Transcript Plant Science

Plant Science
9.2 Transport in angiospermophytes
Root system
 The root system of a plant must supply
sufficient water and mineral ions.
 For this reason it has developed a large
surface area due to branching.
 In addition, there are tiny root hairs that
further increase the surface area.
Root system
Root hair cell structure
Ways in which mineral
ions move into roots
 Diffusion – if the concentration of a mineral in
the soil solution is greater than its
concentration in a root hair cell, then the
mineral may enter the root hair cell by diffusion.
 Mass flow of water – some mineral ions enter
the root along with water. They may then be
carried along in solution to the cytoplasm and
eventually xylem vessels.
Ways in which mineral ions
move into roots contd.
 Fungal hyphae - A mycorrhiza is a mutualistic
relationship between a fungus and a plant root.
 The fungus functions like a root by growing into
the soil and absorbing nutrients for the plant.
The plant provides the fungus with products of
photosynthesis (sugar).
 Many plants do not do well or do not grow at all
without the fungi. Approximately ninety percent
of all plants develop mycorrhizae.
Mineral ion transport
 If the concentration of a mineral in the
soil solution is less than that in a root hair
cell, it may be absorbed by ACTIVE
TRANSPORT.
 Most minerals are absorbed in this way.
 Because active absorption requires
energy, the rate depends on respiration.
Support in terrestrial
plants
 Thickened cellulose – cellulose is made
from microfibrils and polysaccharides that
lend mechanical strength.
 Cell turgor – cells that are full of water,
with the cytoplasm pushing outwards on
the cell wall, are firm and lend support.
 Lignified xylem – the walls of xylem
vessels are strengthened with lignin.
Transpiration
 Transpiration is the loss of water vapour
from the leaves and stems of plants.
 Transpiration causes a flow of water from
the roots, through stems to the leaves.
This is called the TRANSPIRATION
STREAM.
Transpiration stream
 When explaining how water is carried
upwards in the transpiration stream, the
following factors must be considered:
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Structure of xylem vessels
Transpiration pull
Cohesion
Adhesion
Evaporation
Structure of xylem
vessels
 Function for water transport and support.
 Consist of vessels and tracheids, both dead
tissue.
 Made of long cells joined end to end to allow
water to flow in a continuous column.
 The end walls have broken down to give an
uninterrupted flow of water.
 Lignified walls prevents collapse under large
tension forces.
 Very narrow to increase capillary action.
Evaporation and
Transpiration pull
 Water evaporates from the cells of the
spongy mesophyll layer of the leaf.
 The water that evaporates is replaced by
water from the xylem vessels in the leaf.
 Low pressure, or suction, is created
inside xylem vessels when water is
pulled out.
 This is called TRANSPIRATION PULL.
Cohesion and adhesion
 The transmission of the transpiration pull
through xylem vessels depends on the
cohesion of water molecules, due to Hbonding.
 Adhesive forces exist between the water
molecules and the walls of the xylem
vessels. This is known as capillarity.
 Transport of water in xylem is PASSIVE.
Transpiration contd.
Measuring transpiration
Guard cells
 Guard cells can regulate transpiration
by opening and closing stomata.
 The hormone abscisic acid causes the
closing of stomata. (eg when water is
scarce)
Guard cells
Factors affecting rate of
transpiration
 Light – guard cells close the stomata at
night so transpiration is much greater
during the day.
 Temperature – heat is required for
evaporation of water from the surface of
cells, so as temperature rises, so does
rate of transpiration. Higher temperatures
also increase rate of diffusion and reduce
relative humidity of the air outside the
leaf.
Factors affecting
transpiration contd.
 Humidity – water diffuses out of the leaf
when there is a concentration gradient
between the air spaces within the leaf
and the air outside. The lower the
humidity outside the leaf, the steeper the
gradient and therefore the faster the rate
of transpiration.
Factors affecting
transpiration contd.
 Wind – when air is still, pockets of
saturated air form near the stomata,
reducing transpiration. Wind blows away
the saturated air, thus increasing
transpiration rate.
Adaptations of xerophytes
 Plants adapted to grow in very dry
habitats are called XEROPHYTES. Eg.
giant cactus (Cereus giganteus).
 Surface area of leaves reduced to spines
to limit transpiration.
 Very wide-spreading network of shallow
roots to maximise water absorption after
rains.
Adaptations of xerophytes
 Thick waxy cuticle covering stem.
 Thick stems containing water storage tissue.
 Vertical stems to absorb sunlight early and late
in the day, but not at midday when sunlight is
most intense.
 CAM physiology, whereby the stomata open
during cooler nights instead of the intense heat
of the day.
More adaptations….
 Deep roots to access lower water levels.
 Rolled leaves to trap humid air near
stomata.
 Reduced numbers of stomata.
 Stomata sunken in pits surrounded by
hairs to trap saturated air thus lowering
transpiration rate.
Translocation
 Translocation is the movement of
substances around the plant in the
phloem tubes. This could be sugars
produced in photosynthesis, amino acids,
or chemicals from pesticides.
 Translocation occurs through sieve tubes
and is an ACTIVE process requiring ATP.
Translocation contd.