chapter 36 transport in plants
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Chapter 36 Transport in Plants. How do plants move materials from one organ to the other ?. Question ?. Levels of Plant Transport. 1. Cellular 2. Short Distance 3. Long Distance. The transport of solutes and water across cell membranes. Types of transport: 1. Passive Transport - PowerPoint PPT PresentationTRANSCRIPT
Question ?
How do plants move materials from one organ to the other ?
Levels of Plant Transport
1. Cellular
2. Short Distance
3. Long Distance
Cellular Transport
The transport of solutes and water across cell membranes.
Types of transport:
1. Passive Transport
2. Active Transport
3. Water Transport
1. Passive Transport
Diffusion and Osmosis. Requires no cellular energy. Materials diffuse down
concentration gradients.
Problems
Usually very slow. How can diffusion be
assisted? Transport Proteins
Ex. K+ channel
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)
2. Active Transport
Requires cell energy. Moves solutes against a
concentration gradient. Ex: Proton Pumps
Proton Pump
Uses ATP to move H+ out of cells.
H+ creates a membrane potential.
H+ allows cotransport.
Membrane Potentials
Allow cations to moved into the cell.
Ex: Ca+2, Mg+2
Allow anions to move by co-transport.
Ex: NO3
Summary
3. Water Transport
Osmosis - water moves from high concentration to low concentration.
Water Potential
The potential energy of water to move from one location to another.
Abbreviated as
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.
Water Potential
Has two components: Pressure potential:
Solute potential:
Comment
See the Ts lab handout for more on water potential.
Bulk Flow
The movement of water between two locations due to pressure or tension.
Bulk Flow
Much faster than osmosis. Tension (negative pressure)
pulls water from place to place.
May cause bulk flow against the diffusion gradient.
Plant Vacuoles
Create Turgor Pressure against the cell wall.
Affect water potential by controlling water concentrations inside cells.
Tonoplast
Name for the vacuole membrane.
Has proton pumps. Comment – genetic
modification of these pumps gives plants salt tolerance.
Proton Pumps
Drives solutes inside the vacuole.
Lowers water potential
()inside the vacuole.
Result
Water moves into the vacuole. Vacuole swells. Turgor pressure increases.
Turgor Pressure
Important for non-woody plant support.
Wilting: Loss of turgor pressure. Loss of water from cells.
Flaccid Turgid
Aquaporins
Water specific facilitated diffusion transport channels.
Help water move more rapidly through lipid bilayers.
Aquaporins with GFP
Short Distance Transport
1. Transmembrane route
2. Symplast route
3. Apoplast route
1. Transmembrane
Materials cross from cell to cell by crossing each cell's membranes and cell walls.
2. Symplast
The continuum of cytoplasm by plasmodesmata bridges between cells.
3. Apoplast
Extracellular pathway around and between cell walls.
Long Distance Transport
Problem: diffusion is too slow for long distances.
Answer: tension and bulk flow methods.
Root Hairs
Main site of absorption of water and minerals.
Comment - older roots have cork and are not very permeable to water.
Root Cortex
Very spongy. Apoplast
route very common.
Problem
Can't control uptake of materials if the apoplast route is used.
Solution
Endodermis with its Casparian Strip.
Casparian Strip
Waxy layer of suberin. Creates a barrier between the
cortex and the stele. Forces materials from
apoplast into endodermis symplast.
Endodermis
Casparian Strip
Result
Plant can now control movement of materials into the stele.
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.
Xylem Sap Transport Methods
1. Root Pressure
2. Transpiration (Ts)
Root Pressure
Root cells load minerals into xylem.
Water potential () is
lowered. Water flows into xylem.
Result
Volume of water in xylem increases
Xylem sap is pushed up the xylem tissues creating root pressure.
Comments
Root Pressure: limited way to move xylem sap.
Most apparent at night.
Excess water may leave plant through Guttation.
Transpiration (Ts)
Evaporation of water from aerial plant parts.
Major force to pull xylem sap up tall trees.
TCTM Theory
Transpiration Cohesion Tension Mechanism
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.
As water evaporates:
Cohesion: water molecules sticking together by H bonds.
Adhesion: water molecules sticking to other materials (cell walls etc.).
Result
The loss of water from the leaves creates “tension” or negative pressure between the air and the water in the plant.
Tension causes:
Xylem sap to move to replace the water lost from the mesophyll cells.
Xylem Sap
Is “pulled” by the resulting tension all the way down the plant to the roots and soil.
Ts Summary
Xylem sap moves along a continual chain of water potential from: air leaf stem roots soil
Factors that Affect Transpiration Rate
1. Environmental
2. Plant Structures
Multiple Layer Epidermis
Stomatal Crypt
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.
Environmental Factors
1. Humidity
2. Temperature
3. Light
4. Soil Water Content
5. Wind
Plant Structure Factors
1. Cuticle
2. Stomate Number
3. Hairs
Stomates
Openings in the epidermis that allow water and gas exchange.
Controlled by Guard Cells. Control rate of Ts and Ps.
Guard Cells
Turgid: Swell - open stomata. Flaccid: Shrink - close stomata. Size of the cells is a result of
turgor pressure changes.
Turgid - Open Flaccid - Closed
Turgor Pressure of Guard cells
Controlled by K+ concentrations.
K+ Movement
Regulated by proton pumps and K+ channels.
Controlled by: Light (Blue) CO2 concentrations Abscisic Acid (water stress)
Comment
Plant must balance loss of water by transpiration with CO2 uptake for Ps.
Phloem Transport Moves sugars (food). Transported in live cells.
Ex: Sieve & Companion Cells
Source - Sink Transport
Model for movement of phloem sap from a Source to a Sink.
Source
Sugar production site Ex: Ps
Starch breakdown in a storage area.
Sink
Sugar uptake site. Ex: Growing areas
Storage areas Fruits and seeds
Comment
The same organ can serve as a source or a sink depending on the season.
Result
Phloem transport can go in two directions even in the same vascular bundle.
Xylem Transport: In Contrast to Phloem
Usually unidirectional. Endodermis prevents back
flow. Dead cells.
Phloem Loading at the Source:
1. Diffusion
2. Transfer Cells
3. Active Transport
Phloem Loading
Transfer Cells
Modified cell with ingrowths of cell wall to provide more surface area for sugar diffusion.
Result
Sugar loaded into phloem.
Water potential ()
decreases. Bulk flow is created.
Bulk Flow
Movement of water into phloem.
Pressure forces phloem sap to move toward the sink.
At the Sink:
Sugar is removed. Water potential is raised. Water moves out of phloem
over to xylem.
Phloem: summary
Source - builds pressure. Sink - reduces pressure. Pressure caused by:
Sugar content changes Water potential changes
Comment Plants move materials
without "moving" parts, unlike animals.
Summary
Know various ways plants use to move materials.
Know how Ts works and the factors that affect Ts.
Know how phloem transport works.