figure 37.2 the uptake of nutrients by a plant: an overview xylem

1

Xylem and Phloem Flow in Plants

1. Nutrient requirements of plants

2. Overview of flow in plants

3. Lateral (short-distance flow)

4. Long-distance flow in the xylem

5. Phloem flow

Figure 37.2 The uptake of nutrients by a plant: an overview

What makes up the mass ofplants?

1. Water makes up most of theweight of a living plant

2. Carbon (organic)compounds make up most ofthe dry weight of plants

Thus, plants get most of theirmass from water and air, notfrom soil mineral nutrients(inorganic ions).

Table 37.1 Essential Nutrients in Plants

Nutrients and Xylem Flow in Plants





Diffusion vs. Bulk Flow

• Diffusion: Net movement down aconcentration gradient due to the randommotion of individual molecules. (Note:solutes may move independently ofwater.)

• Bulk flow: Movement of water and solutestogether due to a pressure gradient.

• Osmosis: movement of water in or out

• Physical forces drive the transport ofmaterials in plants over a range of distances

• Transport in vascular plants occurs on threescales– Transport of water and solutes by individual

cells, such as root hairs– Short-distance transport of substances from

cell to cell at the levels of tissues and organs– Long-distance transport within xylem and

phloem at the level of the whole plant

2

Figure 36.2 An overview of transport in whole plants (Layer 4)

Note:Short-distance, or“lateral,” flow of fluidsin plants (e.g. from cellto cell, or into the rootform the soil) happensby diffusion

Long-distance flow(i.e. through the xylemand phloem) can onlyhappen by “bulk flow,”i.e. movement that isdriven by pressure

Selective Permeability ofMembranes: A Review

• The selective permeability of a plant cell’splasma membrane– Controls the movement of solutes into and out

of the cell• Specific transport proteins

– Enable plant cells to maintain an internalenvironment different from their surroundings

The Central Role of Proton Pumps• Proton pumps in plant cells

– Create a hydrogen ion gradient that is a formof potential energy that can be harnessed todo work

– Contribute to a voltage known as a membranepotential

CYTOPLASM EXTRACELLULAR FLUID

ATP

H+

H+ H+

H+

H+

H+H+

H+Proton pump generates membrane potentialand H+ gradient.

–

––

–

– +

+

+

+

+

• Plant cells use energy stored in the protongradient and membrane potential to drive thetransport of many different solutes

+CYTOPLASM EXTRACELLULAR FLUID

Cations ( , for example) are driven into the cell by themembrane potential.

Transport protein

K+

K+

K+

K+

K+ K+

K+

K+

–

–

– +

+

(a) Membrane potential and cation uptake

–

–

+

+

• In a mechanism called cotransport: atransport protein couples the passage ofone solute to the passage of another

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

NO3–

NO 3 –

NO3–

NO 3–

NO3

–

NO3 –

–

–

– +

+

+

–

–

– +

+

+

NO3–

(b) Cotransport of anions

H+of through acotransporter.

Cell accumulates anions ( , for example) by coupling their transport to theinward diffusion

H+

H+

H+

H+

H+H+

H+

H+ H+

H+

SS

S

S

S

Plant cells canalso accumulate a neutral solute,such as sucrose( ), bycotransporting down thesteep protongradient.

S

H+

–

–

–

+

+

+

–

–

++–

H+ H+S+

–(c) Contransport of a neutral solute

• The “coattail” effect of cotransport– Is also responsible for the uptake of the sugar

sucrose by plant cells

3

Effects of Differences in WaterPotential

• To survive– Plants must balance water uptake and loss

• Osmosis– Determines the net uptake or water loss by a

cell– Is affected by solute concentration and pressure

• Water potential– Is a measurement that combines the effects of

solute concentration and pressure– Determines the direction of movement of water

• Water– Flows from regions of high water potential to

regions of low water potential

Water Potential• Water moves from areas of higher (more

positive) potential to lower (more negative)• Pressure potential is created by physical

pressure on water (positive) or avacuum/sucking (negative)

• Solute potential is created by a higherconcentration of solutes (= lowerconcentration of water)

Water Potential• Water moves from areas of higher (more

positive) potential to lower (more negative)• Pressure potential is created by physical

pressure on water (positive) or avacuum/sucking (negative)

• Solute potential is created by a higherconcentration of solutes (= lowerconcentration of water)

Ψ= Ψs + ΨpWater pot. = solute pot. + pressure pot.

