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Ch. 37 Book Review Guide. Extremely Detailed

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Angela B Lange

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Water and Sugar Transport in Plants
-Stomata are open during the day so that gas exchange can occur between the atmosphere
and the cells inside the leaf.
-while stomata are open so that gas exchange can take place, the moist interior of the lead
is exposed to the dry atmosphere: as a result, large quantities of water evaporate from the
leafs surface.
-Water loss is a side effect of photosynthesis.
-The evaporation can actually be beneficial under some conditions because it cools the plant,
just as sweating cools your body.
37.1 Water potential and water movement
-Loss of water via evaporation from the aerial parts of a plant is called transpiration.
-Transpiration occurs whenever two conditions are met:
1. stomata are open (usually met during the day when photosynthesis is soccuring)
2. the air surrounding leaves is drier than the air inside the leaves (usually met
whenever humidity is less than 100%)
-plants replace water that is lost from leaves with water that is absorbed by roots.
-water moves from the roots to the leaves passively
-movement occurs because of differences in the potential energy of water.
-Water potential indicates the potential energy that water has in a particular environment
compared with the potential energy of pure water.
-Differences in water potential determine the direction that water moves. Water always
flows from areas of high water potential to areas of lower water potential
-Water potential is key to understanding the movement of water throughout the plant.

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-Because water potential is high in soil and roots but low in leaves and the atmosphere,
there is a water potential gradient between roots and shoots.
-For water to move UP the plant, the water moves DOWN the gradient. When it does so, it
replaces water lost to transpiration.
What Factors Affect Water Potential
-Isotonic: Solute in cell= solutes outside of cell
-Hypotonic: solutes in the cell >solutes outside the cell water flows in cell
-Hypertonic: solutes in the cell<solutes outside the cell water flows out of cell
-The concentration of the solution is important, because when solutions are separated by a
selectively permeable membrane, such as the plasma membrane of a cell, water passes
through the membrane from regions of low solute concentration to regions of high solute
-This movement of water across membranes, in response to differences in water potential is
called osmosis.
-The tendency for water to move in response to differences in solute concentrations is called
solute potential or osmotic potential.
-The solute potential of a solution is defined by its solute concentration relative to pure
water. If water contains a high concentration of solutes, then is has a low solute potential
compared with pure water.
-When an animal cell is placed in a hypotonic solution and water enters the cell via osmosis,
the volume of the cell increases until it bursts.
-This does not happen to a plant cells however if a plant cell swells in response to
incoming water, its plasma membrane pushes against the relatively rigid cell wall. The cell
wall resists ex[ansion of the cell volume by pushing back. The Force exerted by the wall is
called Wall pressure.
-As water moves into the cell, the pressure inside (Turgor pressure) increases until
wall pressure is induced.
-Cells that are firm and that experience wall pressure are said to be turgid.
-Pressure potential: a term used for all physical pressures on water.

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-Summary: Water moves in response to (1) differences in the concentration of solutes, (2)
pressure. Water potential is primarily made up of solute potential and pressure potential.
Calculating Water Potential (from Ppt. not book)
-Water potential ( ) equals the pressure potential
(p) plus the solute potential (s): 
= p + s 
- Water potential is measured in megapascals (MPa)
Water Potential in soils, plants, and the atmosphere
-In soil, the water contains relatively fre solutes and is not normally under much pressure
thus water potential tends to be high. (High when moist. Low if extremely dry)
-Roots have are high in solutes and have considerable turgor pressure
-Leafs are similar to roots
-Atmosphere has a low water potential
High Water PotentialLow water
Soil: -0.3MPa Root: -0.6MPa Leaf: -0.8MPa
Atmosphere: -95.2MPA
(high if moist, (medium-high) (depends on transpiration rate
(changes with
low if extremely dry) low when stomata are open)
-When an entire tissue loses turgor, it is said to wilt
-Turgor pressure is required for growth to occur; without it cells can not expand once cell
division is complete. In addition turgor pressure provide structural support.
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