Transport in Plants

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Transport in Plants

• Explain the need for transport systems in
  multicellular plants in terms of size and surface
  areavolume ratio
• Describe, with the aid of diagrams and
  photographs, the distribution of xylem and phloem
  tissue in roots, stems and leaves of
  dicotyledonous plants
• Describe, with the aid of diagrams and
  photographs, the structure and function of xylem
  vessels, sieve tube elements and companion cells

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Transport in Plants

• Plants need a transport system so that cells deep
  within the plants tissues can receive the
  nutrients they need for cell processes
• The problem in plants is that roots can obtain
  water, but not sugar, and leaves can produce
  sugar, but cant get water from the air

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What substances need to be moved?

• The transport system in plants is called vascular
  tissue
• Xylem tissue transports water and soluble
  minerals
• Phloem tissue transports sugars

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The Vascular Tissues

• Xylem and phloem are found together in vascular
  bundles, that sometimes contain other tissues
  that support and strengthen them

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Root vs. stem vs. leaf

• The vascular bundle differs depending on if it is
  a root or stem

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Root

• The vascular bundle is found in the centre
• There is a large central core of xylem- often in
  an x-shape
• This arrangement provides strength to withstand
  the pulling forces to which roots are exposed
• Around the vascular bundle are cells called the
  endodermis which help to get water into the xylem
  vessels
• Just inside the endodermis is the periycle which
  contains meristem cells that can divide (for
  growth)

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Stem

• The vascular bundles are found near the outer
  edge of the stem
• The xylem is found towards the inside of each
  vascular bundle, the phloem is found towards the
  outside
• In between the xylem and phloem is a layer of
  cambium
• Cambium is a layer of meristem cells that divide
  to make new xylem and phloem

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Leaf

• The vascular bundles (xylem and phloem) form the
  midrib and veins of the leaf
• A dicotyledon leaf has a branching network of
  veins that get smaller as they branch away from
  the midrib
• Within each vein, the xylem can be seen on top of
  the phloem

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Phloem
Xylem
Stem
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A Xylem B Phloem C/D Upper/Lower epidermis
Leaf
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Xylem vessel wall
Xylem vessel lumen
Phloem
Endodermis
Starch grains
Root
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Structure of Xylem

• Used to transport water and minerals from roots
  to leaves
• Consists of tubes for water, fibres for support
  and living parenchyma cells

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Xylem vessels

• Obvious in dicotyledonous plants
• Long cells with thick walls containing lignin
• Lignin waterproofs walls of cells and strengthens
  them
• Cells die and ends decay forming a long tube
• Lignin forms spiral, annular rings or broken
  rings (reticulate)
• Some lignification is not complete and pores are
  left called pits or bordered pits, allowing water
  to move between vessels or into living parts

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Adaptations of Xylem to Function

• Xylem can carry water and minerals from roots to
  shoot tips because
• Made of dead cells forming continuous column
• Tubes are narrow so capillary action is
  effective
• Pits allow water to move sideways
• Lignin is strong and allows for stretching
• Flow of water is not impeded as there are no end
  walls, no cell contents, no nucleus, lignin
  prevents tubes collapsing

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Structure of Phloem

• Function to transport sugars from one part to
  another
• Made of sieve tube elements and companion cells

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Sieve Tubes

• Sieve tube elements not true cells as they have
  little cytoplasm
• Lined up end to end to form a tube
• Sucrose is dissolved in water to form a sap
• Tubes (known as sieve tubes) have a few walls
  across the lumen of the tube with pores (sieve
  plates)

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Companion cells

• In between sieve tubes
• Large nucleus, dense cytoplasm
• Many mitochondria to load sucrose into sieve
  tubes
• Many plasmodesmata (gaps in cell walls between
  companion cells and sieve tubes) for flow of
  minerals

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Water route between cells

• Apoplast between cell walls of neighbouring
  cells
• Symplast through plasma membrane and
  plasmodesmata to cytoplasms from cell to cell
• Vacuolar same as symplast, but also through
  vacuoles

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Water uptake from the soil

• Epidermis of roots contain root hair cells
• Minerals absorbed by active transport using ATP
• Minerals reduce the water potential in the cell
  cytoplasm (more negative) so water is taken up by
  osmosis

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Movement across the root

• Active process occurring at the endodermis (layer
  of cells surrounding the xylem, some containing
  waterproof strip called casparian strip)
• Casparian strip blocks the apoplast pathway
  (between cells) forcing water into the symplast
  pathway (through the cytoplasm)
• The endodermis cells move minerals by active
  transport from the cortex into the xylem,
  decreasing the water potential (more negative),
  thus water moves from the cortex through the
  endodermal cells to the xylem by osmosis
• A water potential gradient exists across the
  whole cortex, so water is moved along the
  symplast pathway (through cytoplasm) from the
  root hair cells across the cortex and into the
  xylem

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Casparian Strip

• Blocks the apoplast pathway (cell walls)
• Water and dissolved nitrate ions have to pass
  into the cell cytoplasm through cell membranes
• There are transporter proteins in the cell
  membranes that actively transport nitrate ions
  into the xylem lowering the water potential (more
  negative)
• Water enters xylem down concentration gradient
  and cannot pass back

