Transport in Plants

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Chapter 36 Transport in Plants
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Question ?

• How do plants move materials from one organ to
  the other ?

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Levels of Plant Transport

• 1. Cellular
• 2. Short Distance
• 3. Long Distance

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Cellular Transport

• The transport of solutes and water across cell
  membranes.

• Types of transport
• 1. Passive Transport
• 2. Active Transport
• 3. Water Transport

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1. Passive Transport

• Diffusion and Osmosis.
• Requires no cellular energy.
• Materials diffuse down concentration gradients.

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Problems

• Usually very slow.
• How can diffusion be assisted?
• Transport Proteins
• Ex. K channel

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Potassium Channel

• Found in most plant cell membranes.
• Allow K but not Na to pass.
• Often gated to respond to environmental stimuli
  (see cell signaling)

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2. Active Transport

• Requires cell energy.
• Moves solutes against a concentration gradient.
• Ex Proton Pumps

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Proton Pump

• Uses ATP to move H out of cells.
• H creates a membrane potential.
• H allows cotransport.

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Membrane Potentials

• Allow cations to moved into the cell.
• Ex Ca2, Mg2
• Allow anions to move by co-transport.
• Ex NO3

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Summary
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3. Water Transport

• Osmosis - water moves from high concentration to
  low concentration.

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Water Potential

• The potential energy of water to move from one
  location to another.
• Abbreviated as y

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Problem

• Cell wall creates a pressure in the cells.
• Water potential must account for this pressure.
• Pressure counteracts the tendency for water to
  move into plant cells.

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Water Potential

• Has two components
• Pressure potential yr
• Solute potential yp
• y yr yp

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Comment

• See the Ts lab handout for more on water
  potential.

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Bulk Flow

• The movement of water between two locations due
  to pressure or tension.

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Bulk Flow

• Much faster than osmosis.
• Tension (negative pressure) pulls water from
  place to place.
• May cause bulk flow against the diffusion
  gradient.

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Plant Vacuoles

• Create Turgor Pressure against the cell wall.
• Affect water potential by controlling water
  concentrations inside cells.

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Tonoplast

• Name for the vacuole membrane.
• Has proton pumps.
• Comment genetic modification of these pumps
  gives plants salt tolerance.

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Proton Pumps

• Drives solutes inside the vacuole.
• Lowers water potential (yp ) inside the vacuole.

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Result

• Water moves into the vacuole.
• Vacuole swells.
• Turgor pressure increases.

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Turgor Pressure

• Important for non-woody plant support.
• Wilting
• Loss of turgor pressure.
• Loss of water from cells.

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Flaccid
Turgid
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Aquaporins

• Water specific facilitated diffusion transport
  channels.
• Help water move more rapidly through lipid
  bilayers.

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Aquaporins with GFP
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Short Distance Transport

• 1. Transmembrane route
• 2. Symplast route
• 3. Apoplast route

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1. Transmembrane

• Materials cross from cell to cell by crossing
  each cell's membranes and cell walls.

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2. Symplast

• The continuum of cytoplasm by plasmodesmata
  bridges between cells.

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3. Apoplast

• Extracellular pathway around and between cell
  walls.

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Long Distance Transport

• Problem diffusion is too slow for long
  distances.
• Answer tension and bulk flow methods.

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Root Hairs

• Main site of absorption of water and minerals.
• Comment - older roots have cork and are not very
  permeable to water.

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Root Cortex

• Very spongy.
• Apoplast route very common.

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Problem

• Can't control uptake of materials if the apoplast
  route is used.

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Solution

• Endodermis with its Casparian Strip.

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Casparian Strip

• Waxy layer of suberin.
• Creates a barrier between the cortex and the
  stele.
• Forces materials from apoplast into endodermis
  symplast.

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Casparian Strip
Endodermis
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Result

• Plant can now control movement of materials into
  the stele.

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Mycorrhizae

• Symbiotic association of fungi with roots of
  plants.
• Help with water and mineral absorption (replaces
  root hairs in some plants).
• May also prevent toxins from entering the plant.

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Mycorrhizae
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Xylem Sap

• Solution of water and minerals loaded into the
  xylem by the endodermis.
• Endodermis - also prevents back flow of water and
  minerals out of the stele.

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Xylem Sap Transport Methods

• 1. Root Pressure
• 2. Transpiration (Ts)

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Root Pressure

• Root cells load minerals into xylem.
• Water potential (yp) is lowered.
• Water flows into xylem.

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Result

• Volume of water in xylem increases
• Xylem sap is pushed up the xylem tissues creating
  root pressure.

