Transcript Long-Distance Transport in Plants

Long-Distance Transport
in Plants
Biology 1001
November 21, 2005
1. Introduction
 Roots absorb water and minerals from the soil,
and shoots absorb light and carbon dioxide for
photosynthesis
 Vascular plants use xylem to transport water
and minerals from roots to shoots and phloem
to transport sugars from where they are
produced or stored to where they are needed
for growth and metabolism
 Long-distance transport occurs by bulk flow,
the pressure-driven movement of a fluid
 We consider four transport functions in plants
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Absorption of water and minerals by roots
Ascent of water and minerals in the xylem
The control of transpiration
Translocation of sugars in the phloem
Figure 36.2!
2. Absorption of Water and Minerals by Roots
 Short-distance or lateral transport from the soil, through the
epidermis and cortex, to the xylem
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Precedes ascent via bulk flow up the tracheids & vessels of the xylem
 There are two routes of cell-to-cell transport in plants: a living
route called the symplast through the protoplasts, and a nonliving route called the apoplast through the cell walls and
intercellular spaces
Lateral Transport of Minerals and Water in Roots –
Figure 36.9!,!!
Lateral Transport in Roots
 Water & minerals in soil solution must first cross the epidermis
of the root, usually near its tip
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Root hairs greatly increase the surface area for uptake of soil solution
Uptake of soil solution by the hydrophilic walls of epidermal cells
provide entry into the apoplast
Soil solution travels via the apoplast towards the stele, where it can
divert into the symplast at any time by crossing plasma membranes of
cortical cells
 Cortical cells increase the surface area for exposure to soil solution
and concentration of minerals in the symplast
Alternatively, some soil solution crosses the plasma membrane and
directly enter the symplast at the root hair
Mycorrhizae also greatly increase the surface area of roots exposed to
soil solution
The Role of the Endodermis
 All materials travelling through the cortex of the root must enter the xylem
before being transmitted to the rest of the plant
 The endodermis, the innermost layer of cells in the root cortex, acts as a
checkpoint for selective passage of minerals from cortex to vascular tissue
 Minerals in the symplast continue through the plasmodesmata of the
endodermal cells into the vascular tissue
 Minerals that reach the endodermis via the apoplast encounter a waxy
suberin belt called the Casparian Strip which forces them to cross a
plasma membrane before entering the endodermis
 In this way all minerals passing into the vascular tissue are screened by a
selectively permeable plasma membrane
 To enter the xylem water & minerals re-enter the apoplast, as xylem cells lack
protoplasts
3. The Ascent of Water & Minerals in the Xylem
 Water & minerals ascend from roots to shoots through the xylem
 The mechanism is a type of bulk flow caused by transpiration,
the loss of water vapour from the leaves and other aerial parts of
the plant
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Causes xylem sap to rise as much as 100 m in certain trees
A single tree can transpire 200L of water per hour in the summer
 Transpiration generates a negative pressure (tension) from above
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Best explained by the Transpiration-Cohesion-Tension Mechanism
 A small amount of xylem sap is pushed upward during the night
by root pressure, resulting in guttation
The Transpiration-Cohesion-Tension Mechanism
 The Transpiration-Cohesion-Tension Mechanism starts with
transpiration pull generated in a leaf, Figure 36.12!
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Water vapour in the air spaces between mesophyll cells diffuses
through the stomata because the air outside the leaf is drier
This water is replaced from the water lining the surface of mesophyll
cells in the air spaces, creating surface tension (negative pressure)
This tension pulls water from the xylem – relies on adhesion of water
molecules to hydrophilic cell wall components and cohesion to each
other
Cohesion and Adhesion in the Ascent of Xylem Sap
 The transpirational pull generated in the leaf
is transmitted along the water column in the
xylem, all the way to the roots and into the
soil solution, by cohesive and adhesive
forces
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Cohesion is the attraction of water molecules
to each other by hydrogen bonding
Adhesion is the adherence of water
molecules to the hydrophilic components of
cells such as xylem
 Note that the bulk flow caused by the
negative pressure of transpiration moves the
water and the minerals together up the
xylem, from the roots to the shoots
 Xylem sap ascent is ultimately solar
powered!
Figure 36.13!