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Transport in Plants
Ch. 36
Transport of Xylem
Fluid rises in the xylem without requiring energy
 Fluid rises AGAINST gravity
Two ways this happens:
 Pushed up by root pressure
 Results from water flowing into stele from the
ground because of high mineral content in root
 Can only push xylem sap up a little bit
 Guttation: escape of water droplets on leaves that
can be seen as morning dew
Pulled up by transpirational pull
 Much stronger force
 Transpiration: evaporation of water from leaves
 Causes tension (negative pressure) in the
 Due to the cohesion of water you can pull a
large amount of water up through the xylem
 Cohesion: binding together of similar
molecules, usually by hydrogen bonds
 For each molecule of water that is lost by
transpiration, another is gained at the roots
 Factors that can change rate of transpiration:
 Humidity
 Closing stomates
 Light Intensity
Controlling Transpiration
Guard cells control opening and closing of stomates
Cell walls of guard cells are not uniform in thickness
When guard cells become turgid they curve, causing the
stomates to open
 Due to radially oriented cellulose microfibrils
When guard cells become flaccid the stomata close
Things that cause stomates to open:
 Low CO2 levels in the leaf: happens when
photosynthesis begins
 Increase in potassium ions in guard cells: causes
water to diffuse into them, making them turgid
 Stimulating the Blue light receptor: causes proton
pumps in the guard cell’s membrane to turn on, which
causes uptake of potassium ions (see above)
Things that cause the stomates to close:
 Lack of water: causes guard cells to lose their turgor
 Hot Temperatures: starts cellular respiration, which
increases CO2 levels in the leaves (see above)
 Abscisic Acid: made in mesophyll cells due to
dehydration, signals guard cells to close stomates
Translocation of Phloem Sap
Translocation: transport of food (sugar) in a plant
Phloem sap travels from sugar source to sugar sink
Sugar source: plant organ in which sugar is being
Ex: Mature leaves
Sugar sink: plant organ that is a net consumer or
storer of sugar
Ex: Growing roots and fruit
Water and Mineral Absorption
Lateral movement: movement of water and nutrients
across a plant
Symplast: system of cytoplasm in cells connected by
 When water reaches the endodermis it can continue
to the xylem through the symplast
Apoplast: system of cell walls and intercellular
spaces that allows extracellular movement of water in
a plant
 When water reaches the endodermis it must pass
through the endodermis by diffusion
Helping Out the Roots
Mycorrhizae: Symbiotic structures constructed from
plants roots and filaments (hyphae) of a fungus
 These structures increase the amount of nutrients a
plant can absorb
 This helps older regions of roots that no longer have
root hairs, thus poor absorption
Rhizobium: Symbiotic bacteria nodules that live on
roots and fix nitrogen gas from air into a nitrogenous
form that the plant requires.