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Water is absorbed through the roots, specifically through root
Transport of water in the xylem
Is less tension in the xylem so the diameter of the
hairs
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hollow, thick-walled tubes called xylem vessels
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This is bephyll and finally out into the atmosphere
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The energy for this is supplied by the sun and is
Movement of water up the stem in the xylem:
ken and so the water molecules can no longer
therefore passive
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The
stick together
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water evaporates from the mesophyll cells due
leak out, as would be the case if it was under
the air spaces next to the stomata
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Instead air is drawn in = consistent
ter potential gradient from the air spaces through the stowith it being under tension
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Water molecules form hydrogen bonds between
mata to the air
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Xyrounding air
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Water forms a continuous, unbroken column
across the mesophyll cells and down the xylem water
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By changing the size of the stothat xylem forms a series of continuous, unbroken
matal pores, plants can control their rate of transpiration
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As water evaporates from the mesophyll cells in
tubes from root to leaves
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This is replaced
A column of water is therefore pulled up the xylem as
by water reaching the mesophyll cells from the xylem ether 5
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Water movement occurs
because:
6
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mesophyll cells lose water to the air spaces by evapothe name cohesion-tension theory)
ration due to heat supplied by the sun
2
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The loss of water from these lowers their water potential

4
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When transpiration is at its
greatest (during the day), there is more tension
(negative pressure) in the xylem
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At night (at its lowest) there

Measurement of water uptake using a potometer:
Almost impossible to measure transpiration because its difficult to condense and collect all the water vapour that leaves
all the parts of a plant
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About 99% of the water taken
up by a plant is lost during transpiration, which means that
the rate of uptake is almost the same as the rate at which
transpiration is occurring
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The rate of water loss in a plant can be measured using a
potometer
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a leafy shoot is cut under water
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2
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3
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4
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5
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6
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7
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Measurements continue

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