Transport in Vascular Plants

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Transport in Vascular Plants

• Chapter 36

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Transport Occurs on 3 Scales

• transport of water solutes by individual cells
  (ex root hairs)
• short-distance transport from cell to cell (ex
  sieve-tube loading)
• long-distance transport within xylem phloem

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Scale 1 Active Transport

• proton pumps chemiosmosis
• energy from ATP is used to pump H ions out of
  cell
• as H ions flow back down their concentration
  gradient, energy is released that is used to
  transport other molecules into the cell
  (cotransport)
• in addition, the membrane potential that results
  from the active transport of H out of the cell
  can be used to draw in other cations like K

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Scale 1 Passive Transport

• osmosis
• in plant cells, the direction that water diffuses
  is controlled not only by solute concentration by
  also by the physical pressure inside the cell
• the rate at which water diffuses is controlled by
  aquaporins
• the affect of solute pressure on osmosis
  water potential (?)
• free water moves from areas of higher ? to areas
  of lower ?

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Review of Water Potential

• ? ?S ?P
• ? 0 in an open container of pure water
• ?S is proportional to the of dissolved solutes
• as solute increases, ?S decreases
• thus, ?S is always
• ?P can be either or
• water in dead xylem cells is usually under
  negative pressure
• water in living cells is usually under positive
  pressure b/c cell contents push the plasma
  membrane against the cell wall creating turgor
  pressure

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Scale 2 Cell to Cell

• transport routes between cells
• transmembrane (P.M. ? cell wall ? P.M.)
• symplast through plasmodesmata
• apoplast along byways provided by the continuum
  of cell walls

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Transport into Roots

• soil solution can enter root hair follow
  symplast route to cortext
• or, soil solution can flow into the hydrophilic
  walls of the epidermis travel along the
  apoplast route to the cortex
• when the soil solution reaches the innermost
  layer of cells in the root cortex (endodermis),
  it is prevented from entering directly into
  vascular tissue by a waxy Casparian strip

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Scale 3 Bulk Flow

• movement of fluid driven by pressure
• in phloem, high positive pressure at the source
  forces sap to the sink
• process is called translocation
• in xylem, pressure (tension) due to transpiration
  drives transport of xylem sap upward from roots

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Translocation

• sieve-tubes always carry sugars from source to
  sink
• mature leaves are the primary sugar source
• sinks consumers or storers or sugar (ex
  growing roots, buds, stems, fruits)
• phloem loading can occur by
• active transport via proton pumping cotransport
• via symplast or a combination of symplast
  apoplast
• sap moves thru a sieve tube by bulk flow driven
  by positive pressure

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Transport from Roots to Shoots

• due to transpiration loss of water vapor from
  leaf by diffusion evaporation
• evaporation creates a negative ? in leaves
  causing water molecules to be pulled from the
  hydrated part of the leaf and, ultimately, the
  xylem root tips
• cohesion adhesion facilitate the process
• cohesion between water molecules creates a
  continuous chain of water prevents the
  chain from breaking
• adhesion of water molecules to xylem walls
  offsets the downward pull of gravity

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Rate of Transpiration

• about 90 of water loss occurs thru stomata
  therefore, stomata are important in regulating
  the rate of transpiration
• stomatal density the fewer of stomata, the
  less water loss
• opening/closing of stomata by guard cells
  controls water loss
• when guard cells take in water by osmosis they
  become more turgid bowed which increases pore
  size (opens stomata)
• when guard cells lose water, they become flaccid
  the pore closes

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Role of K in Stomatal Opening

• stomata open when K from neighboring epidermal
  cells accumulates in guard cells
• this is because the increase in solute
  decreases ?, causing water to enter
• the K fluxes are coupled to a membrane potential
  generated by proton pumps
• cues to open
• light (due to blue-light receptors that stimulate
  the proton pumps)
• CO2 depletion
• internal clock (circadian rhythm)
• cues to close
• water deficiency
• abscisic acid

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Xerophytes

• plants adapted to arid conditions
• leaf modifications
• small, thick (decreases surface area)
• thick cuticle
• highly reflective leaves
• hairy leaves (holds moisture)
• stomata located in depressions
• metabolic adaptation CAM photosynthesis
• take in CO2 at night so stomata can close during
  the day