Transport
in Plants
Two
types
Short-Distance
Transport
Long-Distance
Transport
Short-Distance
Transport
Cellular
Cell
to cell
Involves
osmosis, diffusion, active transport
Long-Distance
Transport
One
plant part to another (leaf to root)
Transport
of sap in xylem and phloem
Involves
bulk flow
Bulk
flow
Definition: Bulk
flow is the movement of a fluid driven by pressure
Diffusion,
osmosis and active transport at the cellular level create pressures
Four
Transport Functions
Absorption
of water and minerals by roots
Transport
of xylem sap (transport of water and minerals by bulk flow in the hollow
[dead] pipes of the xylem)
Control
of transpiration (loss of water by evaporation from the stomata of leaves)
Translocation
of phloem sap (the transport of sugars by bulk flow from the leaves to
other parts of the plant)
Overview
of Water and Mineral Transport
5steps
Entering
the root
Crossing
the cortex
Crossing
the selective barrier at the endodermis (enter the stele )
Entering
the xylem
Flowing
up the xylem
Absorption
of Water and Minerals by Roots
Entering the Root
Travel
by two pathways
Symplast:
the cytoplasm within cells (inside cell membrane)
Apoplast:
spaces between cells (includes cell walls)
Symplast
is living: Apoplast is nonliving
Absorption
of Water and Minerals by Roots
Entering the Symplast of Root
Root
hairs, mycorrhizae, epidermal cell membrane
Root
hairs at the growing tips of root (the primary root)
Water
enters by osmosis
Minerals
by diffusion or active transport
Absorption
of Water and Minerals by Roots
Entering the Apoplast of Root
Most
common route
Cell
walls are hydrophilic (sucks water in)
Water
and dissolved minerals enter together
Soak
into space between epidermal cells into cortex apoplast
Absorption
of Water and Minerals by Roots
Crossing the Cortex
The Apoplastic route
Water
and minerals flow together as a solution
Sucked
along by the hydrophilic cell walls
Continuous
film from soil water to stele
Cannot
enter stele by apoplast
Absorption
of Water and Minerals by Roots
Crossing the Cortex
the Symplastic route
Cell
to cell by plasmodesmata
Cytoplasmic
extensions joining cells
Continuous
pathway of cytoplasm
Can
enter symplast anywhere in cortex from apoplast
Large
surface area for entry to cortex
Selective
uptake of minerals entering the symplast
Water
follows by osmosis
Absorption
of Water and Minerals by Roots
Crossing the Endodermis
Endodermis
is barrier for apoplastic water and minerals
Prevents
leakage from the stele
Space
between cells of epidermis is blocked
Casparian
strip, a belt of suberin (a waxy material)
All
must enter via the symplast
Selective
uptake of minerals occurs
(
e.g. K+ is taken up Na+ is not)
20.1.3
Absorption of Water and Minerals by Roots: Entering the Xylem
-the
xylem which consists of dead cells ( lacking cytoplasm and cell wall= no
protoplast) are part of the apoplast
-hence
the last step is to leave the symplast of the endodermis to enter the apoplast
of the xylem
-minerals
are transferred out of the symplast of the endodermis and living parenchyma
of the stele by both diffusion and active transport creating a hypertonic
solution in the apoplast which pulls the water into the apoplast by osmosis
-water
(carrying dissolved minerals) in the apoplast now freely enters the vessel
elements and tracheids of the xylem