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Water movement up stem

• Root pressure minerals move into xylem by active
  transport, forcing water into xylem and pushes it
  up the stem
• Transpiration Pull loss of water at leaves
  replaced by water moving up xylem.
  Cohesion-tension theory- cohesion between water
  molecules and tension in the column of water
  (which is why xylem is strengthened with lignin)
  means the whole column of water is pulled up in
  one chain
• Capillary action adhesion of water to xylem
  vessels as they are narrow

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How water leaves the leaf

• Through stomata
• Tiny amount through the waxy cuticle
• Water evaporates from the cells lining the cavity
  between the guard cells, lowering water potential
  and meaning that water enters them by osmosis
  from neighbouring cells, which is replaced by
  further neighbouring cells and so on

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Transpiration

• Loss of water vapour from upper parts of the
  plant
• Water enters leaf from xylem and passes to
  mesophyll cells by osmosis
• Water evaporates from surface of mesophyll cells
  to form water vapour (air spaces allow water
  vapour to diffuse through leaf tissue)
• Water vapour potential rises in air spaces, so
  water molecules diffuse out of the leaf through
  open stomata

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Transpiration three processes

• Osmosis from xylem to mesophyll cells
• Evaporation from surface of mesophyll cells into
  intercellular spaces
• Diffusion of water vapour from intercellular
  spaces out through stomata

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Water use in plant

• Photosynthesis
• Cell growth and elongation
• Turgidity
• Carriage of minerals
• Cools the plant

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Measuring transpiration

• Potometer is used to estimate water loss

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Factors affecting transpiration

• Leaf number more leaves, more transpiration
• Number, size, position of stomata more and
  large, more transpiration, under leaf, less
  transpiration
• Cuticle waxy cuticle, less evaporation from leaf
  surface
• Light more gas exchange as stomata are open
• Temperature high temperature, more evaporation,
  more diffusion as more kinetic energy, decrease
  humidity so more diffusion out of leaf
• Humidity high humidity, less transpiration
• Wind more wind, more transpiration
• Water availability less water in soil, less
  transpiration (e.g. in winter, plants lose leaves)

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Too much water loss

• Less turgidity
• Non-woody plants wilt and die
• Leaves of woody plants die first then it will die
  if water loss continues

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Xerophytes

• Smaller leaves reducing surface area e.g. pine
  tree
• Densely packed spongy mesophyll to reduce surface
  area, so less water evaporating into air spaces
• Thick waxy cuticle e.g. holly leaves to reduce
  evaporation
• Closing stomata when water availability is low
• Hairs on surface of leaf to trap layer of air
  close to surface which can become saturated with
  water, reducing diffusion
• Pits containing stomata become saturated with
  water vapour reducing diffusion
• Rolling the leaves so lower epidermis not exposed
  to atmosphere also traps air which becomes
  saturated
• Maintain high salt concentration to keep water
  potential low and prevent water leaving

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Marram Grass
Leaf rolled up to trap air inside
Thick waxy cuticle to reduce water evaporation
from the surface
Trapped air in the centre with a high water
potential (less negative)
Hairs on lower surface reduce movement of air
Stomata in pits to trap air with moisture close
to the stomata
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Movement of Sugars

• Translocation movement of assimilates (sugars
  and other chemicals) through the plant
• Source a part of the plant that releases sucrose
  to the phloem e.g. leaf
• Sink a part of the plant
• that removes sucrose from
• the phloem e.g. root

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Sucrose Entering the Phloem

• Active process (requires energy)
• Companion cells use ATP to transport hydrogen
  ions out of their cytoplasm
• As hydrogen ions are now at a high concentration
  outside the companion cells, they are brought
  back in by diffusion through special
  co-transporter proteins, which also bring the
  sucrose in at the same time
• As the concentration of sucrose builds up inside
  the companion cells, they diffuse into the sieve
  tubes through the plasmodesmata (gaps between
  sieve tubes and companion cell walls)

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Sucrose movement through phloem

• Sucrose entering sieve tube lowers the water
  potential (more negative) so water moves in by
  osmosis, increasing the hydrostatic pressure
  (fluid pushing against the walls) at the source
• Sucrose used by cells surrounding phloem and are
  moved by active transport or diffusion from the
  sieve tube to the cells. This increases water
  potential in the sieve tube (makes it less
  negative) so water moves out by osmosis which
  lowers the hydrostatic pressure at the sink

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Movement along the phloem

• Water entering the phloem at the source, moving
  down the hydrostatic pressure gradient and
  leaving at the sink produces a flow of water
  along the phloem that carries sucrose and other
  assimilates. This is called mass flow. It can
  occur either up or down the plant at the same
  time in different phloem tubes

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Evidence for translocation

• Radioactively labelled carbon from carbon dioxide
  can appear in the phloem
• Ringing a tree (removing a ring of bark) results
  in sugars collecting above the ring
• An aphid feeding on the plant stem contains many
  sugars when dissected
• Companion cells have many mitochondria
• Translocation is stopped when a metabolic poison
  is added that inhibits ATP
• pH of companion cells is higher than
• that of surrounding cells
• Concentration of sucrose is higher at
• the source than the sink

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Evidence against translocation

• Not all solutes move at the same rate
• Sucrose is moved to parts of the plant at the
  same rate, rather than going more quickly to
  places with low concentrations
• The role of sieve plates is unclear

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