1. Transport system in
multicellular plants
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2. Need for transport systems
 Plants have thin, flat leaves which present large
surface area to the sun
 Relatively easy for CO2 and O2 to diffuse into and out
of the leaves, reaching and leaving every cell quickly
enough so that there is no need for a transport system
for these gases
 Plants have two transport systems:
 For carrying mainly water and inorganic ions from roots to the
parts above the ground
 For carrying substances made by photosynthesis from the
leaves to the other areas
 However, fluids don’t move as rapidly as blood does in
a mammal, nor is there an obvious pump such as the
heart
 Neither plant transport system carries O2 and CO2
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3. Distribution of xylem and phloem tissue
in roots of dicotyledonous plants
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4. Distribution of xylem and phloem tissue
in stems of dicotyledonous plants
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5. Distribution of xylem and phloem tissue
in stems of dicotyledonous plants
Sclerenchyma:
Plant tissue whose function is to strengthen and support, composed
of thick-walled cells that are heavily lignified (toughened).
Parenchyma:
Plant tissue composed of loosely packed, more or less spherical cells,
with thin cellulose walls. Although parenchyma often has no
specialized function, it is usually present in large amounts, forming a
packing or ground tissue.
Collenchyma:
Plant tissue composed of relatively elongated cells with thickened cell
walls, in particular at the corners where adjacent cells meet.
It is a supporting and strengthening tissue found in non-woody
plants, mainly in the stems and leaves.
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6. Distribution of xylem and phloem tissue
in leaves of dicotyledonous plants
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7. Distribution of xylem and phloem tissue
in leaves of dicotyledonous plants
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8. Transpiration
 The loss of water vapour by diffusion down a
water potential gradient from a plant to its
environment
 Mostly takes place through the stomata on the
leaves
 Transport of water
 Water from soil – root hair
 Root – xylem tissue
 Xylem tissue – stem
 Stem - leaves
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9. From soil to root hair
 Water moves into the root hairs down a water
potential gradient through a partially
permeable membrane into the cytoplasm and
vacuole of the root hair cell
 Very fine root hairs provides a large surface
area in contact with soil water, increasing the
rate of water absorbed
 Micorrhizas
 associations formed by fungi located in or on roots
which serve a similar function to root hairs
 act like a mass of fine roots which absorb nutrients,
especially phosphate
 Some plants growing on poor soils are unable to
survive without these fungi
 The fungi receive organic nutrients from the plant
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11. From root hair to xylem
 Water moves down the water potential gradient across the root
(water potential inside xylem vessels < water potential in root
hairs)
 Two possible routes through the cortex:
 Apoplast pathway – water seeps across the root from cell wall to
cell wall without entering cytoplasm of cortical cells
 Symplast pathway – water moves into the cytoplasm or vacuole of
cortical cell and into adjacent cells through plasmodesmata
 Apoplast pathway barred at stele (endodermis have a thick,
waterproof, waxy band of suberin in cell walls – Casparian
strip) due to impenetrable barrier to water
 Only way to cross the endodermis is through the cytoplasm of the
cells
 Suberin deposits become more extensive as endodermal cells get
older except in passage cells (gives plant control over inorganic
ions and may help with generation of root pressure)
 Once across the endodermis, water continues to move down
water potential gradient across the pericycle and towards the
xylem vessels
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13. From xylem to leaf
 Water constantly moves out of xylem vessels
down a water potential gradient either into the
mesophyll cells or along their cell walls as
water evaporates from the cell walls
 The removal of water from top of xylem
vessels reduces the hydrostatic pressure
(pressure exerted by a liquid)
 Pressure difference causes water to move up
the xylem vessels (water up a straw)
 Xylem vessels have strong, lignified walls to
stop from collapsing due to tension
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14.  Movement of water up through xylem vessels
is by mass flow (all water molecules move
together as a body of liquid)
 Helped by water molecules attracted to each
other (cohesion) and to the lignin in the walls
of xylem vessels (adhesion)
 If an air bubble forms in the continuous column
of water, column breaks and difference in
pressure cannot be transmitted through the
vessel (air lock)
 Adaptive features:
 Small diameter of xylem vessels – prevents breaks
 Pits in vessel walls
 Allow water to move out (bypass air lock)
 Air bubbles cannot pass through pits
 Allows water to move out of xylem vessels to surrounding
living cells
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16.  Root pressure
 Plants may also increase pressure difference
by raising the water pressure at the base of
the vessels
 By active secretion of solutes (active
transport) into water in xylem vessels in root
 Solutes lowers water potential, draws in
water and increases water pressure
 Water transport is a passive process
fuelled by transpiration from the leaves
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18. From leaf to atmosphere -
transpiration
 Air inside leaf (spaces around mesophyll) is
usually saturated with water vapour from water
around mesophyll cell walls
 If there is a water potential gradient between
the air inside leaf and outside, water vapour
will diffuse out through small pores (stomata)
– transpiration
 Increase in the water potential gradient
between the air spaces in the leaf and the air
outside will increase rate of transpiration
 Transpiration cools leaves
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20. Factors that affect transpiration rate
 Temperature, wind speed, light intensity or humidity
 Rate of water vapour leaving leaves vs. Rate of water
taken up by stem
 High proportion of water taken up is lost by
transpiration
 Rate at which transpiration is happening directly
affects the rate of water uptake
 Potometer
 Completely water-tight and airtight (no leakage of water and
no air bubbles break the continuous water column)
 Submerge in water
 Cut the end of the stem with a slanting cut
 Position of the meniscus at set time intervals is
recorded. Plot a graph of distance moved against time.
