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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
cells
 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
xylem
 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
produced


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
plasmodesmata
 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.
Mycorrhizae
Rhizobium