THE UPTAKE AND TRANSPORT OF WATER AND MINERALS

OBJECTIVES….

• The uptake of ions by active transport• The entry of water into plant roots in terms of

water potential• The structure of xylem vessels• The ascent of water in plants• Transport in the phloem

Vascular Plants: Stems

• Function of stems– Support, transport of water & food • Most stems grow upward• Some stems grow sideward

– Types of stems• Green• Woody

– Transport of materials• Xylem & phloem

Overview of transport in vascular plant

Plant Transport Tissues

Xylem – transport water from roots to shoots vessel elements tracheids

Phloem – transport sugar from they produced/stored that needed for growth

sieve tube member companion cells

Springwood

Summer WoodVascularcambium

phloemCork cambium

CORK

Phloem rays

3 scales of plant transport

1) Intracellular – transport of water and solutes by individual cell (ex: root hairs)2) Short distance – cell to cell at the levels of tissues and organs (ex: loading of sugar from

photosynthetic leaf cells into sieve tubes)3) Long distance – xylem and phloem at the

whole level of plant

Uptake of Ions by Active transport

Active transport –pumping of solutes across membranes against electrochemical gradient

proton pump provide energy for solute transport- uses ATP to pump ion H+ out of the cell - concentration of ion H+ is higher outside of

cell than inside membrane potential

• Inside of cell negative in charge relative outside

• Contributes to voltage

• Two forms of potential energy used to drive the transport of solutes

a) Uptake of K+ ions by root cells

b) Cotransport of ions -cell accumulates anions

(NO3 -) by coupling their transport to the inward diffusion of ion hydrogen through cotransporter

c) Plants accumulate neutral solutes (sucrose) by cotransporting H+ down steep proton gradient

Effects of Water Potential

• Water uptake or loss by the process of osmosis

• Since plant cell has cell wall cause physical pressureCombined effects of solute concentration and physical pressure into measurement

= water potential (ψ)

Water Potential

• Determines the direction of movement of water

• free water (not bound to solutes or surface) moves from regions of higher water potential to region of lower water potential (free flow)

Water Potential of pure water???

• ψ s is solute potential (osmotic potential)

• ψP is pressure potential. Physical pressure on a solution. It can be negative or positive relative to atmospheric pressure (ex: water in dead xylem cells of transpiring plant less than -2MPa).

Water Potential Equation:Ψ = ψ s+ ψP

0.4 M Sucrose Solution

Aquaporin proteins

* Protein that helps to transport water molecules across vacuole and plasma membrane

* No effect on water potential gradient or the direction of water flow but the rate of water diffusion

3 major compartment of vacuolated plants:a) Cell wallb) Cytosolc) Vacuole

In most plants, cell wall and cytosol are continuous from cell to cell Plasmodesmata connects the cytosoliccompartments of neighboring cells formingcontinuous pathway between cells

• 2 pathway : a) symplast pathway b) Apoplast pathway

Symplast Pathway

• Through the cytoplasm • Water enters the root hair cells across the partially permeable

membrane by osmosis• Water moves from higher in the soil to the lower in the cell• Water moves across the root from cytoplasm to cytoplasm

down the gradient• It passes from one cell to the other via plasmodesmata• Water moves into the xylem by osmosis• The only way across the endodermis• Normally the most important pathway

The Apoplast Route

• Water moves through the cellulose cell wall and intercellular spaces

• The permeable fibres of cellulose do no resist water flow

• Water cannot pass the endodermis by this route• Because the Casparian strip in the endodermis cell wall

is impermeable to water• So all water must pass the endodermis via the

cytoplasm• Therefore it is under cellular control• Apoplast route is important when transpiration rates are

high as it is faster and requires no energy

* Casparian strip - is made of suberin, which is impermeable to water. Water is unable to pass through the endodermis by the apoplast route

Bulk Flow in Long Distance Transport- Movement of fluid driven by pressure- water and solutes move through tracheids

and vessels of xylem ;sieve tube of the phloem

Lateral Transport of minerals & water in roots

1) Uptake of soil solution by the hydrophilic walls of root hairs provide access to the apoplast.

2) Minerals & water cross the plasma membrane of root hairs enter the symplast

3) Casparian strip block the passage of water and dissolved minerals. Only minerals in the symplast pathway pass into vascular cylinder

4) Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls.

