Chapter 29
Phloem and Stomata
You Must Know
• How phloem sap moves through plants.
• How and why stomata open and close.
Concept 29.7: Sugars are transported from
sources to sinks via the phloem
• The translocation of
phloem sap through sieve
tubes by bulk flow is driven
by positive pressure.
© 2014 Pearson Education, Inc.
Phloem
• The products of
photosynthesis are
transported through
phloem by the process of
translocation.
Animation: Phloem Translocation Summer
Right click slide / Select play
Animation: Phloem Translocation Spring
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• Bulk flow differs from diffusion
– It is driven by differences in pressure potential,
not solute potential.
– It moves the entire solution, not just water or
solutes.
– It is much faster.
© 2014 Pearson Education, Inc.
Mesophyll cells have lots of chloroplasts
Apoplast
Symplast
Mesophyll cell
Sieve-tube
Element
(part of the
phloem)
High H+ concentration
H+
Proton
pump
S
H+
Mesophyll
cell
Bundlesheath cell
Phloem
parenchyma cell
Loading of sucrose into phloem
Cotransporter
H+
Low H+ concentration
Sucrose
S
(b) A chemiosmotic mechanism is
responsible for the active transport of
sucrose.
Figure 29.22
Vessel
(xylem)
Sieve Source cell
tube
(leaf)
1 Loading of sugar
(phloem)
Sucrose
H2O
2
Bulk flow by positive pressure
Bulk flow by negative pressure
1
2 Uptake of water
3 Unloading of sugar
Sink cell
(storage
root)
3
4
H2O
4 Recycling of water
Sucrose
Stomata: Major Pathways for Water Loss
About 95% of the water a plant loses escapes through stomata.
Guard cell
Figure 29.19a
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
(a) Changes in guard cell shape and stomatal opening and
closing (surface view)
EXTRACELLULAR
FLUID
H+
Hydrogen
ion
H+
H+
CYTOPLASM
H+
H+ Proton
pump
H+
H+
H+
(a) H+ and membrane potential
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
H2O
H2O
H2O
H2O
K+
H2O
H 2O
H2O
H2O
H2O
H2O
(b) Role of potassium ions (K+) in stomatal opening and closing
Stimuli for Stomatal Opening and Closing
• Generally, stomata open during the day and close
at night to minimize water loss.
• Stomatal opening at dawn is triggered by
– Light
– CO2 depletion
– An internal “clock” in guard cells
• All eukaryotic organisms have internal clocks;
circadian rhythms are 24-hour cycles