3/18/2015
Cellular water relations and phloem transport
(Stern Chapter 9)
- Diffusion, turgor pressure, and osmosis
- Phloem transport
- Water potential
Major learning goals:
Understand what you need for osmosis, turgor P
Understand concept of water potential
Understand how phloem transport works
Diffusion, osmosis, turgor pressure
A water strider. What does this tell us about water?
Photo: B. Pratt
Diffusion, osmosis, turgor pressure
Osmosis
-Movement of water through a semipermeable membrane
(where is the semipermeable membrane in the cell?)
Diffusion
-Osmosis can generate turgor pressure (cell on left)
Figure from Stern
textbook
Time
Diffusion, osmosis, turgor pressure
Aquaporins
Turgor pressure provides structural support to
herbaceous (non-woody) plants
Water
Aquaporins
Before
After watering
Semi permeable membrane
Illustration by J. Laur
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3/18/2015
Water potential ()
Water flow through cell membranes is facilitated by
aquaporins.
Outside of cell
Water molecules
 determines the direction of w. movement in living systems;
Water moves from less negative to more negative water
potentials
 = zero for pure water in an open container at sea level and
room temperature (by definition)
Plasma
membrane
Illustration: U. Hacke
Cytoplasm
Aquaporin
Short movie
http://www.youtube.com/watch?v=7EGPtMqZ7pY
Water potential ()
Water potential ()
 determines the direction of w. movement in living systems;
Water moves from less negative to more negative water
potentials
Expressed in units of pressure, specifically Megapascal (MPa)
 = zero for pure water in an open container at sea level and
room temperature (by definition)
Seawater at 20°C has solute potential of ~ -2.5 MPa
 is determined by solute concentration and pressure
 = S + P
0.1 MPa = 1 bar = 14.7 PSI
Now let’s look at some examples.
pressure potential, can be positive or negative
solute potential, depends on concentration of
solutes and is either zero (no solutes) or a
negative number
Water potential ()
Pure water
Water potential ()
S = 0 MPa (no solutes)
S = -2.5 MPa (salt ions)
P = 0 MPa (no external
pressure other than
atmospheric pressure)
P = 0 MPa (no external
pressure other than
atmospheric pressure)
What’s the ?
Salt solution
What’s the ?
(Remember:  = S + P)
Case #1
Case #2
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3/18/2015
Water potential ()
Phloem transport
What may saline soil mean for water uptake?
What about mangrove plants?
Figure from Stern
textbook
• In terms of sugar
production, what
organ is producing the
most?
• What organ(s) are
producing few or little
sugars?
• The phloem transports
sugars to non/lowproducing areas
Figure from a textbook
http://prairieecologist.com/2011/11/09/
salt-marshes-in-nebraska/
Phloem transport
Sieve tube
element
Phloem transport
• Another example of the importance of
turgor pressure
• Pressure flow hypothesis (Münch,
1930) states that phloem sap (water +
sugar) is moved by a P gradient
Figure from a
textbook
• Maintained by loading at source and
unloading at sink.
A = source
B = sink
Companion
cell
Phloem transport
Image from E. Münch (1927) as shown in Knoblauch & Peters (2010) PCE
Can you explain this illustration?
Phloem transport
• The direction of transport may
change
• The phloem can also be used like a nervous system
in animals, propagating electrical impulses
Mimosa pudica – sensitive plant
Figure from Stern
textbook
http://www.potomitan.info/photo/mimosa_pudica2.jpg
https://www.youtube.com/w
atch?v=GZIGRvoENvw
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