Transcript Xylem

Review:
• Three regions
– Meristematic
– Elongation
– Maturation
Root Anatomy
• Root hairs and
branching greatly
increase the
surface through
which water and
dissolved minerals
can be absorbed.
Functions of Roots
• Absorption TODAY
• Anchorage
• Storage
• Conduction of water
• Hormone synthesis
TOMORROW
Uptake of Ions by the Roots – WHAT?
Root hairs bring
nitrates,
phosphates,
sulphates, and a
variety of other
essential inorganic
nutrients into the
plant
Why are these
nutrients
needed?
Uptake of Ions by the Roots – HOW?
• Root interception
– Root grows and intercepts ions
• Simple diffusion
– Ions move down their concentration gradient
– No energy expense by plant
– Example: K+
• Mass flow
– Bulk flow of water into the root “carries” ions to root
– Delivers N, Ca, Mg, S
• Active transport
– Ions move against their concentration gradient
– Requires a specific protein “pump” in the cell membrane
– Energy expense by plant
Water Transport into the Roots
Water flows from
Epidermis
Root Cortex
Endodermis
Xylem
SOIL
via osmosis
Water Transport into the Roots
• Cells are usually
interconnected via
plasmodesmata.
• Their plasma forms a
continuum (a
“symplast”).
Water Transport into the Roots
• The transport of substances from cell to cell via
plasmodesmata is called symplastic
transport.
Water Transport into the Roots
•
Apoplastic transport is when water is transported between
cells through the extra-cellular space.
Water Transport into the Roots
•
The endodermis
functions as a last
checkpoint for
the passage of
dissolved minerals
into the vascular
tissue
Water Transport into the Roots
•
Minerals already in the symplast when they reach the
endodermis continue through the endodermis and
pass into the xylem
Water Transport into the Roots
•
Minerals that reach the
endodermis via the
apoplast encounter a
waxy layer on the
endodermis that blocks
their entry into the
xylem called the
Casparian strip.
Water Transport into the Roots
•
The only way to get through the endodermis is
through the symplastic route.
Water Transport into the Roots
•
Water and minerals
following the
apoplastic route
must first cross the
plasma membrane
of the endodermis
cells and enter the
symplast system.
This ensures that no minerals
can enter the vascular system
with out first being “checked” by
the plasma membrane
Water Transport into the Roots
•
Once through
the endodermis
layer, the water
and minerals are
now free to
enter the xylem
tracheids and
vessels.
The Casparian Strip blocks
Uptake of water through
passage of water unless it
root epidermis by osmosis
is moving via the symplast
Water that does not cross the
Water
that crosses the
root hair plasma
membrane
hair plasma
enters the root
apoplast
system membrane
enters the symplast system
Water moves into the
xylem for transport
up the plant