Transportation in Plants-Mine

8/8/2019 Transportation in Plants-Mine

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Transportation in Plants

The mechanisms of the

movement of water and glucose


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Water Potential

It is water potentialcombined with adhesive

and cohesive forces that

allow water to move UP the

plant against the pull of

gravity

TRANSPIRATION is the

escape of water molecules

in the form of vapor that

contributes most totransporting water from the

roots to the shoots

Highest water

potential

Lowest waterpotential


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Water Potential

Pure water has a waterpotential of zero

The more solutes present,the more negative thepotential

Water moves from areas

of high water potential tolow water potential

Water moves in byosmosis from the roothairs.

Thus the soil has as higherwater potential than theroot hair

The outside airsurrounding the leaf hasthe least water potential ofall!


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Mechanics ofWaterMovement

Water moves by two different pathways toward the center ofthe root.

Apoplast--THROUGH cell walls without ever entering thecells!

Symplast--THROUGH the plasmodesmata between cells


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The pathway along apoplast involves the

movement of water through the nonliving

portions of cells


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The pathway through symplast involves the living

portion of the cells.

It moves from the cytoplasm of one cell to the cytoplasm

of another cell through PLASMODESMATA


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When water reaches the endodermis, it can

continue to the vascular cylinder ONLY thorough

the symplast pathway.


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Figure 36.6 Compartments of plant cells and tissues and routes for lateral transport


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Mechanism ofWaterMovement

1. OSMOSIS

Water moves into root hairs by osmosis from the

soil

The gradient is established by

Constant movement of water out of soil

Higher mineral concentration in the stele by selective

passage of ions through the endodermis

Root pressurethe movement of water into the

root by this concentration gradient forces water

UP the xylem


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This root pressure isevidenced by Guttation

Guttation is when tinydroplets of water and

minerals (sap) is foundat the ends of leaves ofgrasses and small herbsin the early morning.

Root pressure is NOTstrong enough to have amajor effect on watertransport in largerplants!


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2. CAPILLARY ACTION (capillarity)

The rise of liquids in narrow tubes

The height of the column depends on an equilibrium ofthe forces of adhesion, cohesion and gravity


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Cohesionattractive force

between like molecules

Adhesionattractive force

between unlike molecules

Although capillarity aids in

the movement of water UP

a plant, it is transpirationthat is the dominant

driving force for water

movement

Water exhibits hydrogen

bonding so it has a

strong cohesive force


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As water climbs due to adhesion, itbrings other water molecules along due

to its cohesive force as if the moleculeswere holding hands

Water does NOT form a meniscus inxylem which is WHY capillarity is not the

driving force for water moving up a plant


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Figure 36.0x Trees


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3. COHESION-TENSIONTheory

Root pressure andcapillarity make minor

contributions to watermovement.

TRANSPIRATIONtheevaporation of water fromplants through openstomata causing negativepressure or tension todevelop within leaves andxylem tissue.


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The role of the stomata is to balance the intake

of CO2 and the loss of H2O

In other words, balance the rate of

photosynthesis with the rate of transpiration


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Stomata close when temperature are high

Reduces loss of water

C4 and CAM plants have adaptations to continue

photosynthesis when stomata close.


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Stomata open when CO2 concentrations are low

in the leaf

Increases loss of water

Allows photosynthesis to take place


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Stomatal opening is accompanied by diffusion of

K+ ions INTO the guard cells

Water follows the K+ ions into the cells making

them turgid

The filling of the

vacuole with first K+ions and then water

causes the guard

cells to widen the

stomatal opening.


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Mechanism of SUGAR Movement

TRANSLOCATION isthe movement ofcarbohydrates

through phloem froma source [leaf] to asink [where the foodis utilized like a stemor root]

It is described by thepressure flowhypothesis


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1. Sugars enter sieve-tubemembers

Soluble carbohydrates

(glucosenot starch)move from site ofphotosynthesis (palisadeor spongy mesophyll) byactive transport

This establishes aconcentration gradient inthe sieve-tube membersthat is HIGHER than thatat the sink (a root)


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2. Water enters the sieve-

tube members as a

result of the solutes

moving in What happens to the

pressure within the

tube?


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3. Pressure in the sieve-tube

members at the source moves

water and sugars to the sieve-

tube members at the sink

through sieve-tubes. In other

words.

When water floods the sieve-tube,

the pressure builds

So, water is forced to keepmoving since the cell wall is too

rigid to expand

There is nothing for water to do

but move through the nearest

escape hatch


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4. Pressure is reduced in sieve-tube members at the sink assugars are removed forutilization by nearby cells.

As water and sugars move by

bulk flow from source

sink,pressure builds at the sink

But, a sink is an area wherecarbohydrates are being utilized

So, sugar is being removed byactive transport

This increases the waterconcentration within the sieve-tube members

Water then diffuses OUT of thecell into the xylem relieving thepressure


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Any cell can act as a

sink IF it stores sugar

as starch since starch

is INSOLUBLE Any cell can act as a

source IF it breaks

down starch and

forms SOLUBLEsugars

Glucose

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Transportation in Plants-Mine