Plant Physiology

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Plant Physiology

• To recall the basic plant cell structure and
  specialised cells.
• To be able to describe the structure of a root.
• To be able to describe and explain how
  transpiration occurs and what effects its rate.
• To what Xerophytes are and how they are adapted
  to survive dry conditions.
• To know the difference between xylem and phloem
  tissue.
• To be able to explain translocation and relate
  this to Mass-Flow hypothesis
• To be able to use experimental data to explain
  these processes

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Using your knowledge of water potential describe
and explain how water gets into the root and to
the xylem of a plant.
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How does transpiration occur
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The potometer
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Factors effecting rate of transpiration

• Light intensity-changes photosynthesis rate and
  therefore uptake of water.
• Humidity-amount of moisture in air surrounding
  leaves effects diffusion gradients.
• Air movement-physically removes water molecules
  from stomata-draws water through.
• Temperature-varying evapouration from leaf.

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How plants hold on to water
A XEROPHYTE is a plant which is adapted to a dry
environment
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Xylem
These form the network of tubes which carry water
and dissolved minerals UP the plant from the
roots. They develop to be hollow cells which
strengthen their cell walls with LIGNIN. They
either have holes or no end cell walls allowing
the MASS-FLOW of water through the plant
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Phloem
Phloem carry sugar and other dissolved solutes
around the plant they can transport this sap
both UP and DOWN. Phloem are living cells
without nuclei, they comprise SIEVE TUBE CELLS
and SIEVE PLATES between the cells. Bordering the
phloem are COMPANION CELLS which actively
transport substances into the phloem.
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Plasmodesmata
Plasmodesmata are links between cells. They allow
the cytoplasms between cells to be connected and
pass through the cell wall. They are found
throughout the plant including the symplast
pathway
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Translocation

• This is the method of moving the sap (sugar and
  dissolved solutes) through the phloem
• The solutes move from a SOURCE (area where sugar
  is produced-usually leaves) to a SINK (area which
  cannot produce its own sugar-usually roots).
• As flowers and buds cannot photosynthesis these
  are also sinks, which is why translocation can
  occur both up and down the plant.

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Mass-Flow Hypothesis
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Diagram to show mass flow
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How do we investigate these systems?
There are 3 main methods of analysing movement of
substances through the plant and you need to be
aware of how they work, and be able to analyse
data from these experiments.

• Ringing
• Radioactive tracers
• Puncture tapping

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Ringing
As the phloem is the outermost of the vascular
tissue it can be removed, leaving the xylem
intact. Over time the sugar moving downwards
builds up and bulges above the ring. This
indicates the phloem translocates
downwards. There is also reduced growth below the
ring as no sugars reach the roots, whereas the
leaves are unaffected.
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Radioactive tracers

• Many elements can occur with different numbers of
  neutrons e.g 14C compared to 12C. These behave
  the same way but as they are radioactive we can
  observe where they move using radioactive paper
  or a geiger counter.

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Puncture tapping
Phloem is under high pressure and illustrates the
sap moving downwards
Xylem is under low pressure as being drawn
upwards and this method demonstrates the rate of
this occurring
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Using Aphids

• Aphids (greenfly) can be used to puncture tap.
• Once they have inserted their sylet (specialised
  mouthpart like a needle) they can be removed
  leaving the sylet in the phloem.
• The enzymes secreted prevent the stylet being
  blocked so the sap oozes out and solute
  concentrations can be analysed