LG 3 – Plant Transport

Plant Material TransportMaterial Transport –Passive and Active Transport – Water Movement in Plants –

Transport in Roots Water in Roots – Mineral Active Transport – Transport of Water and Minerals in Xylem –

Mechanical Properties of Water Transpiration – Cohesion-tension Mechanisms of Water Transport – Cohesion-Tension in Tallest Trees -

Leaf Anatomy Stomata – Physiology of Stomata- Arid Adaptations –

Transport of Organic Substances in Phloem Organic Compounds – Sources and Sinks -

Unit IIPlants

Learning Goal 3Examine how materials are

transported throughout the body of a plant.

Plant Material Transport

• Material transport – Short distances between cells– Long distances between roots and leaves

(xylem and phloem)

Fig. 32.2, p. 739Stepped Art

Sugar fromSugar fromphotosynthesisphotosynthesis

Mineral ions

a. Short distance transport across cell membranes into roots

Minerals

Water and solutes fromsoil enter plant roots bypassive or active transportthrough the plasmamembrane of root hairs.

Water and mineral ions travelfrom root hairs into xylemvessels by passing throughor between cells (black arrowinto/out of xylem).

Xylem:transport ofH2O and O2

b. Transport in vascular tissues

Phloem:transport ofsugars Vascular tissue

distributes substancesthroughout the plant,sometimes overgreat distances.

c. Long distance transport throughout the plant

Sugar fromphotosynthesis

Cells load and unloadorganic molecules(including CO2) into andout of phloem (purplearrows to/from phloem).

Passive and Active Transport

• Passive transport requires no metabolic energy– Substance moves down concentration or

electrochemical gradient or by membrane potential

• Active transport requires metabolic energy (ATP)– Substance moves against gradient

Water Movement in Plants• Bulk flow of water due to pressure

differences

• Xylem sap – Dilute water movement from roots to leaves

• Osmosis – Passive movement of water across cell

membrane

• Water potential (Ψ) – Driving force (pressure and/or solutes)

Transport in Roots

Water in RootsApoplastic pathway: Water does not cross cell

membrane, includes dead xylem transport

Symplastic pathway: Water moves through plasmodesmata (openings between plant cells).

Transmembrane pathway:

Water moves between living cells through cell membranes.

Casparian strip in root endodermis forces apoplastic water to symplast

Fig. 32.6, p. 743

In the transmembranepathway (black), water that enters the cytoplasm moves between livingcells by diffusing acrosscell membranes, includingthe plasma membrane and perhaps the tonoplast.

In the apoplastic pathway(red), water moves throughnonliving regions–the continuous network of adjoining cell walls and tissue air spaces. However, when it reaches the endodermis, it passes through one layer of living cells.

In the symplastic pathway(green), water passes into and through living cells. After being taken up into root hairs water diffuses through the cytoplasm and passes from one living cellto the next through plasmo-desmata.

Epidermis

Root hair

Cell wallTonoplast

Plasmodesma

Air spaceEndodermis withCasparian strips

Xylemvesselin stele

Root cortex

Casparian Strips in Roots

Fig. 32.7, p. 744

a. Root b. Stele in cross section(stained)

Exodermis

Rootcortex

Stele

Abutting walls ofendodermal cells

Primaryxylem

Primaryphloem

In root cortex, water moleculesmove through the apoplast,around cell walls and throughthem (arrows).

Endodermal cells withCasparian strip

Stele

Endodermis

c. Casparian strip (from above)

Fig. 32.7, p. 744

Sieve tubes in phloem

d. Movement of water into the stele

Endodermis (one cell thick)

Pericycle (one or more cells thick)

Stele

Tracheids and vessels in xylem

Route water takesinto the stele

Waxy, water-impervious Casparian strip (gold) in abutting walls of endodermalcells that control water and nutrient uptake

Radialwall regionimpregnatedwith suberin

Wall of endodermalcell facing root cortex

Transverse wall regionsimpregnated with suberin

Mineral Active Transport

• Most minerals for growth are more concentrated in root than in soil– Active transport into symplast– Active transport at Casparian strip across

membrane

• Minerals loaded into apoplast of dead xylem in root stele– Transported long distance to other tissues

Transport of Water and Minerals in the Xylem

• Mechanical properties of water have key roles in its transport

• Leaf anatomy contributes to cohesion-tension forces

• In the tallest trees, the cohesion-tension mechanism may reach its physical limit

