Plant Transport

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Plant Transport
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Types of Transport

• Xylem- Transports water and minerals up from
  roots
• Phloem - Transports sugar (sap) down from leaves
  to storage or up from storage to where needed

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Xylem Transport

• Transpiration-Cohesion-Tension Theory
• Water molecules leave through stomates by
  transpiration
• Water is brought up in a continuous column
  through xylem because of the pull of transpiration

Water pulled up xylem is similar to water being
sucked through a straw
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Figure 36.11 Ascent of water in a tree
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Xylem Cells

• Xylem cells are dead, with just tough cell walls
  remaining. This is important so that the
  negative sucking pressure does not cause the
  walls of the xylem to cave in.

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Water Properties

• Due to hydrogen bonding, the water molecules
  stick to each other (cohesion) and also to the
  surface of the xylem cells (adhesion)

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Figure 36.7 Lateral transport of minerals and
water in roots
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Transpiration

• Transpiration is a necessary evil - stomata
  must be open for gas exchange for photosynthesis,
  but at the same time, 90 of all water taken in
  by roots is lost through these openings

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Regulation of Transpiration

• Guard cells regulate the rate of transpiration
• To open, potassium is pumped into guard cells and
  water follows by osmosis. The plump guard cells
  bow outward and open
• To close, water diffuses outward and the cells
  relax and flop together, closing the stoma

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Conditions affecting rate of transpiration

• Under which conditions do you think the most
  transpiration will occur?
• ____Light or ____ Dark?
• ____Dry or ____Humid?
• ____Calm or ____Breezy?
• ____Hot or ____Cold?

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Phloem Transport

• Phloem sieve tube cells are living and rely on a
  diffusion mechanism to pass materials from cell
  to cell
• Phloem carries sugars made in photosynthesis in
  the leaves down to storage areas in roots and
  stems
• When sugars are needed in the plant, phloem can
  carry sugars up from storage areas to where they
  are needed

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Phloem Transport Pressure Flow Hypothesis
(Source to Sink)

• 1. Glucose from leaf palisade cell (source)
  diffuses into sieve tube element
• 2. Because water concentration in sieve cell
  decreases (water potential decreases), water
  enters by osmosis
• 3. Sieve tube element swells, pressure builds,
  glucose and water diffuse into next cell down and
  so on until molecules reach a storage cell (sink)

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Figure 36.17 Pressure flow in a sieve tube
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Figure 36.12 An open (left) and closed (right)
stoma of a spider plant (Chlorophytum colosum)
leaf
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Figure 36.13a The mechanism of stomatal opening
and closing
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Figure 36.13b The mechanism of stomatal opening
and closing