Compare rates of water uptake under different
conditions.
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22. Xylem vessel element
 Tissue functions in support and transport
 Angiosperms (flowering plants except conifers)
contain several different types of cell:
 Vessel elements and tracheids are involved in
transport of water
 Fibres are elongated cells with lignified walls that
help support the plant (dead cells)
 Parenchyma cells are ‘standard’ plant cells except
they don’t usually have chloroplasts (shapes vary
but often isodiametric)
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23.  Xylem vessels
 Vessels are made up of many elongated
vessel elements arranged end to end
 Lignin is laid down in the cell wall
 As it builds up, cell dies and an empty space
(lumen) is left
 Lignin not laid down at plasmodesmata
areas leaving ‘gaps’ called pits (not open
pores; crossed by permeable, unthickened
cellulose cell wall)
 The end walls of neighbouring vessel
elements break down completely to form a
continuous non-living tube (xylem vessel)
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24.  Tracheids
 Dead cells with lignified walls but without
open ends (don’t form vessels)
 Elongated cells with tapering ends
 They have pits in the walls so water can
pass from one tracheid to the next
 Main conducting tissue only in ‘primitive’
plants i.e. ferns and conifers
 In the root, water which has crossed the
cortex, endodermis and pericycle moves into
the xylem vessels through the pits in their
walls and then moves up the vessels
towards the leaves
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26. Xerophytes
 Plants that live in places where water is in short supply
requiring adaptations to reduce the rate of
transpiration
 The marram grass leaves can roll up, exposing a tough,
waterproof cuticle to the air outside the leaf, while the
stomata open into the enclosed, humid space in the middle of
the ‘roll’. Hairs trap a layer of moist air close to the leaf
surface (reducing diffusion gradient)
 Opuntia is a cactus stems that store water. Leaves are
reduced to spines, which reduce surface area for transpiration
 Sitka spruce have leaves in the form of needles (reducing
surface area available for water loss). Covered in a layer of
waterproof wax and have sunken stomata
 Phlomis italica have ‘trichomes’ that act as a physical barrier
to the loss of water
 The cardon has swollen, succulent stems that store water
and photosynthesise. The stems are coated with wax and
leaves are extremely small
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28. Plant adaptations to arid conditions
Adaptation Examples
Needle-like leaves, bipinnate Mulga (acacias), she-oaks (casuarinas),
leaves, phyllodes hakeas, Xanthorrhoea (grass trees)
Photosynthetic stems She-oaks (Casuarina)
Woody fruits Banksias, hakeas
Waxy leaves Saltbush (Atriplex)
Ephemeral growth Ephemeral plants e.g. paper daisies
(Helipterum), yellow tops (Senecio)
Partially deciduous Eucalypts
Leaf curling Hummock grass (Triodia)
Sunken stomata Hakeas
Water storage Baobab tree, parakeelyas
Hanging leaves Eucalypts (sclerophylls)
Hairy or shiny leaves Banksias, paper flowers (Thomasia)
Water-directing leaves and stem Mulga (acacia), Xanthorrhoea (grass
trees)
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29. Mulga (acacias)
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30. Casuarinas
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31. Hakeas
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32. Banksias
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33. Banksia fruits
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34. Saltbush
Grass trees
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35. Yellow tops
Paper daisies
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36. Eucalypts
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37. Hummock grass
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38. Baobab tree
Parakeelyas
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40. Translocation
 Transport of soluble organic substances within
a plant, for example sugars (assimilates)
 Transported in sieve elements which are
found in phloem tissue along with several
other types of cells including companion cells
(parenchyma and fibres)
 Phloem sap moves by mass flow
 To create the pressure differences needed for
mass flow in phloem, the plant has to use
energy (active process)
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41.  The pressure difference is produced by
active loading of sucrose into the sieve
