Module 3: Transport In Plants Flashcards
Xylem function
Transport water and dissolved minerals up the plant.
Xylem and phloem are found together in vascular bundles in the plant.
Phloem function
Transport sucrose and other assimilates up and down the plant.
Xylem and phloem are found together in vascular bundles in the plant.
Vascular bundles in the root
Found in the centre of the root.
Xylem in X shape in the middle.
Phloem in between arms of xylem X.
Around the vascular bundle is a ring of endodermis.
In the endodermis is a ring of meristem cells called pericycle.
Meristem cells are stem cells- undifferentiated cells that can still divide.
Vascular bundle in stem
Found near outer edge of stem.
Xylem closer to centre of stem.
Phloem near outside.
In between phloem and xylem is cambium- layer of meristematic cells that can differentiate into xylem and phloem.
Vascular bundle in leaves
Form the veins of leaves.
Xylem are above phloem.
Xylem tissue
Transport water and dissolved minerals up a plant (from roots to leaves).
Consist of: xylem vessels, fibres, parenchyma cells.
Long cells in column. They become lignified and cells die, contents and end walls decay forming xylem vessels.
Structure of xylem vessels
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Phloem tissue
Transport sucrose and other assimilates up and down the plant.
Consist of sieve tube elements and companion cells.
Sucrose is transported as sap (dissolved in water).
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Sieve tube elements
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Companion cells
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How does water enter a plant?
Through root hair cells via osmosis
How are root hair cells adapted?
- Root hair cells are found on the epidermal layer of plant roots.
- Hair like projection into the soil provides a large surface area for osmosis and mineral uptake into the roots.
- Thin wall for a short diffusion path
- Many mitochondria to provide energy for the active transport of minerals
How and why does water enter the root hair cells?
• Minerals are actively transported into the root hair cell from the soil
• This decreases the water potential of the root hair cell (below that of the
soil)
• Water moves into the root hair cell via osmosis across the cell surface membrane, down the water potential gradient
Water pathways
Water needs to cross the cortex and endodermis to reach the xylem. This happens via osmosis between cells. This is done in 3 ways: - Apoplast pathway - Symplast pathway - Vacuolar pathway
Apoplast pathway
• Water travels through the cell walls in gaps
between the cellulose fibres
• Water does not cross membranes and so can
carry dissolved minerals with it
Symplast pathway
• Water crosses the cell surface membranes
via osmosis (through aquaporins) and
enters the cytoplasm
• It then can move through plasmodesmata
which links the cytoplasm from one cell to
the next
Vacuolar pathway
• Similar to symplast but water also moves through the vacuoles, not just the cytoplasm
The casparian strip
• On the cell walls of the cells of the endodermis, there is
a strip of waterproof material called suberin known as
the Casparian strip
• The Casparian strip blocks the apoplast pathway
between the cortex and the xylem ‐ water now must
take the symplast pathway
• Minerals must be actively transported from the
cytoplasm into the xylem through carrier proteins in the
cell membranes
• This lowers the water potential in the xylem, so water
now crosses the cell surface membranes from the cortex
into the xylem via osmosis
• Water potential is lowest in the xylem than anywhere
else in the root ‐ this causes the osmosis of water from
the root hair cell from the cortex to the endodermis
Summarise how water moves from the soil to the xylem
• Minerals actively transported into root hair cell (through carrier proteins)
• Water moves via osmosis from soil into root hair cells across cell surface
membrane (through aquaporins) down the water potential gradient
• Water can move via cell walls in the apoplast pathway
• Water can move via the cytoplasm in the symplast pathway, through plasmodesmata, linking the cytoplasm in neighbouring cells
• At the endodermis, the Casparian strip (made of suberin) blocks the apoplast pathway
• This makes the water enter the symplast pathway
• Water potential is most negative in the xylem due to the active transport of minerals into it
• This causes water to move into the xylem from the cells of the endodermis and cortex
How does water move up the stem into the leaves?
Water moves up the xylem vessels from the root.
This is helped in 3 ways:
‐ root pressure ‐ the push from the water entering the xylem vessels in the roots (doesn’t move water far)
‐ capillary action ‐ adhesion (forces of attraction) of water molecules to lignin in narrow xylem vessels can pull the water up the sides of the vessel
‐ transpiration pull ‐ most of the driving force
What is transpiration?
Transpiration is the loss of water by evaporation out of a plant’s leaves via the stomata.
How does transpiration result in the movement of water up a stem?
1) Water evaporates from the surface of the mesophyll cells in the leaf and forms water vapour
2) Water vapour diffuses from a high water potential to a lower water potential out of the leaf, through the stomata
3) More water is drawn from the mesophyll cells via the symplast/apoplast pathways in the leaf replacing the water that has evaporated
4) This occurs via osmosis down the water potential gradient
5) This water is replaced by water from the xylem vessels (moving out via osmosis)
6) The loss of water from the xylem causes a low hydrostatic pressure at the top of the xylem
7) Water moves from a higher pressure (roots) to a lower pressure (down the pressure gradient) under tension
8) Water is therefore pulled up the xylem by mass flow
9) The cohesion of water molecules due to the hydrogen bonds between them causes them to stay as a long unbroken column of water during this process ‐ the transpiration stream
Factors affecting transpiration
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What is a potometer?
A piece of equipment that measures water uptake in a cut shoot.
