Chapter 36: Resource Transport in Vascular Plants

What are the 2 major transport pathways through plants?

- *apoplast*: consists of everything external to the plasma membrane; includes: cell walls, EC spaces, & interior of dead cells such as vessel elements & tracheids - *symplast*: consists of the cytosol of all the living cells in a plant, as well as the plasmodesmata

What are 3 transport routes for water and solutes in plants?

- *apoplastic*: through cell walls and EC spaces - *symplastic*: where water and solutes cross the plasma membrane once and then travel through the cytosol via plasmodesmata - *Transmembrane*: where water & solutes repeatedly cross plasma membranes as they pass from cell to cell

Bulk Flow

- *bulk flow*: movement of a fluid driven by a *pressure gradient* - enhanced by structural adaptations of xylem & phloem cells (still living) --mature tracheids & vessel elements have no cytoplasm, & sieve-tube elements have few organelles in their cytoplasm -- perforation plates connect vessel elements, & porous sieve plates connect sieve-tube elements

Cavitation

- *cavitation*: formation of a water vapor pocket - drought stress or freezing can cause a break in the chain of water molecules through cavitation - xylem sap transport can often continue after cavitation -- can move b/w adjacent xylem cells through pits --can move from xylem to phloem tissue and back again --cavitation can sometimes be repaired --new xylem added by secondary growth

Mechanisms of Stomatal Opening and Closing

- *changes in turgor pressure* open & close stomata --when turgid, guard cells bow outward & pore between them closes --when flaccid, guard cells become less bowed & pore closes - changes in turgor pressure result primarily from the reversible uptake & loss of K+ ions by the guard cells - proton pumps generate the membrane potential required to move K+ across the plasma membrane

Cohesion & Adhesion

- *cohesion*: water molecules are attracted to each other by hydrogen bonds - water molecules exiting the xylem tug on adjacent water molecules down the column - *adhesion*: of water molecules (H bonds) to xylem cell walls helps offset the gravitational force - H bonding forms an unbroken chain of water molecules extending from leaves to the soil

Transport of Water & Minerals into the Xylem

- *endodermis*: innermost layer of cells in root cortex; surrounds vascular cylinder & is the *last checkpoint* for selective passage of minerals from the cortex into the vascular tissue - *casparian strip*: waxy strip of endodermal wall that blocks *apoplastic transfer* of minerals from the cortex to the vascular cylinder - water & minerals in the apoplast MUST cross the plasma membrane of an endodermal cell to enter the vascular cyclinder - endodermis transports needed minerals from the soil into the xylem & keeps out unneeded & toxic substances - endodermis also prevents solutes in the xylem from leaking back into the soil - endodermal cells discharge water & minerals from their protoplasts into their own cell walls - diffusion & active transport are involved in this movement from *symplast to apoplast* - once in the apoplast, water & minerals can enter the tracheids & vessel elements

Short-Distance Transport of Water

- *osmosis*: diffusion of water into or out of a cell that is affected by solute concentration & pressure - *potential*: water's capacity to perform work - *water potential*: determines the direction of movement of water - water flows from regions of higher water potential to regions of lower water potential

Movement from Sugar Sources to Sugar Sinks

- *phloem sap*: aqueous solution that is high in sucrose; travels from sugar source to sugar sink - *sugar source*: organ that is a net producer of sugar, such as mature leaves - *sugar sink*: organ that is net consumer or depository of sugar, such as roots, buds, or fruits - storage organ can be a sugar sink in the summer and a sugar source in the spring

Turgor Pressure

- *turgor pressure*: positive pressure created when the protoplast presses against the cell wall - *protoplast*: living part of the cell including plasma membrane

Adaptations that Reduce Water Loss

- *xerophytes* are adapted to arid climates --some desert plants complete their life cycle during rainy season --others have fleshy stems that store water or leaf modifications that reduce the rate of transpiration --some plants use a specialized form of photosynthesis called *crassulacean acid metabolism (CAM)* in which stomatal gas exchange occurs at night

Part V: Bulk Transport via the Xylem

- *xylem sap*: water & dissolved minerals, is transported from roots to leaves by bulk flow - *transpiration*: evaporation of water from plant's surface; involved in transport of xylem sap - transpired water is replaced as water travels up from the roots

Stomata: Major Pathways for Water Loss

- 95% of water lost by plants through stomata - each stoma is flanked by a pair of guard cells which control diameter & shape of stoma by changing shape - stomatal density is under genetic & environmental control

Water Potential

- abbreviated by psi sign & measured in a unit of pressure called *megapescal* (MPa) - psi = 0 MPa for pure water at sea level & room temp - both solute concentration & pressure affect water potential in plants - water potential = solute potential + pressure potential - *solute potential (s), or osmotic potential*, is directly proportional to molarity - solute potential is always negative - *pressure potential (p)*: is the physical pressure on a solution; can be positive or negative (solution withdrawn by syringe is under negative pressure, when expelled is is under positive pressure)

Pushing Xylem Sap: Root Pressure

- at night, root cells continue pumping up mineral ions into the xylem of the vascular cylinder, lowering the water potential - water flows in from the root cortex, generating *root pressure*, a push of xylem sap - *guttation*: exudation of water droplets on tops or edges of leaves (more water to enter leaves than is transpired) can be caused by root pressure - positive pressure is relatively weak & is a minor mechanism of bulk flow **guttation is different from dew (condensed atmospheric moisture)

Unloading of Sugar

- at sink, sugar molecules diffuse from the phloem to sink tissues & are followed by water

