CHAPTER 36 PLANT TRANSPORT

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CHAPTER 36PLANT TRANSPORT

• I. Absorption of Water Minerals by Roots
• A. Most occurs near tip where epidermis is
  permeable root hairs increase surface area.
  The soil solution (water and dissolved minerals)
  enters epidermal cells via osmosis selective
  transport. The solution travels laterally
  through the cortex via the continuum of cell
  walls. Cells will absorb water and minerals as
  needed.

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Absorption of Water Minerals (cont.)

• B. Water minerals moving through the cell
  walls will eventually reach the endodermis
  which blocks entry into the stele due to the
  Casparian strip. Water minerals that have been
  absorbed by cortex cells (or endodermal
  cells) can move laterally through the endodermis
  and into the stele via plasmdesmata.

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Absorption of Water Minerals (cont.)

• C. Endodermal cells and parenchyma cells within
  the stele discharge water minerals into their
  cell walls via diffusion and active transport.
  Water minerals are now free to move into the
  lumen of the tracheids vessel elements of
  the xylem.

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II. Water Mineral Transport

• A. root pressure - At night when transpiration
  is low, water minerals are still entering the
  stele. Thus water minerals are pushed upward
  in the xylem. Guttation occurs in some plants.
  However in most plants root pressure is not
  enough to balance the water lost in daytime
  transpiration.

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Water MineralTransport (cont.)

• B. transpiration-cohesion-tension mechanism -
  Water evaporates from the stomata through the
  process of transpiration. Water molecules
  exhibit cohesion. Thus as one molecule
  evaporates, the adjacent molecule is pulled to
  the leaf surface. This pull is transmitted from
  the leaves down to the roots. This is aided by
  adhesion of water to the walls of the xylem
  cells.

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III. Control of Transpiration

• A. Morning light triggers the activity of
  proton pumps which use ATP to pump H out of
  the guard cells. This promotes the uptake of
  K. This promotes the uptake of water. This
  causes the guard cells to swell and open the
  stomata.

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Control ofTranspiration (cont.)

• B. At night as photosynthesis continues while
  stomata are closed, CO2 is depleted. Low CO2
  levels also trigger the opening of the stomata.
• C. Plants also use circadian rhythms to control
  the opening of the stomata.
• D. Plants close their stomata in response to
  water deficiency high temperatures (high CO2
  levels)

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IV. Transport of Food

• A. Phloem transports sap to sugar sinks from
  the nearest sugar sources (regardless of their
  locations) through translocation. Sugar travels
  from the source to the phloem by following the
  continuous path of cell walls and/or through
  plasmodesmata. Sugar is loaded into the sieve
  tubes through active transport (proton pumps.)

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Transport of Food (cont.)

• B. Phloem loading results in high solute
  concentration. This causes water to diffuse
  into the sieve tube. This increase in pressure
  causes the sap to flow toward the sink.
• C. At the sink, active transport is used to
  remove sugar from the sieve tube. Thus, water
  diffuses out.