Tracheids - elongated cells in the xylem of vascular plants

that serve in the transport of water and minerals

- All tracheary elements develop a thick lignified cell wall, it lack protoplast when mature (become part of apoplast)

- Water & minerals free to enter

Water & Minerals from Roots to Shoots trough Xylem

1. Root Pressure

2. Transpiration -Cohesion-Tension

3. Capillarity

Root Pressure: Pushing xylem sap• Water is pushed up the xylem by hydrostatic pressure (during

transpiration very low or zero at night)

• Mineral salts are pumped into the xylem vessels in the root by the endodermal cells

• Lowering the in the xylem and creates root pressure

• Water moves in from the surrounding cells by osmosis

• Raising the hydrostatic pressure so pushing water up the xylem• Root pressure is minor mechanism and many plants do not

generate root pressure

Root Pressure: Evidence1. Cut stumps of plants exude water from their cut ends

2. In certain conditions some leaves exude water from their leaves = guttation

3. Pressures recorded by mercury manometers attached to the cut stumps could push water in the xylem up to 30m

Guttation

Transpiration-Cohesion-Tension

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

stoma

cuticle

cuticle

water vapour diffuses into the air down gradient

xylem

lowest in the air

water evaporates from the spongy mesophyll cell surface lowering cell

water moves into cells down gradient by osmosis

water is pulled along the xylem

Transpiration pull

• Water vapor in air spaces of leaf (stomata) diffuses out of the leaf when outside leaf is drier (lower water potential) by diffusion and evaporation – Transpiration

• How does loss if water vapor from leaf translate into pulling force for upward movement of water through plant???

• Negative pressure causes water to move up through xylem • Water brought to leaves via xylem in leaf veins and then

drawn into mesophyll cell walls• Depends on adhesion of water to hydrophilic components

in cell wall

• More evaporation of water increases surface tension, pressure at air water interface becomes negative

• Water molecules from more hydrated parts of the leaves are then pulled toward this area, reduce the tension (higher potential to lower potential)

• These pulling forces are transferred into xylem• Increasing negative pressure at air water interface

causes xylem to lose water to mesophyll cells that diffuse out through stomata

= Transpiration pull

The Cohesion Tension Hypothesis for Movement of Water up the Xylem Vessels

Water evaporates from the spongy mesophyll cells and diffuses into the atmosphere

Transpiration

Lower in the leaf cells

Water moves from down the gradientWater is pulled up xylem vessels

Lower pressure/tension at top of xylem

Cohesive forces between water molecules prevent water column breaking

Water moves across root from soil down gradient

Via the apoplast and symplast paths

Mechanism of Opening and Closing of Stomata

How stomata helps to regulate the transpiration rate????

Translocation of Organic Nutrients through Phloem

Movement from Sugar Sources to Sugar Sinks

• Phloem sap is an aqueous solution consist of sugar (mainly disaccharide sucrose in most species) ,minerals, amino acids and hormones

• The sucrose conc. can be as high as 30% by weight that gives the sap syrupy thickness

• direction of phloem sap is variable• sieve tubes carry sugar from sugar source to

sugar sink• sugar source & sugar sink???

* Sugar source – plant organ that is a net producer of sugar (ex: mesophyll cell)

* Sugar sink – organ that is net consumer or storer of sugar ( ex: buds, stems, tuber). Receives sugar from nearest sources

direction of transport depends on location for each sieve tube – may carry sap in opposite direction

Direction of flow vary by season or developmental stage

• Sugar must be loaded into sieve tube members before exported to sink

a) Move from mesophyll cell to sieve tube members via symplast way trough plasmodesmata

b) Moves by symplastic and apoplastic pathway

• In maize and many other plants, phloem loading needs active transport due to the higher concentration of sucrose in sieve tube members

• uses proton pump and cotransport• phloem unload sucrose at the sink end of sieve

tube• Concentration of free sugar in sink is lower than

in sieve tube because the unloaded sugar consumed during growth & metabolism /converted to starch

1) Loading of sugar into sieve tube at source ↓ Ψ inside sieve tube members. It causes tube to take up water by osmosis

2) Uptake water generates positive pressure that forces sap to flow along the tube

3) Pressure is relieved by unloading sugar & water loss from tube at the sink

4) In the case of leaf-to-root translocation, xylem recycles water from sink to source

THANK YOU!!