Transport in
Plants
AP Biology
2006-2007
General Transport in plants
 H2O & minerals


transport in xylem
transpiration
 evaporation, adhesion & cohesion
 negative pressure
 Gas exchange

photosynthesis
 CO2 in; O2 out
 stomates

respiration
 O2 in; CO2 out
 roots exchange gases within air spaces in soil
 Sugars


transport in phloem
bulk flow
 Calvin cycle in leaves loads sucrose into phloem
 positive pressure
AP Biology
Why does
over-watering
kill a plant?
Start with Water
 Review of the structure of water
AP Biology
Review of Water Potential
  = s + p
AP Biology
Water and minerals must be absorbed
through the roots first before it can
travel in xylem
 Water: Root hairs increase surface
area. Water is small enough to diffuse
through membrane. Transport proteins
called aquaporins also aid in this
AP Biology
Mineral absorption
Minerals cannot pass through the membrane
because they are charged
 Proton pumps

active transport of H+ ions out of cell
 chemiosmosis
 H+ gradient

creates membrane
potential
 difference in charge
 drives cation uptake

creates gradient
 cotransport of other
solutes against their
gradient
AP Biology
Water flow through root
 Porous cell wall



water can flow through cell wall route & not enter cells = apoplast
It can also flow through cells but then must pass through the cell
membrane and plasmodesmata = symplast
However once water reaches endodermis (Casparain strip) it must
enter symplastic route (i.e. go into to cells) before it can enter the
xylem
Casparian strip
AP Biology
Controlling the route of water in root
 Endodermis



cell layer surrounding vascular cylinder of root
lined with impermeable Casparian strip
forces fluid through selective cell membrane
 filtered & forced into xylem cells
Aaaah…
Structure–Function
yet again!
AP Biology
Water is now in the xylem!!! How do we
fight gravity and get to the leaves!
 BULK FLOW! The long-distance

transfer of fluids by pressure
Water is pulled through the xylem by
negative pressure driven by
transpiration (the loss of water at
stomata) at the leaves
AP Biology
Transpiration
 Air outside the leaf is drier (has lower
water potential) then the air inside the
leaf. Therefore water vapor exits the
lead via the stomata.
 The vaporization
of water creates
negative pressure
(tension)
AP Biology
Ascent of xylem fluid
Transpiration pull is generated by leaf as water
exits through the stomata (negative tension is
created). To replace the water that is lost water
from the xylem is pulled into air spaces
AP Biology
In SUM
 The negative pressure

potential created by
transpiration lowers the
water potential.
Therefore water moves
up the xylem from an
area of high water
potential to an area of
low water potential
AP Biology
The structure of water and xylem also
helps this
STRUCTURE OF WATER
 Cohesion: Water molecules help pull each
other up
 Adhesion: Attraction of xylem walls helps
offset the force of gravity
STRUCTURE OF XYLEM
 Since xylem is composed of dead/empty cells
water does not have to pass the cell membrane
 Walls of xylem can adhere to water
AP Biology
How can transpiration be stopped?
Epidermal cell
Guard cell
 Uptake of K+ ions
by guard cells



proton pumps
water enters by
osmosis
guard cells
become turgid
H2O
K+
H2O
K+
 Loss of K+ ions
Nucleus
Chloroplasts
H2O
K+
H2O
K+
K+
H2O
K+
H2O
K+
H2O
K+
H2O
Thickened inner
cell wall (rigid)
by guard cells


AP Biology
water leaves by
osmosis
H2O
K+
guard cells
become flaccid
H2O
K+
H2O
K+
H2O
K+
Stoma open
Stoma closed
water moves
into guard cells
water moves out
of guard cells
Control of transpiration
 Balancing stomate function

always a compromise between
photosynthesis & transpiration
 leaf may transpire more than its weight in
water in a day…this loss must be balanced
with plant’s need for CO2 for photosynthesis
AP Biology
How is K+ pumped into guard cells?
 Light stimulates guard cells to activate
protein pumps which help with the
cotransport of K+ into cells.
 Lack of CO2 opens stomata
 Lack of water in environment

Abscisic acid “tells” stomata to close in
response to lack of water
RECALL CAM PLANTS!!!!!!!!!!!!
AP Biology
Transport of sugars in phloem
 Flow from source (where sugar is made) to sink

(where sugar is needed)
Loading of sucrose into phloem
 flow through cells via plasmodesmata
 proton pumps
 cotransport of sucrose into cells down proton
gradient
AP Biology
Pressure flow in phloem
 Mass flow hypothesis

“source to sink” flow
 direction of transport in phloem is
dependent on plant’s needs

phloem loading
 active transport of sucrose
into phloem
 increased sucrose concentration
decreases H2O potential

water flows in from xylem cells
 increase in pressure due to
increase in H2O causes flow
which in turn pushes sugar
through phloem
AP Biology
can flow
1m/hr
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AP Biology
2006-2007