Quantitative Analysis of WaterPotential

• The addition ofsolutes– Reduces water

potential

0.1 Msolution

H2O

Purewater

ψP = 0

ψS = −0.23

ψ = −0.23 MPaψ = 0 MPa

(a)

• Application of physical pressure– Increases water potential

H2O

ψP = 0.23

ψS = −0.23

ψ = 0 MPaψ = 0 MPa

(b)

H2O

ψP = 0.30

ψS = −0.23

ψ = 0.07 MPaψ = 0 MPa

(c)

4

• Negativepressure(suction)– Decreases

water potential

H2O

ψP = 0

ψS = −0.23

ψ = −0.23 MPa

(d)

ψP = −0.30

ψS = 0

ψ = −0.30 MPa

Water flow in plants iscontrolled by both ofthese forces. Forexample: potassiumconcentration andpressure from the cellwall

• Water potential– Affects uptake and loss of water by plant cells

• If a flaccid cell is placed in an environmentwith a higher solute concentration– The cell will lose water and become plasmolyzed– Cellular potential is greater than environmental

potential

0.4 M sucrose solution:

Initial flaccid cell:

Plasmolyzed cellat osmotic equilibriumwith its surroundings

ψP = 0

ψS = −0.7

ψP = 0

ψS = −0.9

ψP = 0

ψS = −0.9

ψ = −0.9 MPa

ψ = −0.7 MPa

ψ = −0.9 MPa

• If the same flaccid cell is placed in asolution with a lower solute concentration– The cell will gain water and become turgid

Distilled water:

Initial flaccid cell:

Turgid cellat osmotic equilibriumwith its surroundings

ψP = 0

ψS = −0.7

ψP = 0

ψS = 0

ψP = 0.7

ψS = −0.7

ψ = −0.7 MPa

ψ = 0 MPa

ψ = −0 MPa

Transport is also regulated by thecompartmental structure of plant cells

• The plasma membrane– Directly controls the traffic of molecules into

and out of the protoplast– Is a barrier between two major compartments,

the cell wall and the cytosol– Aquaporins are transport proteins that

facilitate water movement across membranes

• A major compartment in most mature plant cells is thevacuole, a large organelle that can occupy as much as90% of more of the protoplast’s volume

• The vacuolar membrane– Regulates transport between the cytosol and the

vacuole

Transport proteins inthe plasma membrane

regulate traffic ofmolecules betweenthe cytosol and the

cell wall.

Transport proteins inthe vacuolarmembrane regulatetraffic of moleculesbetween the cytosoland the vacuole.

Plasmodesma

Vacuolar membrane(tonoplast)

Plasma membrane

Cell wall

Cytosol

Vacuole

Cell compartments. The cell wall, cytosol, and vacuole are the three maincompartments of most mature plant cells.

(a)






5

• In most planttissues– The cell walls and

cytosol arecontinuous fromcell to cell

• The cytoplasmiccontinuum– Is called the

symplast• The apoplast

– Is the continuumof cell walls plusextracellularspaces

Key

Symplast

Apoplast

The symplast is thecontinuum of

cytosol connectedby plasmodesmata.

The apoplast isthe continuumof cell walls andextracellularspaces.

Apoplast

1. Transmembrane route

2. Symplastic route3. Apoplastic route

Symplast

Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another.

(b)

Short distance or lateral transport: movement withintissues and organs radially:

3 routes

• Water and minerals can travel through aplant by one of three routes– Out of one cell, across a cell wall, and into

another cell (transmembrane route)– Via the symplast– Along the apoplast

Bulk Flow in Long-DistanceTransport

• In bulk flow– Movement of fluid in the xylem and phloem is driven

by pressure differences at opposite ends of the xylemvessels and sieve tubes

– Diffusion does not work well over long distances, suchas from roots to leaves

– In xylem it is negative pressure that drives flow(transpiration)

– In phloem it is hydrostatic pressure in one end of thesieve tube that forces sap to the opposite end

• Roots absorb water and minerals from thesoil

• These enter the plant through theepidermis of roots and ultimately flow tothe shoot system through the xylerm

Xylem transport: Root uptakeThe Roles of Root Hairs,

Mycorrhizae, and Cortical Cells

• Much of the absorption of water and minerals occurs near roottips, where the epidermis is permeable to water and where roothairs are located

• Root hairs account for much of the surface area of roots, andgreatly enhance absorption

• Root hairs are extensions of epidermal cells

• The soil solution flows into the hydrophilic cell walls, along theapoplast and into the root cortex

6

• Most plants form mutually beneficial relationshipswith fungi, which facilitate the absorption of waterand minerals from the soil

• Roots and fungi form mycorrhizae, symbioticstructures consisting of plant roots united withfungal hyphae

2.5 mm

Lateral transport of minerals and water in roots

Note the important role of theCasparian strip in the endodermis:blocking the apoplastic pathway intothe stele. Why is this useful to theplant?

