Plant Transport

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Plant Transport

• Moving water, minerals, and sugars

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Chapter 42 Plant Anatomy Nutrient Transport
Quiz this Thursday!

• 42.1 - How Are Plant Bodies Organized How Do
  They Grow? p. 860
• 42.2 - The Tissues and Cell Types of Plants? p.
  862
• 42.3 - The Structures, Functions of Leaves,
  Roots, Stems? p. 865
• 42.4 - How Do Plants Acquire Mineral Nutrients?
  p. 873
• 42.5 - How Do Plants Move Water Upward from Roots
  to Leaves? p. 876

Chapters on Reproduction and Hormones (plant
responses)
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Vascular Tissue
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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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Xylem

• Xylem tissue transports water from roots to
  leaves.
• Xylem vessels are dead at maturity.

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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.

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• Animated tutorial Nutrient uptake

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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.

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Thinking question

• What would happen in a root that had no Casparian
  strip? Why would this be a problem?

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Microbial helpers

• Microbes in the soil help plants absorb
  nutrients
• Mycorrhizal fungi help absorb minerals by
  extending the surface area over which minerals
  are absorbed.

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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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Thinking Question

• Suppose a gardener notices that several of her
  favorite flowering plants are infected with a
  fungal disease. She decides to spray not only the
  plants, but the soil all over her garden to
  prevent the disease. What negative effect could
  this have on her plants?

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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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Thinking question

• If the forces of cohesion-tension theory move
  water up a stem, what happens to water pressure
  at the roots? Will that affect water moving into
  the roots?

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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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• Transpirationhttp//www.kscience.co.uk/animation
  s/transpiration.htm

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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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• Guard Cell Function http//www.phschool.com/scie
  nce/biology_place/labbench/lab9/stomamov.html

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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. Photosynthesis and
  transpiration slow down.

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Thinking question

• Suppose you were testing transpiration in trees
  on a warm, humid summer day, and again on a windy
  day when the air is dry. How would transpiration
  on these two days differ?

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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.

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Pressure-flow 2

• Active transport is used to load sucrose into
  phloem tubes against a diffusion gradient.

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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 and causes the
  sap to move.

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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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Thinking question

• Sugars are often stored in roots, sometimes as
  starch. Why can a root be both a source and a
  sink when it comes to sugar transport?

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• Pressure-Flow animationhttp//www.tvdsb.on.ca/we
  stmin/science/sbioac/plants/sucrose.htm
• Plant transport animated tutorial