Plant%20Transport

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Plant Transport

• Moving water, minerals, and sugars

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Vascular Tissue
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Xylem

• Xylem tissue transports water from roots to
  leaves.
• Xylem vessels are dead at maturity. Why? Think of
  both structural and functional reasons.

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Phloem

• Phloem tissue transports sap (water and sugar)
  from source to sink.
• Phloem vessels are live at maturity, but need
  companion cells. Why?

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Source and Sink

• Source where the sugar starts its journey
  (either where it is produced or stored).
• Sink where sugar ends up (either where it is
  needed or will be stored).

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Transpiration
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Water transport in 3 parts

• Transpiration (or evapo-transpiration) is the
  transport of water and minerals from roots to
  leaves. It involves three basic steps
• Absorption at the roots.
• Capillary action in the xylem vessels.
• Evaporation at the leaf.

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Part 1 Roots

• Roots absorb water and minerals in a 4-step
  process
• Active transport of minerals into root hairs.
• Diffusion to the pericycle.
• Active transport into the vascular cylinder.
• Diffusion into the xylem.

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Mineral and water uptake
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Casparian Strip

• The Casparian strip controls water movement into
  the vascular cylinder of the root.
• Water cannot move between cells. It must move
  through the cells by osmosis. Why is this
  important?

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Microbial helpers

• Mycorrhizal fungi help plants absorb minerals by
  extending the surface area of roots.

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Microbial helpers

• Nitrogen-fixing bacteria in root nodules help
  plants acquire nitrogen.
• N-fixing bacteria are associated mostly with
  legumes and alder trees.

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Step 2 Capillary action

• Cohesion polar water molecules tend to stick
  together with hydrogen bonds.
• Adhesion water molecules tend to stick to polar
  surfaces.

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Capillary action

• Cohesion and adhesion cause water to crawl up
  narrow tubes. The narrower the tube the higher
  the same mass of water can climb.
• Maximum height 32 feet.

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Cohesion-tension theory

• Cohesion between water molecules creates a water
  chain effect.
• As molecules are removed from the column by
  evaporation in the leaf, more are drawn up.

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Back to the roots...

• Pressure differences created by transpiration
  draws water out of the roots and up the stems.
• This creates lower water pressure in the roots,
  which draws in more water.

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Part 3 Evaporation

• Evaporation at the surface of the leaf keeps the
  water column moving.
• This is the strongest force involved in
  transpiration.

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Stomata control

• Guard cells around the stomata are sensitive to
  light, CO2, and water loss.
• Cells expand in response to light and low CO2
  levels, and collapse in response to water loss.

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Stomata

• When stomata are open, evaporation draws water
  out of the leaf. Gas exchange can also occur to
  keep photosynthesis and respiration running.
• When stomata are closed, evaporation cannot
  occur, nor can gas exchange. What happens to
  photosynthesis and transpiration?

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• Water uptake in roots
• Transpiration

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Sugar Transport
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The trouble with phloem

• Phloem tissue is living tissue, unlike xylem.
  When scientists studying how it works cut into
  it, the plants responded by plugging up the
  phloem.

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Aphid helpers

• But aphids can pierce phloem tissue and suck out
  sap without any problem.
• Scientists used aphids to study the flow of sap
  in phloem.

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Sap

• Sap consists of sugar dissolved in water at high
  concentrations usually between 10 and 25.
• Since this is highly concentrated, plants have to
  use active transport to work against a diffusion
  gradient as part of the sap-moving process.

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Pressure-flow theory

• The pressure-flow theory explains how sap moves
  in a plant from source to sink
• Sugars begin at a source and are pumped into
  phloem tube cells.
• Osmosis moves water into the cells and raises
  pressure.
• Pressure moves the sap.

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Pressure flow 1

• The leaf is a source of sugar, since it makes
  sugar by photosynthesis. Glucose and fructose
  made by photosynthesis are linked to make
  sucrose, which does not move easily through the
  cell membranes. Why is this important?

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Pressure-flow 2

• Active transport is used to load sucrose into
  phloem tubes against a diffusion gradient. As
  sugar is loaded into the cell, what else moves in
  on its own? What will happen to the pressure in
  the cell?

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Pressure-flow 3

• The high concentration of sucrose in the sieve
  tube cells of the phloem causes water to move in
  by osmosis, which raises pressure in the cell.
  What happens to the sap?

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Pressure-flow 4

• A developing fruit is one example of a sink.
  Sucrose may be actively transported out of phloem
  into the fruit cells. In a root, sucrose is
  converted into starch, which keeps sugar moving
  in by diffusion.

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Pressure-flow 5

• As the sugar concentration drops in the sieve
  tube cells, osmosis moves water out of the tube.

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Pressure-flow 6

• As water moves out by osmosis, the pressure in
  the sieve tube cells drops. The pressure
  difference along the column of sieve tube cells
  keeps the sap flowing.

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Pressure-flow Review
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• Animated tutorial Phloem Loading