elements (in the photosynthesising leaf)
 This decreases the water potential in the
sap inside sieve element
 Water follows sucrose into the sieve
element (osmosis – down water potential
gradient)
 At other points, sucrose is removed and
water follows by osmosis
 In the leaf water moves into sieve tube,
in the root water moves out
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42.  Pressure difference created causing water
flow from the high pressure area to the
low pressure area, taking with it any
solutes
 Source: any area of a plant in which
sucrose is loaded into the phloem
 Sink: any area where sucrose is taken out
of the phloem
 Sap flows both upwards and downwards in
phloem (xylem always upwards)
 Can only flow one way in any particular
sieve tube at any one time
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44. Loading of sucrose into phloem
 Sucrose in solution moves from mesophyll cell to the
phloem tissue
 May move by the symplast pathway (moving from cell
to cell via plasmodesmata) or apoplast pathway
(travelling along cell walls)
 Companion cells and sieve elements work in tandem
 Sucrose is loaded into a companion cell by active transport
 H+ are moved out of the companion cells using ATP
 Large excess of H+ outside
 Can move back into cell down concentration gradient through
protein which acts as carrier for both H+ and sucrose
 Sucrose molecules are carried through this co-transporter
molecule into companion cell against concentration gradient
for sucrose
 Sucrose molecules can then move from the companion cell
into the sieve tube (through plasmodesmata)
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45. Unloading of sucrose from phloem
 Occurs into any tissue which requires
sucrose but mechanism still unknown
 Probably by diffusion
 In tissue, enzymes convert sucrose into
something else (e.g. invertase hydrolyses
sucrose to glucose and fructose)
 This decreases its concentration and
maintains concentration gradient
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47. Evidence for the mechanism of phloem transport
 Phloem protein is not present in living, active
phloem tissue
 The rate of transport in phloem is about 10,000
times faster than diffusion
 Considerable evidence for the active loading of
sucrose into sieve elements in sources such as
leaves:
 Phloem sap always has a relatively high pH (around
8) – expected if H+ is being actively transported out
of the cell
 There is a difference in electrical potential across
the plasma membrane (-150mV) – consistent with
excess of H+ outside the cell compared with inside
 ATP is present in phloem sieve elements in large
amounts – expected as it is required for active
transport of H+ out of cell
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48. Sieve elements
 Sieve tubes are made up of many elongated
sieve elements, joined end to end vertically to
form a continuous column
 A living cell (cellulose cell wall, plasma membrane
and cytoplasm containing ER and mitochondria)
 Amount of cytoplasm is very small and only forms
a thin layer lining the inside of the wall of the cell
 No nucleus and ribosomes
 Sieve plate: made up of end walls of 2 meeting
sieve elements, perforated by large pores
 In living phloem, pores are always open,
presenting little barrier to the free flow of liquids
through them
 Contents of phloem sieve tubes (phloem sap)
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50. Companion cells
 Each sieve element has one companion
cell lying beside it
 Cellulose cell wall, plasma membrane,
cytoplasm, small vacuole and nucleus
 Number of mitochondria and ribosomes
is larger than normal (metabolically very
active)
 Numerous plasmodesmata pass through
their cell walls (direct contact between
cytoplasms of companion cell and sieve
element
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52. Difference between sieve elements and xylem vessels
 Xylem vessels are dead, translocation through
phloem sieve tubes involves active loading of
sucrose at sources requiring living cells
 Xylem vessels have lignified cell walls, whereas
phloem tubes do not (lignin in cell walls kills
the cell)
 Water flow through dead xylem vessels
unimpeded and strong walls support the plant
 The end walls of xylem elements disappear
completely, whereas those of phloem sieve
elements form sieve plates (probably
supporting structures/allows phloem to seal up
rapidly if damaged/prevents entry of
microorganisms which feed on the nutritious
sap or cause disease)
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