CHAPTER 32 LECTURE NOTES

Land plants require a transport system, because unlike their aquatic ancestors, photosynthetic plant organs have no direct access to water and minerals.

I. The traffic of water and solutes occurs on cellular, organ, and whole-plant levels: an overview of transport

Three levels of transport occur in plants:

1. Uptake of water and solutes by individual cells.

2. Short-distance cell-to-cell transport at the level of tissues and organs.

3. Long-distance transport of sap in xylem and phloem at the whole-plant level.

A. Transport at the Cellular Level

1. Review of Active and Passive Transport of Solutes

The plasma membrane's selective permeability controls the movement of solutes between a plant cell and the extracellular fluids. Solutes may move by passive or active transport .

Passive transport occurs when a solute molecule diffuses across a membrane down a concentration gradient.

No direct expenditure of energy by the cell.
Transport proteins embedded in the membrane may increase the speed at which solutes cross.

Transport proteins may facilitate diffusion by serving as carrier proteins or forming selective channels.

Carrier proteins bind selectively to a solute molecule on one side of the membrane, undergo a conformational change, and release the solute molecule on the opposite side of the membrane.
Selective channels are simply passageways by which selective molecules may enter and leave a cell; some gated selective channels are stimulated to open or close by environmental conditions.

Active transport occurs when a solute molecule is moved across a membrane against the concentration gradient.

Energy requiring process.
The proton pump is an active transporter important to plants.

A proton pump hydrolyzes A TP and uses the energy to pump hydrogen ions (H+) out of the cell.

Produces a proton gradient with a higher concentration outside of the cell.
Produces a membrane potential since the inside of the plant cell is negative in relation to the outside.
This membrane potential and the stored energy of the proton gradient are used by the plant to transport many different molecules.
Potassium ions (K+) are pulled into the cells because of the electrochemical gradient.
Nitrate (NO3 1 enters plant cells against the electrochemical gradient by cotransporting with hydrogen ions.

2. Water Potential and Osmosis

Osmosis results in the net uptake or loss of water by the cell and depends on which component, the cell or extracellular fluids, has the highest water potential.

Water potential (Y) = The free energy of water that is a consequence of solute concentration and applied pressure; physical property predicting the direction water will flow.

Water will always move across the membrane from the solution with the higher water potential to the one with lower water potential.
Water potential is measured in units of pressure called megapascals (MPa); one MPa is equal to ten atmospheres of pressure.
Pure water in an open container has a water potential of zero megapascals (Y= 0 MPa).
Addition of solutes to water lowers the Y into the negative range. Increased pressure raises the Y into the positive range.
A negative pressure, or tension, may also move water across a membrane; this bulk flow (movement of water due to pressure differences) is usually faster than the movement caused by different solute concentrations.

NOTE: You may wish to call attention to Campbell, Figure 32.3.

The effects of pressure and solute concentration on water potential are represented by:

Y = Yp -Ys

· Yp = pressure potential.

· Ys = solute potential or osmotic potential.

· A 0.1M solution has a pressure potential of 0.23 MPa; in an open container, physical pressure is zero; the water potential of this 0.1M solution would be – 0.23 (Y= 0 -0.23 = -0.23). Water would enter this solution due only to osmotic pressure.

· The addition of pressure to the solution could counter the affects of osmotic pressure by stopping net water movement (if P = 0.23) or by forcing water from the solution back into the pure water (if P > 0.3).

· Similar changes would result if a negative pressure were applied to the pure water side of the membrane.

Plant cells will gain or lose water to intercellular fluids depending upon their water potential.

A flaccid cell (P = 0) placed in a hyperosmotic solution will lose water by osmosis; the cell will plasmolyze (protoplast pulls away from the cell wall) in response.
A flaccid cell placed in a hypoosmotic solution will gain water by osmosis; the cell will swell and a turgor pressure develops; when pressure from the cell wall is equal to the osmotic pressure, an equilibrium is reached and no net water movement occurs (P = p; Y= 0).

3. The Role of the Tonoplast

Tonoplast = Membrane surrounding the large central vacuole found in plant cells; is important in regulating intracellular conditions.

