9.1: Transport in the xylem
of plants
Transpiration
The loss of water vapour from leaves
and other aerial parts of the plant.
90% of water absorbed by the
roots is lost by transpiration.
Structure of
the leaf
Note:
Waxy cuticle
Stoma
Guard cells
Spongy mesophyll
Stomata and guard cells
1. Why can stoma only be closed temporarily?
2. What causes the stoma to open and close?
3. Why do they change shape? What substances are
involved?
Structure of stem & root
Potometers
Complete the virtual transpiration lab.
The Xylem
The structure of the xylem leads to very efficient water
transport within the stem, maintaining structure even at very
low pressures. Features include:
-
Long continuous tube
Thickened walls
Walls embedded with lignin (strong polymer)
Non-living (flow of water is passive)
Cohesive properties of water molecules and adhesion
between the water molecules and the walls of the xylem
allow water to be transported efficiently.
Water loss
When water is evaporated from the leaf, adhesion causes water to
be drawn through the cell wall from the nearest available source
(xylem) to replace the lost water.
This evens applies at low pressure, the force of adhesion between the
cell wall and water is strong enough to suck water out of the xylem.
Consequently a pulling force is created by the water in the xylem –
Leaf, stem, roots. This is called transpiration-pull and is strong enough
to pull water up the stem against the pull of gravity.
Transpiration is a passive process, what factors affect the rate of
transpiration?
Active transport
Roots absorb water by osmosis
Mineral concentrations are usually higher in root
tissue compared to the soil. Therefore, protein pumps
in the plasma membrane of root cells active pumps
ions into the plant. There are separate pumps for
each mineral type.
Some plants, e.g. orchids, require minerals which bind
to soil particles so they have formed a relationship
with a fungus. The fungus grows around the root hairs
of the plant, absorbing minerals bound to soil
particles (phosphate), in return, the plant supplies
sugars and nutrients to the fungus.
Water conservation
Adaptations for water conservation include:
1. By being ephemeral – very short life cycle to be completed when water is
plentiful.
2. By being perennial – store water in specialized leaves/stems/roots,
consequently these plants live for a long time.
3. Marram grass (Ammonphila arenaria) has a rolled leaf (see slide 12), Stomata
are in pits, hairs on the inside of the roll. All of these adaptations are an
attempt to reduce water loss by evaporation.
4. Xerophytes are adapted to very dry environments. E.g. cacti.
- Very small leaves (spines)
- Pleats to allow stem to expand/contract
- Waxy cuticle covering stem
- Few stomata
- Stoma open at night
Finally….
Halophytes survive in very salty (saline) soils. Adaptations
include:
1. small, scaly leaves/spines
2. Leaves are shed when water is scarce
3. Water storage in specialized cells in leaf
4. Thick cuticle – layered epidermis
5. Sunken stomata
6. Long roots
7. Able to remove excess salt with specialized cells.