• Water can cross the cortex– Via the symplast or apoplast

• The waxy Casparian strip of the endodermalwall– Blocks apoplastic transfer of minerals from the

cortex to the vascular cylinder

Lateral transport in roots

• Once soil solution enters the roots– The extensive surface area of cortical cell

membranes enhances uptake of water and selectedminerals

• The endodermis is a selective sentry– It is the innermost layer of cells in the root cortex– Surrounds the vascular cylinder and functions as the

last checkpoint for the selective passage of mineralsfrom the cortex into the vascular tissue (via the waxycasparian strip)

– All material must pass via the symplast

Lateral transport in roots






Ascent of water in a tree(long-distance flow). Wateris pulled upward byextreme negative waterpotential generated byevaporation out of stomata(transpiration).

7

Figure 36.12 The generation of transpirational pull in a leaf

Cohesion and Adhesion in theAscent of Xylem Sap

• The transpirational pull on xylem sap– Is transmitted all the way from the leaves to the root

tips and even into the soil solution– Is facilitated by cohesion (water molecules to one

another via their polar bonds) and adhesion (to thehydrophilic vessel walls)

– Small diameter of vessels and tracheids increasesadhesive surface

• The movement of xylem sap against gravity– Is maintained by the transpiration-cohesion-

tension mechanism

– Stomata increasephotosynthesis

– Stomata increase water loss– Closing them reduces

photosynthesis and can leadto overheating in plants

20 µm

At night, some xylem sap is also pushed up by rootpressure: roots pump ions in, and water follows,creating high water potential in roots. Guttation (seebelow) is one consequence of this.

However, root pressure cannot push very much xylemsap. Most upward movement of xylem sap is due totranspirational pull, not push by root pressure.

Phloem flow• Products of photosynthesis, organic nutrients

(sugars), are translocated through the phloem

• In angiosperms the specialized cells are calledsieve tube members (with companion cells)

• In gymnosperms these are sieve cells (withalbuminous cells)

• Phloem sap– Is an aqueous solution that is mostly sucrose– Travels from a sugar source to a sugar sink– Also carries minerals, amino acids and

hormones

• A sugar source– Is a plant organ that is a net producer of sugar,

such as mature leaves• A sugar sink

– Is an organ that is a net consumer or storer ofsugar, such as a tuber or bulb

8

Mesophyll cellCell walls (apoplast)

Plasma membrane

Plasmodesmata

Companion(transfer) cell

Sieve-tubemember

Mesophyll cell

Phloem parenchyma cell

Bundle-sheath cell

• Phloem sap moves from sugarsource to sugar sink

• Sugar must be loaded intosieve-tube members beforebeing exposed to sinks

• In many plant species, sugarmoves by symplastic andapoplastic pathways

Sucrose manufactured in mesophyll cellscan travel via the symplast (blue arrows)to sieve-tube members. In some species,sucrose exits the symplast (red arrow)near sieve tubes and is activelyaccumulated from the apoplast by sieve-tube members and theircompanion cells.

Loading of sucrose into phloem: Notice that it can follow either the symplasticor apoplastic pathway. Loading from the latter requires active transport.Loading occurs in sugar sources, such as mature leaves.

High H+ concentration Cotransporter

Protonpump

ATP

Key

SucroseApoplast

Symplast

H+ H+

Low H+ concentration

H+

S

S

• In many plants– Phloem loading requires active transport

• Proton pumping and cotransport ofsucrose and H+

– Enable the cells to accumulate sucrose

Pressure flow in a sieve tube ofthe phloem is from sugar sourcesto sugar sinks.

Sugar loading at sources andunloading at sinks creates apressure gradient that drives bulkflow of the phloem sap.

Sources: Mature leaves, otherphotosynthetic organs, storageorgans (such as roots)

Sinks: Growing shoots, flowers,developing fruits, root tips

Tapping phloem sap with the help of an aphid, which is a phloem-feeder. Anaphid stylus, minus the aphid, functions as a phloem sap pressure probe(right).

Attach stylus near mature leaves, and another near base of plant.

Where would you predict sap pressure to be higher?

Sieve tube member

Stylet

Things to remember aboutwater potential

• Water will diffuse from areas of low soluteconcentration (high potential) to areas ofhigh solute concentration (low potential)

• Water will move from areas of highpressure to areas of low pressure by bulkflow

• Cell membranes are selective in thesolutes allowed to pass through, somesolutes are actively pumped

figure 37.2 the uptake of nutrients by a plant: an overview xylem

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