question ? u how do plants move materials from one organ to the other ?
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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: Ca+2, Mg+2
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
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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:
Solute potential:
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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
()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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Endodermis
Casparian Strip
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Result
Plant can now control movement of materials into the stele.
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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 () 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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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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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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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 ()
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.