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Comments

• Root Pressure limited way to move xylem sap.
• Most apparent at night.
• Excess water may leave plant through Guttation.

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Transpiration (Ts)

• Evaporation of water from aerial plant parts.
• Major force to pull xylem sap up tall trees.

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TCTM Theory

• Transpiration
• Cohesion
• Tension
• Mechanism

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How does TCTM work?

• Water evaporates from leaves, especially from the
  cell walls of the spongy mesophyll.
• Reason water potential of the air is usually
  much less than that of the cells.

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As water evaporates

• Cohesion water molecules sticking together by H
  bonds.
• Adhesion water molecules sticking to other
  materials (cell walls etc.).

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Result

• The loss of water from the leaves creates
  tension or negative pressure between the air
  and the water in the plant.

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Tension causes

• Xylem sap to move to replace the water lost from
  the mesophyll cells.

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Xylem Sap

• Is pulled by the resulting tension all the way
  down the plant to the roots and soil.

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Ts Summary

• Xylem sap moves along a continual chain of water
  potential from
  air? leaf? stem? roots? soil

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Comments

• Tension is a negative pressure which causes a
  decreased in the size of xylem cells.
• Xylem cells would collapse without secondary cell
  walls.

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Factors that Affect Transpiration Rate

• 1. Environmental
• 2. Plant Structures

Multiple Layer Epidermis
Stomatal Crypt
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Homework

• Read Chapter 36, 39
• Chapter 36 Mon. 4/16
• Test 2 next week Chapters 29, 30, 35, 36. A
  few questions may come from 37, 38 and possible
  39.

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Environmental Factors

• 1. Humidity
• 2. Temperature
• 3. Light
• 4. Soil Water Content
• 5. Wind

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Plant Structure Factors

• 1. Cuticle
• 2. Stomate Number
• 3. Hairs

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Stomates

• Openings in the epidermis that allow water and
  gas exchange.
• Controlled by Guard Cells.
• Control rate of Ts and Ps.

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Guard Cells

• Turgid Swell - open stomata.
• Flaccid Shrink - close stomata.
• Size of the cells is a result of turgor pressure
  changes.

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Turgid - Open
Flaccid - Closed
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Turgor Pressure of Guard cells

• Controlled by K concentrations.

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To Open Stomata

• 1. K enters the guard cells.
• 2. Water potential lowered.
• 3. Water enters guard cells.
• 4. Turgor pressure increases.
• 5. Guard cells swell and Stomata opens.

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To Close Stomata

• 1. K leaves guard cells.
• 2. Water leaves guard cells.
• 3. Turgor pressure decreases.
• 4. Guard cells shrink and Stomata close.

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K Movement

• Regulated by proton pumps and K channels.
• Controlled by
• Light (Blue)
• CO2 concentrations
• Abscisic Acid (water stress)

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Comment

• Plant must balance loss of water by transpiration
  with CO2 uptake for Ps.

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Adaptations for Balance

• C4 Ps
• CAM Ps

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Phloem Transport

• Moves sugars (food).
• Transported in live cells.
• Ex Sieve Companion Cells

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Source - Sink Transport

• Model for movement of phloem sap from a Source to
  a Sink.

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Source

• Sugar production site
• Ex Ps Starch breakdown in a
  storage area.

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Sink

• Sugar uptake site.
• Ex Growing areas Storage areas
  Fruits and seeds

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Comment

• The same organ can serve as a source or a sink
  depending on the season.

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Result

• Phloem transport can go in two directions even in
  the same vascular bundle.

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Xylem Transport In Contrast to Phloem

• Usually unidirectional.
• Endodermis prevents back flow.
• Dead cells.

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Phloem Loading at the Source

• 1. Diffusion
• 2. Transfer Cells
• 3. Active Transport

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Phloem Loading
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Transfer Cells

• Modified cell with ingrowths of cell wall to
  provide more surface area for sugar diffusion.

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Result

• Sugar loaded into phloem.
• Water potential (yp) decreases.
• Bulk flow is created.

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Bulk Flow

• Movement of water into phloem.
• Pressure forces phloem sap to move toward the
  sink.

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At the Sink

• Sugar is removed.
• Water potential is raised.
• Water moves out of phloem over to xylem.

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Phloem summary

• Source - builds pressure.
• Sink - reduces pressure.
• Pressure caused by
• Sugar content changes
• Water potential changes

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Comment

• Plants move materials without "moving" parts,
  unlike animals.

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Summary

• Know various ways plants use to move materials.
• Know how Ts works and the factors that affect Ts.
• Know how phloem transport works.