Ch:36Transport in Vascular Plants

By:Stephanie TuminelloPatrick SingerEsther UrenaMelissa GiammarinoSolange Beckles

Transport in Vascular Plants Occurs at 3 Levels

1-Transport of water and solutes in individual cells

2- short distance transport at organ and tissue level from cell to cell

3-long distance transport in xylem and phloem at the level of the whole plant

Proton Pumps

uses ATP to pump H+ across cell membrane through active transport

Contributes to voltage called membrane potential Membrane potential is separation of charges

across a membrane Plants use membrane potential and energy in the

gradient to transport solutes Cotransport couples passage of one solute to the

passage of another in the opposite direction In plants sucrose is cotransported with H+

Water potential Osmosis is passive transport of water across a

membrane Water potential(Ψ) is the combination of solute

concentration and pressure Water moves from regions of higher water potential to

lower water potential Solute potential is proportional to the amount of

dissolved solute particles Solute particles bind to water molecules and reduce

the amount of free water Pressure potential is the amount of pressure exerted

on the solution

Water Potentials Effect on Cells

Cells that have the same water potential as the surrounding solution are flaccid

If the solution has a lower water potential than the cell water will leave the cell and the cell will plasmolyze

When plasmolyed, the protoplast will shrink and pull away from the cell wall

If the solution has a higher water potential than the cell water will enter the cell and it will become turgid, the ideal state for plants

Water often crosses membranes through proteins called aquaporins

The Three Compartments of Cells

The plasma membrane is selectively permeable barrier between the cell wall and the cytosol

Most plant cells also have vacuoles The tonoplast, which is the vacuolar membrane, regules

molecular traffic between the cytosol and cell sap The tonoplasts H+ gradient is used to move ions across

the vascular membrane Plasmodesmata connect the cytosol of neighboring cells

the continuum of cytosol of neighboring cell is the symplast

The continuum of cell walls and the extercelluar space is the apoplast

3 Routes of Short-Distance Transport In the transmembrane route is when

substances move out of one cell and across the cell wall to enter the neighboring cell

In the symplastic route solutes and water move from cell to cell via the plasmodesmata after entering one cell

In the apoplastic route solutes and water move along the byways provided by the continuum of cell walls

Long-Distance Transport Bulk Flow, how long distance transport

occurs, is the movement of a fluid driven by pressure

Bulk Flow occurs through the tracheids and vessels of the xylem, driven by negative pressure

Transpiration is the evaporation of water from a leaf which reduces pressure and creates tension pulling sap from the roots

Bulk flow occurs in the sieve tubes of the phloem

Roots Absorb Water and Minerals From Soil Most absorption occurs at root tips where the

epidermis is permeable to water. Root hairs are extensions of the epidermal cells. Soil particles coated with water and minerals

attach to root hairs. This solution then flows along the apoplast into the

root cortex. Here water and certain solutes are taken up into

the symplast. Roots and fungi form mycorrhizae, symbiotic

structures consisting of plant roots united with fungal hyphae.

The hyphae absorb minerals and transfers them to the host plant.

The Endodermis The endodermis is the innermost layer of cells in the

root cortex and surrounds the vascular cylinder Last selective barrier for minerals going to the vascular

tissue Casparian strips line the transvverse and radial walls of

the vascular cylinder, making it impervious to water Due to casparian strips minerals must pass through

passively selective plasma membranes The last part of minerals path from the soil to xylem

pathway is the trachieds and vessels of the xylem, which lack protoplasts and thereofre are part of the apoplast

The Ascent of Xylem Sap At night when transpiration is really low,

root cells continue to pump mineral ions into the xylem of vascular plants.

Because of the accumulation of minerals due to the epidermis, there is a lower water potential within the vascular cylinder.

Root pressure is the upward push of xylem sap.

When more water enters the leaf than is transcribed the result is guttation.

Water and Minerals Ascend From Roots to Shoots Through the Xylem

Xylem sap flows upward starting at the roots, then travel throughout the shoot system, into veins that branch out into each leaf.

Transpiration is the loss of water vapor from leaves and other aerial parts.

Plants wilt if the water last through transpiration is not replaced by water traveling up from the roots.

Xylem Sap Ascent By Bulk Flow The movement of fluid in bulk flow is

driven by a water potential difference at opposite ends of a conduit.

