Lesson 2: Transport in Plants: Xylem Flashcards
Transport in the Xylem -
Transpiration 1
The xylem is involved in the transport of
water.
Water is absorbed in the roots, travels up
the stem in the xylem vessels in vascular
bundles to the leaves.
The loss of water vapour from the leaves
and stems of plants is known as
transpiration.
Transpiration also carries minerals
throughout the plants and can cool the
plant in hot climates.
Water is lost due to evaporation through
the stomata, which allow for gas exchange
in the leaf.
Transport in the Xylem -
Transpiration 2
Most plants live in areas where
the humidity is lower in the
environment than in the leaves.
During the day water vapour
leaves the air spaces in the
mesophyll through open
stomata in the lower
epidermis of the leaf and
stem.
The evaporating water is
drawn from the xylem in the
vascular bundles.
The vascular bundles are
continuous with the roots so
a column of water is formed
connecting the roots, stems,
leaves and air spaces. This is
known as a transpiration
stream.
Factors Affecting Transpiration 1
Environmental factors can affect the rate of transpiration in plants.
Light – controls the opening and closing of the stomata. As light increases, stomata open, speeding up transpiration.
Factors Affecting Transpiration 2
Temperature – as the temperature rises, the rate of transpiration also
increases as water evaporates from the air spaces in the spongy
mesophyll and diffuses out the stomata
Factors Affecting Transpiration 3
- Atmospheric Humidity – increases in atmospheric humidity causes a
decrease in transpiration due to the change in concentration gradient
between the airspaces in the spongy mesophyll and the atmosphere.
Factors Affecting Transpiration 4
- Wind Current – increase in wind currents causes an increase in
transpiration due to the removal of the water saturated air near the stomata.
Stomata 1
Transpiration is controlled by the pair of guard cells surrounding each
stomata.
The guard cells change shape to open and close the stomata.
Guard cells open when they gain potassium ions (K+) and water from the
surrounding cells. The cells actively transport potassium in and water
follows by osmosis.
The guard cells have unevenly thick cell walls. The cell walls closer to the
stomata are thicker and the cellulose molecules are oriented such that
when the cell swells (becomes turgid), it bows away from the stomata
creating a larger gap.
Stomata 2
Stomata are generally open during the day and closed at night.
During the day, when light is present, CO2 can be taken in through the
stomata to continue photosynthesis. At night there is no photosynthesis,
the stomata close to preserve water.
Potassium ion movement can be controlled by a hormone called abscisic
acid produced in the roots during drought.
When abscisic acid is present, potassium ions leave the guard cells, water
follows by osmosis and the stomata close.
Cohesion - Tension
The movement of water through the xylem can be
explained through the cohesion – tension theory.
Loss of water vapour in the leaves results in
negative pressure (tension) in the vascular bundles.
Water vapour enters the air spaces in the mesophyll
(parenchyma) to replace the water lost through the
stomata.
Water is drawn up the xylem continuously due to
cohesive forces between the water molecules and
adhesive forces between the water molecule and
the xylem vessel walls.
Water is drawn in from the cortex in the roots to
replace water that is lost in transpiration
The tension caused by transpiration causes water to
be drawn into the roots from the soil.
Roots
The roots of a plant are
responsible for absorbing
water and mineral ions from
the soil.
Surface area is increased
through extensive branching
of the root system and the
development of root hairs.
Mineral Ions
Minerals are found in the soil as salts.
Salts dissolve in the soil water and can move into root cells.
Plants require minerals to make a variety of substances:
Calcium – required for cell walls
Magnesium – used to make chlorophyll
Iron – a cofactor in enzymes
Mineral Ions Enter the Root in Different Ways
Passive Transport
Mass Flow – dissolved minerals move into the root along with the water
Facilitated Diffusion – from the soil into the root hairs, down their
concentration gradient.
Active Transport
When concentration is lower in the soil than the plant. Potassium, nitrate
and phosphate are commonly moved into root hairs by active transport.
Mutualistic Relationships
Close association with a fungus (mycorrhiza), minerals can pass directly from
the fungal hyphae to the root cells.