Contains integral transport proteins that control the movement of solutes between the cytosol and the vacuole.
Has a membrane potential. Proton pumps in the tonoplast help the plasma membrane maintain a low H+ concentration in the cytosol by moving H+ into the vacuole.
Several solutes are transported between the cytosol and vacuole due to this membrane potential and proton gradient.

B. Short-Distance (Lateral) Transport at the Level of Tissues and Organs

Lateral transport is usually along the radial axis of plant organs, and can occur by three routes in plant tissues and organs: (See Campbell, Figure 32.5)

1. Across the plasma membranes and cell walls. Solutes move from one cell to the next by repeatedly crossing plasma membranes and cell walls.

2. The symplast route. A symplast is the continuum of cytoplasm within a plant tissue formed by the plasmodesmata which pass through pores in the cell walls. Once water or a solute enters a cell by crossing a plasma membrane, the molecules can enter other cells by traveling through the plasmodesmata.

3. The apoplast route. An apoplast is the continuum between plant cells which is formed by the continuous matrix of cell walls. Water and solute molecules can move from one area of a root or other organ via the apoplast without entering a cell.

Water and solute molecules can move laterally in a plant organ by anyone of these routes or by a combination through switching from one to another.

C. Long-Distance Transport at the Whole-Plant Level

This type of transport is usually along the vertical axis of the plant (up and down) from the roots to the leaves and vice versa.

Vascular tissues are involved in this type of transport as diffusion would be too slow.

Bulk flow (movement due to pressure differences) moves water and solutes through xylem vessels and sieve tubes.
Transpiration reduces pressure in the leaf xylem; this creates a tension which pulls sap up through the xylem from the roots.
Hydrostatic pressure develops at one end of the sieve tubes in the phloem; this forces the sap to the other end of the tube.

II. Roots absorb water and minerals from soil

Water and minerals enter plants through the following transport pathway:

soil à epidermis à root cortex à xylem. (See Campbell, Figure 32.6)

Soil à Epidermis:

Most absorption occurs near root tips where the epidermis is permeable to water.
Root hairs, extensions of epidermal cells, increase the surface area available for absorption.

Epidermis à Root cortex:

Lateral transport of minerals and water through the root is usually by a combination of apoplastic and symplastic routes.

The apoplastic route exposes parenchymal, cortex cells to soil solution.

=> Soil solution, containing soil particles, water and dissolved minerals, flows into hydrophilic walls of epidermal cells and passes freely along the apoplast into root cortex.

=> Compared to the epidermis, the apoplastic route exposes greater membrane surface area for water and mineral uptake into cytoplasm-

The symplastic route makes selective mineral absorption possible.

=> As soil solution moves along cell walls, some water and solutes cross the plasma membrane of epidermal and cortex cells.

=> Cells cannot absorb a sufficient supply of mineral ions by diffusion alone -the soil solution is too dilute. Active transport permits root cells to accumulate essential minerals in very high concentrations.

=> For example, transport proteins of the plasma membrane and tonoplast actively transport K+ into root cells as Na+ is pumped out.

Root Cortex à Xylem:

Only minerals using the symplastic route may move directly into the vascular tissues. They have been previously selected by a membrane.
Minerals and water passing through apoplasts are blocked at the endoderm is by a Casparian strip (a ring of suberin around each cell in the endodermis) and must enter an endodermal cell.

Water and mineral entrance into the stele must be through the cells of the endodermis.

Casparlan strips ensure that all substances entering the stele pass through at least one membrane, allowing only selected ions to pass into the stele. Also prevents stele contents from leaking back into the apoplast and out into the soil.
Water and minerals enter the stele via symplast, but tracheids and xylem vessels are part of the apoplast.
Endodermal and parenchymal cells selectively discharge minerals into the apoelast so they may enter the xylem. This action probably involves diffusion and active transport.
Those minerals and water which move into the apoplast are free to enter the tracheids and xylem vessels.

III. The ascent of xylem sap depends mainly on transpiration and the physical properties of water

The shoot depends upon an efficient delivery of its water supply.

Xylem sap flows upward at ISm per hour or faster.
Xylem vessels are close to each leaf cell, because veins branch throughout the leaves.

