ enters the cell through stomata and __water vapor__ diffuses from inside the leaf toward outside. Explain. stomata lead to a maze of internal airspaces that expose the mesophyll cells to the CO2 they need for photosynthesis. The air in these spaces is saturated with water 1
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SI Leader: Anita Vijapura Spring 2010 BSC2011 03/04 vapor bc its in contact with the moist walls of the cells. The air outside the leaf is drier (lower water potential than air inside leaf). Therefore, water vapor in the
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Phloem (answers) - SI Leader: Anita Vijapura Spring 2010...

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