Part II: Short-Distance Transport

- controlled by membrane solubility - both active & passive transport occur - in plants, membrane potential is established through pumping H+ ions by proton pumps - in animals, membrane potential is established through pumping Na+ sodium-potassium pumps - plant cells use E of H+ gradients & membrane potential to *cotransport* other solutes by active *transport* - plant cell membranes have *ion channels* that allow only certain ions to pass

Ascent of Xylem Sap

- driving force is *gradient of water potential* - for bulk flow over long distances, the water potential gradient is mainly due to the gradient of the *pressure potential (p)* - transpiration results in the pressure potential at the leaf end of the xylem being lower than the pressure potential at the root end

Self-thinning

- dropping of sugar sinks such as flowers, seeds, or fruits, occurs when there are more sugar sinks than the sources can support

Part III: Long-Distance Transport

- efficient long-distance transport requires bulk flow - *bulk flow*: movement of a fluid driven by a *pressure gradient* - water & solutes move together through tracheids & vessel elements of xylem & sieve-tube elements of phloem - branching veins in leaves ensure that all cells within a few cells of the vascular tissue - *diffusion, active transport* (short distance), & *bulk flow* (long distance) act together to transport resources throughout the plant

Part VI: Stomata

- leaves have large SAs & high surface-to-volume ratios which can increase the rate of photosynthesis or increase water loss through stomata - *guard cells* open & close stomata to help balance water conservation with gas exchange for photosynthesis

Absorption of Water & Minerals by Root Cells

- most water & mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable - root hair account fro much of the SA of roots - water & minerals enter from epidermal cells in the root cortex through 3 transport routes (apoplastic, symplastic, transmembrane routes) - after soil solution enters the root cortex, the extensive SA of cortical cell membranes enhances uptake of water & selected minerals - *active transport*: enables essential minerals to accumulate at much higher concentrations in roots compared to the surrounding soil

Phloem Loading

- often requires *active transport* - proton pumping & cotransport of scurose & H+ enable the cells to accumulate sucrose

Stimuli for Stomatal Opening and Closing

- open during day & close at night to minimize water loss - opening at dawn is triggered by: --light (activates blue-light receptors in plasma membrane of guard cells which stimulate activity of proton pumps) --CO2 depletion: result of photosynthesis --an internal "clock" in guard cells - drought stress can cause stomata to close during daytime - hormone *abscisic acid* produced in response to water deficiency & causes the closure of stomata

Bulk Flow by Positive Pressure

- phloem sap moves through sieve tube by bulk flow driven by positive pressure (*pressure flow*) - phloem sap flows from sources, where pressure is high, to sinks where pressure is low - mechanism of translocation for angiosperm - but, pores b/w phloem cells in nonflowering vascular plants may be too small to permit pressure flow

Part VII: Sugar Transport in Plants

- products of photosynthesis are transported through phloem by the process of *translocation* - in angiosperms, *sieve-tube elements* are the conduits for translocation

Transport of Sugar to Sieve-Tube Elements

- sugar must be loaded in sieve-tube elements before being exported to sinks - depending on species, sugar may move by symplastic or both symplastic & apoplastic pathways - companion cells enhance solute movement b/w apoplast & symplast

Effects of Transpiration on Wilting & Leaf Temp

- sunny, warm, dry & windy conditions cause evaporation & increase transpiration rates - some evaporative loss can occur through cuticle when stomata closed - if uptake & transport aren't sufficient replace water loss, plant will wilt - transpiration results in *evaporative cooling*, which can lower temp of leaf

Secondary Xylem walls

- thick secondary walls prevent vessel elements & tracheids from collapsing under negative pressure

Pulling Xylem Sap: the Cohesion- Tension Hypothesis

- transpiration & water cohesion pull water from roots to shoots - xylem sap is normally under negative pressure, or *tension*

Aquaporins

- transport protein in the cell membrane that facilitate passage of water - opening & closing of aquaporins affect the rate of osmotic water movement across the membrane

Is sap pushed up from the roots or pulled up by the leaves?

- water flows in the root cortex, generating root pressure that pushes water (but positive pressure is relatively weak & is a minor mechanism) - *Cohesion-tension hypothesis*: transpiration & water cohesion pull water from shoots to roots - xylem sap is normally under negative pressure, or *tension* - see transpirational pull

Water Movement Across Plant Cell Membranes

- water potential affects uptake & loss of water by plant cells - if a *flaccid* cell ( in which the protoplast contracts the cell wall but lack turgor pressure) is placed in an environment with a higher solute concentration, the cell will lose water & undergo plasmolysis - *plasmolysis*: occurs when the protoplast shrinks and pulls away from the cell wall **generally, presence of any solute in water, lowers potential - if a flaccid cell is place in a solution with a lower solute concentration, the cell will gain water & becomes *turgid* - turgor loss in plants causes wilting, which can be reversed when the plant is watered ** if cell water potential is higher than surrounding --> lose water (flaccid) **if cell water potential is lower than surrounding--> gain water (turgid)

Transpirational Pull

- water vapor in the air spaces of a leaf diffuses down its water potential gradient & exits the leaf via stomata - as water evaporates, the air-water interface retreats into the mesophyll cell walls - the surface tension of water at air-water interface creates a *negative pressure potential* - negative pressure potential lowers water potential - water molecules are *pulled* from more hydrated ares of the leaf by the negative pressure potential - *cohesion* of water molecules transfers the pulling forces to the water in the xylem - transpirational pull on xylem sap is transmitted from leaves to roots

Part I: Plant Transport System

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Part IV: Transport of Water & Minerals into the Xylem

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