Download AS plant transport - Skinners` School Science

Transport in Plants
• Explain the need for transport systems in
multicellular plants in terms of size and surfacearea-to-volume ratio.
• Describe the distribution of xylem and phloem
tissues in roots, stems and leaves of
dicotyledonous plants.
Transport systems and surface area to
volume ratio
• Single celled organisms have large
surface area: vol ratio and can obtain
all their requirements (oxygen, CO2,
water and minerals) by diffusion. The
sugar they make is available
throughout the cell.
• Large plants need to transport water
and minerals up from the roots.
Sugars need to be moved from the
leaves where they are made to other
areas of the plant. In order to do this
they need transport systems
Tissues that carry out movement of
water, minerals and sugars
• A Tissue is group of similar
cells that carry out a
particular function
• Xylem tissue is made of
Xylem vessels, fibres and
parenchyma cells. Xylem
transports water and
minerals.
• Phloem tissue is made of
Sieve tubes and companion
cells. Phloem transports
sugars.
© Pearson Education Ltd 2008
This document may have been altered from the original
Distribution of vascular tissue in
plant roots
• Xylem forms a cross shape in the centre with areas of phloem
between the arms
• The vascular (transport) tissue is surrounded by a layer of endodermis
• The Pericycle is a layer of meristem (dividing) cells inside the
endodermis
Distribution of vascular bundles
in stem
• In the stem the vascular
bundles of Xylem, Cambium
and Phloem are arranged
around the edge of the stem
• Xylem is always on the inside
of the bundle,
• Cambium is a layer of
meristem cells in the middle
of the bundle
• Phloem is always on the
outside of the bundle
Distribution of vascular tissue in leaves
• In leaves the vascular
tissue is seen as the
midrib and veins in
the leaf
• The xylem is always
closer to the top
surface of the leaf
• The phloem is below
the xylem
Describe the structure and function
of xylem vessels, sieve tube
elements and companion cells.
Xylem tissue
Week 10
Phloem (longitudinal section)
© Pearson Education Ltd 2008
This document may have been altered from the original
Explain, in terms of water potential, the movement
of water between plant cells, and between plant
cells and their environment.
• Pure water has a water potential of zero
• Any solute dissolved in water lowers the
water potential and makes it more
negative
• Water always moves from an area of
higher water potential to an area of lower
water potential
Movement of water into plant cells
• This cell has a water potential of
-500kPa
• It is bathed in pure water with a water
potential of 0kPa
• So water enters the cell down the water
potential gradient (from higher to lower)
by osmosis
• Eventually the pressure exerted by the
water entering the cell equals the
pressure exerted by the cell wall on the
contents, water stops going into the cell
• The cell becomes turgid
A pair of adjacent cells, A and B, have water potentials of -1000kPa and
1200kPa respectively. Which cell will gain water from the other
Movement of water out of plant cells
• If a plant cell is put into a
solution with a much lower
water potential than the cell
then water will leave the cell
by osmosis.
• The vacuole shrinks and then
the cytoplasm shrinks and
becomes smaller in volume
What is in the space between
the cell wall and the plasma
membrane of the plasmolysed
cell?
• Eventually the cell membrane
pulls away from the cell wall
• The cell becomes
plasmolysed.
Movement of water between plant cells
• If adjacent cells have
different water
potentials then water
will move between
them, by osmosis,
down the water
potential gradient
© Pearson Education Ltd 2008
This document may have been altered from the original
Complete the question on
water potential
Movement of water through plants
• A, the Apoplast pathway. Water and
dissolved ions move through the cell
walls between the cellulose molecules
• B, the Symplast pathway. Water goes
into the cell through the plasma
membrane into the cytoplasm. It moves
from cell to cell through the
plasmodesmata
• C, the Vacuolar pathway. Water goes
into the cell through the plasma
membrane into the cytoplasm and then
into the vacuole.
• Describe, with the aid of diagrams, the pathway by which water is
transported from the root cortex to the air surrounding the leaves, with
reference to the Casparian strip, apoplast pathway, symplast pathway,
xylem and stomata.
• Explain the mechanism by which water is transported from the root cortex to
the air surrounding the leaves, with reference to adhesion, cohesion and the
transpiration stream.
© Pearson Education Ltd 2008
This document may have been altered from the original
• Describe, with the aid of diagrams, the
pathway by which water is transported
from the root cortex to the air surrounding
the leaves, with reference to:
the Casparian strip,
apoplast pathway
symplast pathway,
xylem and stomata.
Water movement into the plant
• Water goes into the root cells and
moves across the cortex by osmosis,
water can move by any of the
pathways
• At the Casparian strip in the
Endodermis water is forced out of the
Apoplast pathway and into symplast
or vacuolar pathway
• Water and ions pass through proteins
in the plasma membrane into the
cytoplasm
• Nitrate ions are actively pumped from
the endodermis cells into the xylem
• This lowers the water potential in the
xylem so water follows by osmosis
Movement up the stem and out
of the leaves
• Pumping ions into the xylem forces water to
follow by osmosis
• Water can rise up stems about 3 metres by
this process
• Water evaporates from leaves through the
stomata, water moves through the leaf by
osmosis down the water potential gradient
• Water leaves the xylem creating tension in
the xylem, this is why the xylem needs to be
strengthened with lignin, to prevent
collapse.
• Cohesion between water molecules means
that the whole column of water is pushed
upwards from below and pulled upwards
from above
Loss of water from leaf by Transpiration
• Osmosis moves water
from xylem to palisade
and spongey
mesophyll
• Water evaporates from
the mesophyll cells
into the intercellular
spaces
• Water diffuses from
intercellular spaces out
through stomata
Factors that increase transpiration rates
• Number of leaves
• More leaves = larger surface area
•
• More stomata = more spaces for
evaporation
Number of
stomata
• Cuticle
• More cuticle= slower evaporation
• Light
• Stomata open in sunlight
• Temperature
• Higher temp = faster evaporation,
faster diffusion through stomata
• Relative humidity
• Lower gradient slows water loss
• Air movement/ wind • Maintains water potential gradient
• Little water in plant closes stomata
• Water available
and reduces water loss
Animation link
• http://www.kscience.co.uk/animations/tran
spiration.swf