• Root pressure contributes to upward water movement in some plants

• Stomata regulate the loss of water by transpiration

• In dry climates, plants exhibit various adaptations for conserving water

Mechanical Properties of Water

• Transpiration – Evaporation of water out of plants– Greater than water used in growth and metabolism

• Cohesion-tension mechanism of water transport– Evaporation from mesophyll walls– Replacment by cohesion (H-bonded) water in xylem– Tension, negative pressure gradient, maintained by

narrow xylem walls, wilting is excess tension

Fig. 32.8, p. 746

The driving force of evaporation into dry air

Upper epidermisVeinMesophyll

Growingcells also

removesmall

amountsof water

from xylem.

Cohesion in the xylem of roots, stems, and leaves

Xylem

Vascularcambium

Phloem Water uptake ingrowth regions

Water uptake from soil by roots

Stoma

Stelecylinder

Endodermis Cortex Watermolecule

Roothair

1 Transpiration is the evaporation of water molecules from above ground plant parts, especially at stomata. The process puts the water in the xylem sap in a state of tension that extends from roots to leaves.

2 The collective strengthof hydrogen bonds amongwater molecules, which areconfined within thetracheids and vessels inxylem, imparts cohesion tothe water.

3 As long as watermolecules continue toescape by transpiration, that tension will drive the uptake of replacement water molecules from soil water.

Cohesion-Tension in Tallest Trees

• Transpiration follows atmospheric evaporation– Driving forces: Dryness and radiation– Tallest trees (>110m) near physical limit of

cohesion

• Root pressure occurs in moist to wet soils– Moves water up short distances

• Guttation – Water movement under pressure out leaves

Guttation

Leaf Anatomy

• Stomata

• Transpiration losses of water must be regulated to prevent rapid dessication– Cuticle limits H2O loss but also prevents CO2

uptake– Water is always lost when stomata open for

photosynthesis

Fig. 32.10, p. 748

Guard cell

a. Open stoma b. Closed stoma

Chloroplast(guard cellsare the onlyepidermalcells thathave theseorganelles)

Stoma

Guard cell

Physiology of Stomata

• Stomata must balance H2O loss and CO2

uptake by responding to many signals, biological clock

• Stomata open to increase photosynthesis– Increasing light (blue)

– Decreasing CO2 concentration in leaf

• Stomata close under water stress– Abscisic acid is hormonal signal for closure,

synthesized by roots and leaves

Arid Adaptations

Xenophytes have adaptations to aridity

Thickened cuticle, sunken stomata, water storage in stems

Transport of Organic Substances in the Phloem

• Organic compounds are stored and transported in different forms

• Organic solutes move by translocation

• Phloem sap moves from source to sink under pressure

• Organic Compounds• Translocation

– Long-distance transport of substances via phloem

– Phloem flow under pressure, moves any direction

• Macromolecules broken down into constituents for transport across cell membranes

• Phloem sap composed of water and organic compounds that move through sieve tubes

Transport of Organic Substances in the Phloem

Sources and Sinks

• Source: Any region of plant where organic substance is loaded into phloem– Companion and transfer cells, use free

energy• Sink: Any region of plant where organic

substance is unloaded from phloem• Pressure flow mechanism moves substance by

bulk flow under pressure from sources to sinks– Based on water potential gradients

Fig. 32.15, p. 752

Sieve tube of the phloem

Source(for example,

mature leaf cells)

Sink(for example,developingroot cells)

bulkflow

Solute

Water

3 The pressurethen pushes solutes by bulk flow betweena source and a sink, with water moving into and out of the system all along the way.

5 Solutes areunloaded into sink cells, and the water potential in thosecells is lowered.Water moves out of the seive tube and into sink cells.

1 Active transportmechanisms movesolutes into thecompanion cellsand then into thesieve tube, againstconcentrationgradients.

2 As a result ofthe increased solute con-centration, the water potential isdecreased in thesieve tube, andwater moves in by osmosis, increasing turgor pressure.4 Pressure andsolute con-centrationsgradually decrease between the source and the sink as substances move into the sink from phloem.

LG 3 Vocab Terms

1. Passive Transport -

2. Active Transport -

3. Osmosis -

4. Water Potential -

5. Apoplastic vs Symplastic Pathway -

6. Casparian Strip -

7. Cohesion-Tension Mechanism -

8. Source -

9. Sink -

10. Pressure Flow Mechanism -