1. Chapter 29
Phloem and Stomata

2. You Must Know How phloem sap moves through plants.
How and why stomata open and close.

3. Concept 29.7: Sugars are transported from sources to sinks via the phloem
The products of photosynthesis are transported through phloem by the process of translocation.
The translocation of phloem sap through sieve tubes by bulk flow is driven by positive pressure.
Phloem
In angiosperms, sieve-tube elements are the conduits for translocation.
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4. Bulk flow, the movement of a fluid driven by pressure.
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.
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5. Loading of sucrose into phloem
Mesophyll cells have lots of chloroplasts
Apoplast
Symplast
Mesophyll cell
Mesophyll
cell
Phloem
parenchyma cell
Sieve-tube
Element
(part of the
phloem)
(b) A chemiosmotic mechanism is
responsible for the active transport of
sucrose.
High H+ concentration
Low H+ concentration
Proton
pump
Cotransporter
Sucrose S H+
Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube elements. In some species, sucrose exists the symplast near sieve tubes and travels through the apoplast (red arrows). It is then actively accumulated from the apoplast by sieve-tube elements and their companion cells.
A storage organ can be both a sugar sink in summer and sugar source in winter.
Sugar must be loaded into sieve-tube elements before being exported to sinks.
Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways.
Companion cells enhance solute movement between the apoplast and symplast.
In most plants, phloem loading requires active transport.
Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose.
At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water.
Loading of sucrose into phloem 5

6. Sieve tube (phloem) Vessel (xylem)
Figure 29.22
Sieve
tube
(phloem)
Bulk flow by positive pressure 1 2
Loading of sugar
H2O
Sucrose
Source cell
(leaf) 1
Vessel
(xylem) 2
Uptake of water
Bulk flow by negative pressure
Unloading of sugar
Sink cell
(storage
root)
Sucrose 3
Figure Bulk flow by positive pressure (pressure flow) in a sieve tube
Recycling of water
H2O
Sucrose 4 3 6

8. Stomata: Major Pathways for Water Loss
About 95% of the water a plant loses escapes through stomata.
Guard cell
Leaves generally have broad surface areas and high surface-to-volume ratios.
These characteristics increase photosynthesis and increase water loss through stomata.
Guard cells help balance water conservation with gas exchange for photosynthesis.
Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape.
Stomatal density is under genetic and environmental control. 8

9. cellulose microfibrils
Figure 29.19a
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Guard cells turgid/Stoma open
Guard cells flaccid/Stoma closed
Changes in turgor pressure open and close stomata .
When turgid, guard cells bow outward and the pore between them opens.
When flaccid, guard cells become less bowed and the pore closes.
The radial orientation of cellulose microfibrils in the cell walls cause the guard cells to increase more in length than width when turgor increases. Since the two guard cells are tightly joined at their tips, they bow outward when turgid, causing the stomatal pore to open.
(a) Changes in guard cell shape and stomatal opening and
closing (surface view) 9

10. Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
CYTOPLASM
EXTRACELLULAR
FLUID
Proton
pump
Hydrogen
ion
(a) H+ and membrane potential H+
Guard cells turgid/Stoma open
Guard cells flaccid/Stoma closed
H2O
H2O
H2O
H2O
H2O K+ K+
H2O
Changes in turgor pressure result primarily from the reversible uptake and loss of potassium ions (K) by the guard cells. Proton pump actively pumps hydrogen ions out of the guard cell. The resulting voltage (membrane potential) drives K+ into the cell through specific membrane channels. The absorption of K+ causes the water potential to become more negative within the guard cells, and the cells become more turgid as water enters by osmosis. Stomatal closing results from a loss of K+ from guard cells to neighboring cells, which leads to an osmotic loss of water.
H2O
H2O
H2O
H2O
(b) Role of potassium ions (K+) in stomatal opening and closing 10

11. 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 11