plant transportchapter 37

• Transport systems in plants• Three levels of transport occur in plants:

• Uptake of water and solutes by individual cells.• cell-to-cell transport (lateral transport)• long-distance transport

transport

O2

CO2water and minerals

Lightwater

O2CO2

uptake by cellsCYTOPLASM EXTRACELLULAR FLUID

ATP

H+

H+

H+H+

H+

H+

H+H+

H+

Proton pump generates membrane potential and gradient.

+

+

+

+

+

_

_

_

_

_

• Transport at the cellular level• Passive transport

• transport proteins• Carrier proteins • Selective channels

• active transport• proton pump • cations pulled into cell

CATIONS can now diffuse into the cell

• Water potential and osmosis• osmotic potential• hyperosmotic conditions -- plasmolysis • hypoosmotic conditions -- turgor pressure

• other aspects of water potential• osmotic, pressure, matrix • pure water --> ψ = 0

• solutes --> negative• pressure --> positive• negative pressure?

• water moves from high potential to low potential

lateral transport• mechanisms/pathways

• transmembrane route • symplast route• apoplast route

Cell wall

Cytosol

Vacuole

Plasmodesma

Vacuolar membrane

Plasma membrane

Transmembrane route

Apoplast

Symplast

Symplastic route

Apoplastic route

lateral transport• anatomical considerations --

• Root hairs• mycorrhizae • large surface area of cortical cells

• pathway of water:• soil - epidermis - root cortex - xylem

lateral transport• Soil to epidermis• Epidermis to root

cortex• routes:• apoplastic route• symplastic route --

• diffusion not enough -- active transport

• Root cortex to Xylem• Casparian strip

• suberin• tracheids and xylem

vessels = apoplast

Pathway along apoplast

Casparian strip

Endodermal cell

Pathwaythroughsymplast

Casparian strip

Apoplasticroute

Symplasticroute

Roothair

EPIDERMIS CORTEX ENDODERMIS

Vessels(xylem)

Stele(vascular cylinder)

long distance transport• Bulk flow -- long-distance

transport• vertical axis of the plant• diffusion?• transports water and solutes

• Cohesion and adhesion of water • transmission of negative

pressure• Cohesion • adhesion (matrix pressure)• small diameter of vessels

Xylem sap

Mesophyll cells

Stoma

Water molecule

Atmosphere

Adhesion by hydrogen bonding

Cellwall

Xylemcells

Cohesion by hydrogen bonding

Water molecule

Root hair

Soil particle

Water

Wa

ter

po

ten

tia

l gr

ad

ien

t

TRANSPIRATION

COHESION AND ADHESION IN THE

XYLEM

WATER UPTAKEFROM SOIL

• control of transpiration• Guard cells

• turgid -- guard cells "buckle" • flaccid -- guard cells sag

stomata• open at dawn in response to three cues:

• light • decrease of CO2 in leaf air spaces• internal clock of guard cells

• stomata may close during daytime if:• water deficiency resulting in flaccid cells• mesophyll production of abscisic acid• high temperature

stomata

• photosynthesis-transpiration compromise• anatomical considerations related to transpiration

• large interior surface areas• stomata more concentrated on bottom of leaves• waxy cuticle on surface of leaves

• Benefits of transpiration• pulls water• Assists in mineral transfer from roots to shoots• evaporative cooling

• Wilting?

transpiration xylem sap ascension• Xylem sap

• provides water to all cells of the plant -- provides nutrients (minerals)

• transpiration • Pushing xylem sap: root pressure

• Root pressure --> guttation

• Pulling xylem sap• transpiration-cohesion-tension mechanism

• Transpirational pull

xylem sap ascension

Cuticle Xylem

Upperepidermis

Mesophyll

Lower epidermis

Cuticle

Air space

Microfibrils in cell wall ofmesophyll cell

Stoma

Microfibril (cross section)

Water film

Air-water interface

• PHLOEM SAP BULK FLOW• translocation

• sieve-tube members• form long sieve tubes.• sieve plates

• phloem sap • sucrose• minerals• amino acids • hormones

phloem sap

phloem sap• Source-to-sink transport

• Source • Sink • depend on season.

• tubers? • Direction of flow in phloem can change

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2

34

Vessel(xylem)

Sieve tube(phloem)

SOURCE CELL (LEAF)

SINK CELL (STORAGE ROOT)

Sucrose

H2O

H2O

Bu

lk fl

ow

by

neg

ati

ve p

ress

ure

H2O

Sucrose

Bu

lk fl

ow

by

po

siti

ve p

ress

ure

1Loading of sugar

2Uptake of water

3Unloading of sugar

4Water recycled

phloem sap• Phloem loading and unloading

• loading• symplast • usually apoplastic routes• active transport

• unloading• sometimes active transport• sometimes diffusion

• Bulk flow from sap• pressure-flow mechanism

1

2

34

Vessel(xylem)

Sieve tube(phloem)

SOURCE CELL (LEAF)

SINK CELL (STORAGE ROOT)

Sucrose

H2O

H2O

Bu

lk fl

ow

by

neg

ati

ve p

ress

ure

H2O

Sucrose

Bu

lk fl

ow

by

po

siti

ve p

ress

ure

1Loading of sugar

2Uptake of water

3Unloading of sugar

4Water recycled