No energy is expended in lifting xylem sap by bulk flow.

Absorption of sunlight causes water to evaporate from mesophyll cells, driving transpiration, lowering water potential

Accent of Xylem Sap Cohesion and adhesion facillitate the long-distance

transport of xylem sap from the leaves to the roots into the soil solution

The cohesion of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water molecules separating

The strong adhesion of water molecules to the hydrophilic walls of xylem cells aids in offsetting the downward pull of gravity

Transpirational pull can extend down to the roots only through an unbroken chain of water molecules.

Cavitation is the formation of a water vapor pocket in a vessel and can cause the chain to break.

Transpirational Pull The air in the airspaces are used to express the mesophyll to the

carbon dioxide it needs to perform photosynthesis is saturated with water vapor because it is in contact with the most walls of the cells

Since the air inside the leaf has a lower water potential then the air outside the leaf, water vapor in the air enters the space of leaf diffuses down its water potential gradient and exits the leaf through the stomata. This is called transpiration

The leading hypothesis as to how loss of water vapor from a leaf translates into the pulling force for upward movement of water through a plant is that negative pressure that causes water to move up through the xylem develops at the air water interface in the mesophyl wall

Transpirational pull depends on some of the special properties that water possesses, such as adhesion, cohesion, and surface tension

Negative pressure lowers water potential the negative water potential of leaves provides the pull in transpirational pull

Stomata help regulate the rate of transpiration

Leaves have very large surface areas to increase the rate of photosynthesis

Leaves have about 20 to 30 times more internal surface area than outside surface area due to the cells irregular shape

Effects Of Transpiration On Wilting And Leaf Temperature Usually transpiration draws up water

from the roots quickly enough to replace the water lost by evaporation

In some extended periods of drought water cannot be drawn up as fast as it evaporates, when this happens wilting occurs

Transpiration also causes evaporative cooling which lowers the temperature of the leaf as much as 10 or 15 degrees compared with the surrounding air to prevent enzyme denaturation

Stomata The amount of water lost by a leaf

depends both on the number of stomata and average size of the pores

The stomatal density is under both environmental and genetic controlling factors

Guard cells buckle outward when turgid, increasing the size of the pores between them, thus more water is lost

Stomatal Opening and Closing The active transport of H +out of guard cells creates

voltage that drives k + into guard cells When k + accumulates stomata open, and close when k

+ leaves the cell A depletion of CO2 in the air spaces of the leaf occurs

during the beginning of photosynthesis in the mesophyll will result in the stomata opening

The stomata will also open due to the internal clock of the guard cells

Cycles with intervals of 24 hours are circadian rhythms Environmental stresses, such as water deficiency, can

cause the stomata to close even during the day Guard cells regulate photosynthesis and transpiration

on a moment-to-moment basis on many different stimuli

Xerophyte Adaptations that Reduce Transpiration Xerophytes are plants adapted to dry climates Several leaf modifications help reduce the rate of

transpiration: Small, thick leaves reduce surface area relative to leaf

volume limiting water loss. Hairy leaves trap a boundary layer of water. Stomata are located on the underside of the leaf in

clusters protecting them from the wind. They use CAM, crassulacean acid metabolism, to attain

CO2. mesophyll cells transform CO2 into organic acids during the night allowing the stomata to close during the day when its hotter.

During the day, sugars are synthesized by the C3 photosynthetic pathway.

Movement of Sugar Translocation is the movement of organic

nutrients in the plants In angiosperms sieve tube membranes are the

conduits for translocation Phloem sap is much different form xylem sap

and contains large amounts of sugar A sugar source is a plant organ that is a net

producer of sugar A sugar sink is a net consumer of sugar, most

notably mature leaves

Movement of Sugar (cont’d) Direction of flow in sugar tubes depends on the

surrounding plant parts and external factors such as season

Sometimes sugar travels through cells through the symplastic or apoplastic pathways

Special cells called transfer cells have special ingrown walls that improve solute transfer

Phloem lets sucrose out at the sink end of the seive tube

The concentration of free sugar is always lower in the sink then in the tube because sugar in the sink is either absorbed or converted into insoluble polymers

Pressure Flow

In angiosperms sugar moves through sieve tubes through bulk flow called pressure flow

Pressure increases at the source end and decreases at the sink end which causes water to flow from the source to the sink, carrying sugar with it