CHAPTER 36

TRANSPORT IN PLANTS

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Overview of Transport in a Vascular Plant

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TERMS TO BE REVIEWED1. passive transport 10. chemiosmosis2. transport proteins 11. proton pump3. active transport 12. membrane potential4. cotransport 13. turgor pressure5. water potential 14. plasmolysis6. osmosis7. solute concentration8. aquaporins9. tonoplast

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I. INTRODUCTION1. Most water and mineral absorption occurs in the cells at the tips of

the roots (Cellular Level).2. Root hairs are modified epidermal cells that are specialized for

water absorption.They absorb soil solution which consists of water molecules and dissolved mineral ions that are not bound tightly to soil particles.

3. Soil solution flows through the hydrophilic walls of epidermal cells and travels along the cell walls and the intercellular spaces into the root cortex4. Movement of soil solution into the cell involves: osmosis, diffusion,

active transport, proton pumps, aquaporins, water potential, cotransport, transport proteins

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II. THREE MAJOR PATHWAYS OF TRANSPORT IN PLANTS

A. Transport is also regulated by the compartmental structure of plant cells

B. The plasma membrane directly controls the traffic of molecules into and out of the protoplastThe plasma membrane is a barrier between two major compartments, the cell wall and the cytosol

C. The third major compartment in most mature plant cells is the vacuole, a large organelle that occupies as much as 90% or more of the protoplast’s volume The vacuolar membrane regulates transport between the

cytosol and the vacuole5

Cell Compartments:

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Cell wallCytosolVacuole

D. In most plant tissues, the cell wall and cytosol are continuous from cell to cellThe cytoplasmic continuum is called the symplastThe cytoplasm of neighboring cells is connected by channels called plasmodesmata

The apoplast is the continuum of cell walls and extracellular spaces

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E. Water and minerals can travel through a plant root by three routes: (Short-Distance Transport)Transmembrane route: out of one cell, across a cell wall,

and into another cellSymplastic route: via the continuum of cytosolApoplastic route: via the cell walls and extracellular spaces

F. Long-Distance Transport at the Whole Plant LevelInvolves movement along the vertical axis (up and down)Involves bulk flow which is the movement of fluid in xylem

and phloem driven by pressure differences at opposite ends of xylem vessels and sieve tubes

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Transport Routes Between Cells

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III. ABSORPTION OF WATER AND MINERALS BY ROOTS

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1. Pathway:Epidermis cortex endodermis stele (xylem)

2. Mineral ions enter epidermal cells by diffusion and active transport using carrier proteins

3. Movement usually a combination of apoplastic and symplastic routes

4. Only minerals and water using the symplastic route move directly into xylem

5. Minerals and water using apoplastic route are blocked at the endodermis by the Casparian strip and must enter an endodermal cell to move into xylem

Lateral Transport

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6. Mycorrhiza hyphae absorb water and selected minerals and can enable older regions of the roots to supply water and minerals to the plants.

Mycorrhiza

IV. TRANSPORT OF XYLEM SAP

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A. Ascent of xylem sap depends on:1. transpiration-the loss of water vapor from leaves and other

aerial parts of the plant2. physical properties of water-cohesion and adhesion

B. Xylem sap (composed of water and minerals) flows upward against gravity

C. Xylem vessels are close to each leaf cellD. Water must move upward to replace that lost by transpiration E. Pushing Xylem Sap: Root Pressure

1. Usually occurs at night when transpiration is low2. Root pressure (upward push of xylem sap) is generated by

accumulation of minerals in stele which lowers the water potential and forces fluid up the xylem

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3. More water entering leaves than is transpired can result in guttation (discharge of water droplets at the leaf margin)

F. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism

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1. Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata by transpiration.

2. Transpiration produces negative pressure in the leaf, which exerts a pulling force on water in the xylem, pulling water into the leaf3. Involves:

Cohesion AdhesionTension (Negative Pressure)

TRANSPIRATION

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4. Cavitation—formation of water vapor pockets in xylem that breaks the chain of water molecules and the pull is stoppedOnce the water chain is broken the xylem vessels is no longer functional

Can occur during drought stress or freezing5. Ascent of xylem sap is ultimately solar powered.

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Ascent of Xylem Sap

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V. CONTROL OF TRANSPIRATION

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A. Introduction1. About 95% of water taken in is lost by transpiration through the stomata2. Amount of water lost by a leaf depends on the number of stomata and the average size of the opening3. Guard cells, by controlling the size of stomata, help conserve

water4. Benefits of transpiration:

a. Assists in mineral transfer from roots to shootsb. Reduces leaf temperatures

5. If transpiration exceeds delivery of water by xylem, plant wilts.6. Rate of transpiration is greatest on a sunny, warm, dry, and windy day

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7. Stomata are more concentrated on bottom of leaf away from the sun to reduce evaporation

8. Waxy cuticle also prevents water lossB. How Stomata Open and Close

1. Guard Cells control stomatal diameter by changing shape.

Turgid Flaccid

Turgid Guard Cells Flaccid Guard Cells

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2. When guard cells take in water, they become turgid and the gap between cells increases.

3. When guard cells lose water, they become flaccid and the gap between cells decreases.

4. Changes in turgor pressure results primarily from the reversible uptake and loss of K+ by guard cells

Stomata open—guard cells accumulate K+ and gain waterStomata closed—guard cells lose K+ and lose water

5. Generally, stomata are opened during the day and closed at night.

Cells Turgid/Stoma Open Cells Flaccid/Stoma Closed

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Cells Turgid/Stoma Open Cells Flaccid/Stoma Closed

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6. Stoma open at dawn because:Light stimulates guard cells to accumulate K+ and become

turgidDecrease of CO2 because of PS

Internal clock of guard cells (circadian rhythms—cycles that have intervals of approximately 24 hours)

7. Stoma close during daytime because of:Water deficiency (environmental stress)Production of abscisic acid

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C. Adaptations to Reduce Transpiration1. Xerophytes—plants adapted to arid climates 2. Modifications of xerophytes:

small, thick leaves thick cuticle store water in fleshy stems during rainy season stomata concentrated on lower leaf surface stomata may be located in depressions CAM pathway for PS

VI. TRANSPORT IN PHLOEM

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A. Introduction1. Translocation—transport of organic products of PS by

phloem throughout the plant-in angiosperms it involves sieve-tube members-direction varies

2. Phloem sap—aqueous solution that is mostly sucrose 3. Sugar source—an organ that is a net producer of sugar

such as mature leaves4. Sugar sink—an organ that is a net consumer or storer of

sugar, such as a tuber or bulb

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B. Movement of phloem sap1. Phloem sap moves from source to sink2. Direction of flow depends on location of sugar source

and sugar sink which depends on the season3. Sugar is first loaded into sieve-tube members before

moving to the sink.4. Can involve:

Symplastic and/or apoplastic routesTransfer cells—modified companion cells with

structures which increase cells’ surface area and enhance transfer between apoplast and symplast

Active transport and cotransport

Loading of Sucrose into Phloem

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5. Sugar will diffuse from phloem to the sink.6. Water follows by osmosis.

C. Pressure Flow (Bulk Flow) of Phloem Sap in Angiosperms1. Phloem sap moves by bulk flow driven by positive pressure (pressure flow)2. Higher levels of sugar at the source lowers the water potential and causes water to flow into the tube3. Xylem recycles the water from sink to source.

Pressure Flow in a Sieve Tube

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