Download 20 BLY 122 Lecture Notes (O`Brien) 2009 VII. Chapter 37—Water

BLY 122 Lecture Notes (O’Brien) 2009
VII. Chapter 37—Water and Sugar Transport in Plants
I. Key Concepts
A. Plants do not expend energy (ATP) to move water
1. Water leaves plant when stomata are open (= transpiration)
2. Transpiration pulls water up through xylem vessels from roots
B. Plants use energy to move sugars
1. Sugar moved from sites of photosynthesis (= sources) to storage sites (= sinks)
2. ATP spent to “load” sugars into phloem vessels against a concentration gradient
II. Water Potential and Water Movement (37.1)
A. Transpiration is the loss of water from aerial plant parts. (Transpiration is in some
ways analogous to evaporation of animal sweat.)
B. Water movement from soil to atmosphere through plant tissues follows a water
potential gradient.
C. Water uptake is passive
1.
OSMOSIS (Review pp. 105-107): Movement of water from a region of high
concentration to a region of low concentration
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No ATP required
D. What factors Affect Water Potential? (Important definitions)
1. Solution: Liquid in which something is dissolved
2. Solute: Substance that is dissolved
The following 2 terms are relative, that is they are used when comparing two fluids.
3. Hypotonic: A solution is hypotonic to a cell, if the solution has a lower salt
concentration
than the fluid in the cell.
4. Hypertonic: In the above situation, the cell’s cytoplasm is hypertonic to the solution
outside.
During osmosis, water will cross a semi-permeable membrane from the hypotonic solution
and dilute the hypertonic fluid.
5.
SOLUTE POTENTIAL of a solution (= Measure of its capability to supply water)
a. Property of a solution resulting from its solute (= dissolved material) content
b. Describes its ability to absorb or give up water by osmosis.
c. If solute potential is NEGATIVE: water moves into cell (Solution outside is
hypotonic to cell)
d. If ZERO: no net movement (= Isotonic)
e. If POSITIVE: water leaves cell (Solution outside is hypertonic to cell)
6.
Water potential is the tendency of water to move from one location to another.
a. Water potential is determined by solute potential and pressure potential.
b. Water moves from regions of high water potential to regions of low water
potential.
c. Dissolved solutes in plant cells lower the water potential, and water moves
across the cell membrane by osmosis from regions of high water potential.
d. When no membrane is present, water moves from areas of high pressure to
areas of low pressure (Figs. 37.1a-b; 37.3)
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E.
Water potential differences between soil, plants, and air establish a gradient that
determines the direction of water flow.
1. Soil usually has high water potential relative to plants. (Soils yield water to plants
unless the soils are salty.)
2. Air has low water potential relative to plants. (Air tends to withdraw water from
plants if stomata are open.)
3. Dissolved solutes in plants cells lower the water potential (= make cell
hypertonic) and water moves into cell by osmosis from external regions of high
water potential (= outside is hypotonic)
4. When no membranes is present, water moves from areas of high pressure to
areas of low pressure (such as would occur inside xylem vessels.)
5. Plants gain water from soil and lose it to the atmosphere. (Fig. 37.4)
III. How Does Water Move from Roots to Shoots (37.2)
A. Water Movement via Capillary Action
1. CAPILLARY ACTION: Movement of water up a narrow tube (Fig. 37.10a)
2. Water molecules bind to one another in all directions via hydrogen bonding
3. At surface, water molecules can only bind with molecules below them creating a Ushaped MENISCUS due to SURFACE TENSION (Fig. 37.10b)
4. COHESION: Since all water molecules are connected by hydrogen bonds, pulling on
surface molecules will be transmitted to molecules below
5. ADHESION: Water molecules form stronger hydrogen bonds to non-water molecules
than water, such as glass or cells walls or narrow vessels in plants
B. Cohesion-Tension Theory of Water Movement
1. Theory proposes that water is pulled up xylem by the surface tension generated at
the interface between the atmosphere and water inside the leaf. (Fig. 37.11)
2. Details
a. A steep water potential gradient is created when stomata pores open and the
humid leaf interior is exposed to the dry air.
b. Water exits the leaf, and menisci form at the air-water interface.
c. Menisci deepen as more water is lost, resulting in a “pull” or tension on the
water column.
d. The cohesiveness of water molecules due to hydrogen bonding leads to the
transmission of the tension throughout the plant, from leaf cells through water
in the hollow xylem and on to the water in the soil.
IV. Water Absorption and Water Loss (37.3)
1. Dry-habitat plants may have sunken stomata surrounded by hairs and thickened
cuticle. (Fig. 37.16b)
2. CAM and C4 photosynthetic pathways enable plants to survive in hot, dry
environments efficiently. (See Chapter 10)
3. Some dry-habitat plants have extremely low root water potentials.
IV. Translocation (37.4)
A. Sugars move from location where sugar enters phloem (= the source), to location
where sugar is removed (= the sink). (Figs. 37.17 & 37.18)
B. Sugars move within the living sieve-tube elements of phloem tissue. (Fig 37.19)
C. Pressure-Flow Hypothesis
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1. Sugars are transported from source to sink along a turgor pressure gradient in
phloem.
a. High sugar concentration in phloem sap at source leads to movement of water
from xylem into phloem, increasing turgor pressure.
b. Low sugar concentration in phloem sap at sink reduces turgor pressure, and
water moves from phloem sap to xylem.
c. Water flows in continuous loop driven by water potential gradients between
xylem and phloem.
d. Sugars move in one direction by bulk flow along turgor pressure gradient in
phloem. (Fig 37.20)
2. Phloem Loading
a. Sucrose is transported into phloem vessels at source
b This occurs against a concentration gradient and requires energy (ATP).
c. Two membrane proteins, a proton pump and a proton-sucrose co-transporter,
are involved in phloem loading. (Fig 37.22)
3. Phloem Unloading
a. Unloading of sucrose at sink (either in root or in new leaf) occurs against a
concentration gradient
b. Requires hydrolysis of ATP. (Fig 37.25)
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