plant transport - dr. michael belanich
TRANSCRIPT
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Transport in Plants
• Three levels:
– at cellular level
– lateral transport (short-distance)
– whole plant (long distance)
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Transport in Plants
Cellular Level:
• Diffusion
– movement from an area of high concentration
to an area of lower concentration
• Active transport
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Transport in Plants
Cellular Level:
• Osmosis:
– passive transport of water across a
semipermeable membrane
– In plants it depend on solute concentration and
pressure (due to the cell wall)
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Transport in Plants
Cellular Level:
• Water potential
– Addition of solutes lowers
– Increasing pressure raises
• Water moves from high water potential to
low water potential
• Turgor pressure
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Figure 36.3 Water potential and water movement: a mechanical model
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Figure 36.4 Water relations of plant cells
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Figure 36.5 A watered tomato plant regains its turgor
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Transport in Plants
Lateral Transport
How water & dissolved minerals get into
roots...
• through the cells of the root
• along the extracellular pathway consisting
of cell walls
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Figure 36.8 Mycorrhizae, symbiotic associations of fungi and roots
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Lateral transport of minerals and water in roots
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Casparian Strip
• A waxy material that surrounds endodermal
cells
– prevents material from crossing the endodermis
between cells
• Substances must enter the cells of the
endodermis in order to pass into the
vascular cylinder
– Allows selectivity
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Transport in Plants
Long Distance Transport
• Diffusion too slow
• Instead substances move by bulk flow
– the movement of fluid due to pressure
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Xylem
• dead at functional maturity
• Only the cell walls remain to form tubes,
connected by pores, through which water
can move.
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Xylem
• Xylem sap brings minerals to leaves and
water to replace what is lost by transpiration
– Transpiration is the evaporation of water from
leaves or other aerial parts of the plant
• rates of >15 m per hour
• distances of 100m in the tallest trees.
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Movement of Xylem Sap
• Pushed or Pulled?
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Movement of Xylem Sap
Pushed…
• by root pressure
– minerals actively pumped into the xylem
– causes water to move in by osmosis
– positive pressure is generated
– When transpiration is low
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Figure 36.9 Guttation
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Movement of Xylem Sap
Pulled…
• by transpiration-cohesion
• Water is pulled out of the xylem to replace
the water that is lost from leaves through
stomata
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Movement of Xylem Sap
Pulled…
• transpiration pull is translated all the way to
the roots by
– cohesion
– Adhesion
• Costs no energy to transport xylem sap up
to leaves
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Phloem
• cells that are living at functional maturity
• Phloem sap consists primarily of sugar,
primarily sucrose (30%)
– hormones, amino acids, minerals
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Phloem Sap
• Direction of flow in phloem is variable
• Bulk Flow or Mass Flow Hypothesis
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Phloem Sap
• flows from sugar sources
– where sugar is produced by photosynthesis or
the breakdown of starch
– Leaves, storage organs
• to sugar sinks
– that consume sugar
– non green plant parts, growing shoots and roots,
fruits
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Phloem Sap
• sugars are actively loaded into phloem at
the source
– water follows and there is a high pressure
• sugars are actively transported out of the
phloem at the sink
– water follows and pressure is lower
• sap flows from high to low pressure
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Eudicot Root
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