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Water
Sugar
Diffusion is only
good for short
distances
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Xylem – transports water
and minerals from roots
to leaves
Phloem – transports the
water soluble products of
photosynthesis (sucrose
and amino acids) from
leaves to other area of the
plant
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Xylem Vessels
Dead cells with pits
No end walls
Are main water conducting cells
Lignin (a waterproof chemical)
replaces the cellulose cell wall
◦ Provides support
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XylemTracheids:
◦ Dead cells which taper at ends
◦ Also conduct water (but not as well
adapted as they still have end walls)
◦ Contain lignin
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Supporting fibres (sclerenchyma
cells)
◦ Dead cells
◦ For support
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Xylem parenchyma
◦ Living cells that act as packing
tissue
◦ Aid in support when turgid
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Tracheids and vessels form a continuous
system of channels for water transport
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Lignin helps support the vessels and is
arranged in different patterns.
1.
2.
3.
No cytoplasm or organelles to resist water
movement
No end walls of cells leaving a clear path
The secondary cell wall is contains lignin
making the sides impermeable to water
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Sieve tube elements
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Companion cells
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Phloem parenchyma
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Phloem fibres
◦ Living cells linked end to end for transport
◦ End walls called sieve plates have pores to allow
fluid (containing organic compounds) to pass
through
◦ No nucleus and few organelles
◦ Many plasmodesmata to companion cells
◦ Have dense cytoplasm with a large nucleus, many
organelles (many mitochondria and ribosomes)
◦ Nucleus controls activities of sieve tube element.
◦ Ribosomes allow production of enzymes and
carriers.
◦ Mitochondria produce ATP for active transport in
sieve tube element
◦ Living cells with tin walls
◦ Act as packing tissue between other cells
◦ Stores starch
◦ Elongated , tapered cells with thick walls for
support
• Root hairs provide a large surface area for
absorption and have permeable walls.
• The solution of water and mineral salts
moves down the water potential gradient
into the root.
• Once in the cells, there are 3 possible
pathways for water:
A. Apoplast
B. Symplast
C. Vacuolar
• Water travels through the cells walls
• This is the main route as it has the least
resistance
• This route is blocked by at the endodermis by
the Casparian band made of water-proof
suberin
• Water must pass across the plasma membrane
and continues along the symplast route
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Water enter the cells (by osmosis) and then
travels through the cytoplasm and
plasmodesmata
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Water can also travel through the cell vacuoles (the
vacuolar pathway)
Not a main pathway
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At the endodermis ions are actively taken up into
the xylem (to by-pass the Casparian band)
This lowers the water potential in the xylem,
causing water to be drawn through the
endodermis
This produces a positive hydrostatic pressure
inside the xylem, forcing water upwards
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The mass flow hypothesis
suggests that there is a
passive flow of sucrose
from source to sink
Source= leaves are a
source of sugars
Sink = growing tissues
does not account for all
observations such as
movement in opposite
directions at the same
time and at different
rates
Other hypotheses have
been proposed; including
diffusion and cytoplasmic
streaming
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water column in the xylem is held up by
cohesion between water molecules and the
adhesion between the water molecules and
the hydrophilic lining of the xylem vessels
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is the loss of water
from the leaves which
gives rise to the
transpiration stream
Cohesion-tension
theory
◦ Cohesion= water
molecules have a strong
attracted to each other
◦ Tension= as water
evaporates at leaf this
creates a tension (pull)
of the water column up
the xylem
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water lily, live with their roots submerged in the
mud at the bottom of a pond and have floating
leaves on the surface
Hydrophytes have little need for support or
transport tissues, have little or no cuticle and
stomata only on the upper surface of their leaves.
There are large air spaces present in both stem
and leaf tissue
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have adapted to living under conditions of
low water availability
Ex: Marram Grass
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leaf shape
sunken stomata
thick cuticle
hairs
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plants of temperate regions and flourish in
habitats with adequate water supply.
They need to survive unfavourable times of
the year by shedding their leaves, surviving
underground or as dormant seeds.
Transporting water and nutrients
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Plant transport tissues
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Plant anatomy
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Parts of a plant transport system
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What is transpiration?
Transpiration is the loss of water from the leaves of a plant.
Most of this occurs from the underside of a leaf, where there
are many stomata in the epidermis.
Most plants control their water
intake by opening and closing
their stomata. This happens
when water levels change in
the guard cells around each
stoma. This occurs either
passively by osmosis, or by
active transport of solutes.
Transpiration rates also vary naturally in response to
environmental factors such as temperature and humidity.
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What is water potential?
Water tends to move from areas of high water concentration
to areas of low water concentration. This is osmosis.
Water also tends to move
from areas of high
hydrostatic pressure to
areas of low hydrostatic
pressure. It is also affected
by gravity and electrostatic
forces, such as those that
cause surface tension.
The collective term for the tendency of water to move due
to any of these effects is water potential.
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Cohesion–tension theory
Water is a polar molecule, meaning that
its positive and negative charges are not
evenly distributed. The oxygen atom has
a slight negative charge, while the two
hydrogen atoms are slightly positive.
This means that, in the xylem, water
molecules spontaneously arrange so that
positive and negatively charged poles lie
next to each other.
This causes the molecules to cohere, or
stick together, so that as some leave a
plant by transpiration, others are pulled
up behind them.
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Transpiration rates
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Factors affecting transpiration
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The transpiration stream
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Conserving water
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What is root pressure?
Water is usually drawn up a plant by the tension resulting
from transpiration and cohesion between water molecules.
In some situations, such
as 100% humidity, a
plant is unable to
transpire. Instead, water
can be transported by
positive pressure from
below. This is known as
root pressure.
Solutes are actively transported into the roots of the plant,
causing water to enter by osmosis. This increases the
hydrostatic pressure in the root, forcing water up the stem.
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The transpiration stream
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What is translocation?
Translocation is the movement
of nutrients around a plant. The
term includes the movement of
minerals, which can be dissolved
in water and transported in the
xylem, but usually refers to the
transport of sugars, amino acids,
and other organic molecules in
the phloem.
Translocation can occur in either
direction in the phloem – it is
bidirectional. It is an active
process, requiring energy, unlike
water transport in the xylem.
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The pressure flow hypothesis
The most widely accepted explanation of sap movement
in plants is the pressure flow hypothesis.
According to the theory, sap moves through phloem vessels
due to differences in hydrostatic pressure. This is a similar
effect to root pressure.
Evidence for this effect
includes the excretion of sap,
or honeydew, by an aphid
when it taps a phloem vessel
to feed. The sap is forced
through the aphid’s body,
demonstrating that the sap in
the phloem is under pressure.
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Translocation of sugars
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Understanding translocation
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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