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VII. Plant Transport
A. Transport into the roots
1. Most minerals (solutes) and some water are
actively transported into root hairs (ATP)
2.
These minerals are transported cell to cell
toward the stele through plasmodesmata
3. Most water, on the other hand, is carried
between cells, through the cell walls
4. Therefore the endodermis and Casparian
strip control the entry of water and solutes
into the vascular cylinder (stele)
*Because of root organization all water and
solutes must pass through a plasma membrane at
some point, making selection of solutes possible
B. Transport of water (xylem sap)(2ft./min.)
1. Transpiration - evaporation from
leaves
a. 99% of absorbed water is lost
through evaporation (95% from stomata)
b. Why?
i. cooling of plant
ii. CO2 uptake
iii. adequate mineral supply to leaf
2. Root pressure
a. Water potential is higher in soil than in root
due to mineral uptake (mineral are trapped
in stele by endodermis)
b. Therefore water diffuses into stele
c. Therefore water is forced up xylem in stele
3.Transpiration-cohesion-tension mechanism
C. Transport of food (translocation)
- phloem sap (1m./hr.)
- primarily sucrose
1. Source
Sink
- leaves
- roots
- fruit
- roots
- shoot tip
2. Pressure-flow mechanism
a. Sucrose is actively loaded into companion cell
(cotransport with proton (H+) pump) (ATP)
b. Sucrose immediately diffuses into sieve tube
member through plasmodesmata
c. Water diffuses into hypertonic sieve tube mem.
d. Resulting pressure causes flow of sucrose
solution through sieve tube (bulk flow)
e. At sink sucrose is actively transported out and
water follows osmotically
Companion
cell
Sieve tube
member
VIII. Stomata
- most common on undersurface of leaves
A. Guard cell structure
1. Inner cell wall thicker
2. Unlike epidermal cells they have chloroplasts
3. Radial microfibrils in cell walls, thus they can
only elongate
B. Factors affecting opening and closing of stomata
1. Light and dark
2. CO2 concentration in leaf
3. Circadian rhythm of plant itself
C. Steps in the process of stomatal opening are the
following:
1. Blue-light receptors in guard cell membrane
stimulate proton pump to pump out H+,
2. which makes the cytosol negatively charged,
3. which causes K+ to diffuse in, [K+]
4. which causes water to diffuse in osmotically, [H2O]
5. which causes an increase in guard cell turgor,
6. which causes guard cell elongation,
7. which causes stomatal opening.