IV. TRANSPORT OF WATER (AND IONS)
Figure 38. Transport of water and ions in the root.
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mmaries/3510.html
A. In the roots
1. Movement of water
a. The symplast pathway
i. symplast: the cytoplasms of all the living cells in the root
ii. water moves into root hairs by osmosis
iii. water moves by osmosis from the epidermis to the xylem through connections
between the cells called plasmodesmata
b. the apoplast pathway
i. apoplast: the cells walls and the spaces outside of the cell membranes
ii. water easily moves toward the endodermis from the epidermis
iii. at the endodermis, the Casparian strip, which is impermeable to water
prevents further movement of water from this pathway
iv. water then enters the symplast pathway
c. the accumulation of water in the xylem produces root pressure
i. strong enough to push water partially up the xylem but not strong enough to
explain water movement in the xylem of tall plants
ii. explains why sap oozes out of cut trees and why guttation occurs
a. guttation: loss of water from the leaves, but not via transpiration through
the stomata
Figure 39.
Guttation. In herbaceous plants, water
passes through specialized
groups of cells at the edges of the leaves; it
is visible here as small
droplets around the edge of the leaf in this
strawberry plant
(Fragaria ananassa).
2.Movement of mineral ions
a. Minerals required by plants
Table 2. Plant nutrients.
http://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06_39.pdf
Table 3. Plant nutrients and symptoms of deficiency.
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mmaries/3610.html
b. mineral concentration in the soil is usually lower than in the cells of the plant
i. these minerals are brought into the cells by active transport
c. some minerals enter the roots by facilitated diffusion
d. some minerals enter the roots by diffusion
e. minerals then move either through the symplast or the apoplast
B. In the stem (Transpiration-Cohesion Tension Theory)
Here is an animation:
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ials/life9e_3501_xylem_trans.html
Figure 40. Water transport through the xylem.
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mmaries/3510.html
1. cohesion: the attraction of molecules to other molecules of the same type, relatively
strong in water
2. adhesion: the attraction of molecules to other molecules of a different type
3. as the plant grows, a continuous column of water is formed in the xylem
4. transpiration of water through the leaves produces a tension that pulls the column of
water upwards
5. movement of water does not require energy
6. ions are dissolved in the water and follow the movement of the water
C. Regulation Of Stomatal Opening And Closing (and thus Regulation of Transpiration)
Figure 41. Regulation of stomatal opening and closing.
1. light stimulates active transport of protons (H+ ions) out of the guard cells
2. creates an electrochemical gradient so that K+ ions move into the cell
3. Cl- ions follow the K+ ions
4. water follows by osmosis
5. the cell swells and its turgor pressure increases
6. causes the stomata to open
7. when water loss is high, abscisic acid (produced in the roots but stored and released
by mesophyll cells) triggers closing of stomata
a. cause an increase in the pH of the cytoplasm
b. Ca2+ ions are released from the vacuole into the cytoplasm
c. stimulates a loss of negative ions, especially NO3- and Cl- from the cell
d. stimulates a loss of K+ ions from the cell
e. loss of solutes results in a decrease in osmotic pressure
f. water moves out of the cell
g. stomata close
8. Factors affecting the rate of transpiration
a. light: stomata open in light and close without light
b. temperature: transpiration increases with increasing temperature
c. wind: transpiration increases with increasing wind
d. humidity: transpiration decreases with increasing humidity
e. moisture of the soil: transpiration increases with soil moisture
f. carbon dioxide surrounding the leaf: transpiration decreases with increasing carbon
dioxide concentration
9. a potometer is a device used to measure the rate of transpiration
Figure 42. A potometer.
http://www.ekcsk12.org/faculty/jbuckley/apbio/transpirationactivities.htm
D. Adaptations of Xerophytes
1. xerophytes: plant that lives in dry environments
2. deep roots to reach water far underground
3. shallow spreading roots to collect rain from as great an area as possible
4. reduced leaves to reduce the surface area for transpiration
a. leaves reduced to spines or needles
5. rolled leaves, leaf hairs and sunken stomata so that evaporated water is trapped close
to the surface of the leaf
a. increases the humidity near the leaf surface so less water is lost by transpiration
6. stomata opening at night and closed during the day (opposite of the normal)
a. plants called CAM plants
7. water storage tissues in the leaves or stems (succulents)
8. decreased density of stomata
9. thicker cuticles
VI. TRANSPORT OF PRODUCTS OF PHOTOSYNTHESIS
Here is an animation:
http://content.bfwpub.com/webroot_pubcontent/Content/BCS_3/Sadava_9e/Animated%20Tutor
ials/life9e_3502_pres_flow_mod.html
Figure 43. Movement of material in the phloem.
A. contents of phloem: concentrated solution of sucrose (mainly), other sugars, amino acids,
hormones, ions, etc.
1. called sap
2. movement of solutes in phloem is called translocation
B. Pressure-Flow Hypothesis
1. source: the site of photosynthesis (spring/summer) or the site of solute storage/roots
(winter)
2. sink: the tissues that need nutrients such as fruits, seeds, roots (spring/summer) or
leaves/stems (early spring)
a. another way of saying this is any place that the sugars (etc.) are actively being
used
3. sugars are loaded into the sap at the source
4. causes an increase in the solute concentration of the phloem sap
5. water follows by osmosis
6. increases the pressure in the phloem column
7. causes the sap to move towards the sink
8. sugars are removed from the phloem at the sink
9. water returns to the xylem by osmosis
10. the pressure at the sink decreases