Transport in Plants

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Transport in Plants

• AP Biology
• Ch. 36
• Ms. Haut

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Physical forces drive the transport of materials
in plants over a range of distances

• Transport in vascular plants occurs on three
  scales
• Transport of water and solutes by individual
  cells, such as root hairs
• Short-distance transport of substances from cell
  to cell at the levels of tissues and organs
• Long-distance transport within xylem
  and phloem at the
  level of the whole plant
• A variety of physical processes are
  involved in the different types
  of transport

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Transport at Cellular Level

• Relies on selective permeability of membranes
• Transport proteins
• Facilitated diffusion
• Selective channels (K channels)
• Aquaporinswater-specific protein channels that
  facilitate water diffusion across plasma membrane

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Transport at Cellular Level

• Proton pumps
• create a hydrogen ion gradient that is a form of
  potential energy
• contribute to a voltage known as a membrane
  potential

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Transport at Cellular Level

• Plant cells use energy stored in the proton
  gradient and membrane potential to drive the
  transport of many different solutes

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Transport at Cellular Level

• In the mechanism called cotransport, a transport
  protein couples the passage of one solute to the
  passage of another

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Effects of Differences in Water Potential

• To survive, plants must balance water uptake and
  loss
• Osmosis determines the net uptake or water loss
  by a cell is affected by solute concentration and
  pressure

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Effects of Differences in Water Potential

• Water potential is a measurement that combines
  the effects of solute concentration and pressure
• Water potential determines the direction of
  movement of water
• Water flows from regions of higher water
  potential to regions of lower water potential

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How Solutes and Pressure Affect Water Potential

• Both pressure and solute concentration affect
  water potential
• The addition of solutes reduces water potential
• The solute potential of a solution is
  proportional to the number of dissolved molecules
• Pressure potential is the physical pressure on a
  solution

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Differences in Water Potential Drive Water
Transport in Plant Cells
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Three Major Compartments of Vacuolated Plant Cells

• Transport is also regulated by the compartmental
  structure of plant cells
• The plasma membrane directly controls the traffic
  of molecules into and out of the protoplast
• The plasma membrane is a barrier between two
  major compartments, the cell wall and the cytosol

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• The third major compartment in most mature plant
  cells is the vacuole, a large organelle that
  occupies as much as 90 or more of the
  protoplasts volume
• The vacuolar membrane regulates transport between
  the cytosol and the vacuole

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• In most plant tissues, the cell walls and cytosol
  are continuous from cell to cell
• The cytoplasmic continuum is called the symplast
• The apoplast is the continuum of cell walls and
  extracellular spaces

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Lateral Transport of Minerals and Water
Casparian stripwaxy material (suberin) that
creates selectivity (only minerals already in
symplast can enter stele)
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The Roles of Root Hairs, Mycorrhizae, and
Cortical Cells

• Much of the absorption of water and minerals
  occurs near root tips, where the epidermis is
  permeable to water and root hairs are located
• Root hairs account for much of the surface area
  of roots

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Mycorrhizae

• Most plants form mutually beneficial
  relationships with fungi, which facilitate
  absorption of water and minerals from the soil
• Roots and fungi form mycorrhizae, symbiotic
  structures consisting of plant roots united with
  fungal hyphae

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Pushing Xylem Sap Root Pressure

• At night, when transpiration is very low, root
  cells continue pumping mineral ions into the
  xylem of the vascular cylinder, lowering the
  water potential
• Water flows in from the root cortex, generating
  root pressure

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• Root pressure sometimes results in guttation, the
  exudation of water droplets on tips of grass
  blades or the leaf margins of some small,
  herbaceous eudicots

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Transportation of Xylem Sap (Water)
Transpiration-Cohesion Theory

• Water evaporates from leaves
• through stomatacreates a low
• pressure at top of water column

• Water replaced by water from
• xylemwater in areas of high
• pressure move to areas of low
• pressure

• Strong cohesion of water with the
• pressure difference helps to pull the
• entire water column up from roots
• to rest of plant

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Transpirational Pull

• Water is pulled upward by negative pressure in
  the xylem
• Water vapor in the airspaces of a leaf diffuses
  down its water potential gradient and exits the
  leaf via stomata
• Transpiration produces negative pressure
  (tension) in the leaf, which exerts a pulling
  force on water in the xylem, pulling water into
  the leaf

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Cohesion and Adhesion in the Ascent of Xylem Sap

• The transpirational pull on xylem sap is
  transmitted all the way from the leaves to the
  root tips and even into the soil solution
• Transpirational pull is facilitated by cohesion
  and adhesion

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• Opening and closing is regulated by turgor
  pressure
• Stoma of most plants open during the day and
  closed during the night

Turgor pressure increases and guard cells expand,
opening the pore
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At night K pumped out of cells
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Organic nutrients are translocated through the
phloem

• Translocation is the transport of organic
  nutrients in a plant

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Movement from Sugar Sources to Sugar Sinks

• Phloem sap is an aqueous solution that is mostly
  sucrose
• It travels from a sugar source to a sugar sink
• A sugar source is an organ that is a net producer
  of sugar, such as mature leaves
• A sugar sink is an organ that is a net consumer
  or storer of sugar, such as a tuber or bulb

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Translocation

• Sugar must be loaded into sieve-tube members
  before being exposed to sinks
• In many plant species, sugar moves by symplastic
  and apoplastic pathways

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Transportation of Food Pressure-flow Hypothesis
At the source end of the sieve tube

• Sugars are made in photosynthetic cells and
  pumped by active
• transport into sieve tubes

• Concentration of dissolved substances increases
  in the sieve tube
• and water flows in by osmosis

• Pressure builds up at the source end of the sieve
  tube

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Transportation of Food Pressure-flow Hypothesis
At the sink end of the sieve tube

• Sugars are pumped out

• Water leaves the sieve tube by osmosis

• Pressure drops at the sink end of the sieve tube

• Difference in pressure causes sugars to move from
  source to sink

Water flows in
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