Water transported up from roots must replace that lost by transpiration.

Transpiration is the evaporation of water from the aerial parts of a plant.
The upward flow of xylem sap also provides nutrients (minerals) to the shoot system.

A. Pushing Xylem Sap: Root Pressure

When transpiration is low, active transport of ions into the xylem decreases the stele's water potential and causes water flow into the stele. This osmotic water uptake increases pressure which forces fluid up the xylem (= root pressure).

Root pressure causes guttation (exudation of water droplets at leaf margins).
The water droplets escape through specialized structures called hydrathode which relieve the pressure caused by more water entering the leaves than is lost by transpiration.

Root pressure is not the major mechanism driving the ascent of xylem sap.

Cannot keep pace with transpiration.
Can only force water up a few meters.

B. Pulling Xylem Sap: The Transpiration-Cohesion- Tension Mechanism

Transpiration pulls xylem sap upward, and cohesion of water transmits the upward-pull along the entire length of xylem.

Transpirational Pull:

Transpirational pull depends upon the creation of negative pressure.
Gaseous water in damp intercellular leaf spaces diffuses into the drier atmosphere through stomata.
The lost water vapor is replaced by evaporation from mesophyll cells bordering the airspaces.
The remaining water film, adhering to the hydrophilic cell walls, retreats into the cell wall pores.
Cohesion in this surface film of water resists an increase in the surface area of the film -a surface tension effect.
The water film forms a meniscus due to the negative pressure caused by the adhesion and cohesion.
This negative pressure pulls water from the xylem, through the mesophyll, toward the surface film on cells bordering the stomata.
Water moves through symplasts and apoplasts to a region of low water potential.
Mechanism results in water from the xylem replacing water transpired through the stomata.

Cohesion and Adhesion of Water:

The transpirational pull on the xylem sap is transmitted to the soil solution. Cohesion of water due to H bonds allows for the pulling of water from the top of the plant without breaking the "chain."
The adhesion of water (by H bonds) to the hydrophilic walls of xylem cells also helps pull against gravity.
The small diameter of vessels and tracheids is important to the adhesion effect.
The upward pull of sap causes tension (negative pressure) in xylem, which decreases water potential and allows passive flow of water from soil into stele.

Cavitation = Formation of a water vapor pocket in xylem.

Breaks the chain of water molecules and the pull is stopped.
Vessels cannot function again unless refilled with water by root pressure. (This can only occur in small plants.)
Pits between adjacent xylem vessels allow for detours around a cavitated area.
Secondary growth also adds new xylem vessels each year.

C. Review: The Long-Distance Transport of Water in Plants by Solar-Powered Bulk

Flow

Bulk flow is the movement of fluid due to pressure differences at opposite ends of a conduit.

The ascent of xylem sap is ultimately solar-powered, by causing evaporative water loss and, thus, negative pressure.

Xylem vessels or chains of tracheids serve as the conduits in plants.
Transpirational pull lowers the pressure at the upper (leaf) end of the conduit.
Osmotic movement of water from cell to cell in roots and leaves are due to small gradients in water potential caused by both solute and pressure gradients.
In contrast, bulk flow through the xylem vessels depends only on pressure.

IV. Guard cells mediate the transpiration-photosynthesis compromise

Transpiration results in a tremendous water loss from the plant. This water is replaced by the upward movement of water through the xylem, and guard cells surrounding stomata balance the requirements for photosynthesis with the need to conserve water.

A. The Photosynthesis- Transpiration Compromise

Large surface areas along a leaf's airspaces are needed for CO2 intake for photosynthesis, but also results in greater surface area for evaporative water loss.

Internal surface area of a leaf may be 10x-30x the external surface area.
Stomata are more concentrated on the bottom of leaves, away from the sun; this reduces evaporative loss.
The waxy leaf cuticle prevents water loss from the rest of the leaf surface.

Transpiration:photosynthesis ratios measure efficiency of water use. This ratio is 9 H2O

Lost/g CO2 assimilated into organic material.

Ratio of 600: 1 is common in C3 plants; 300: 1 in C4 plants.
C4 plants can assimilate CO2at greater rates than C3 plants.

Benefits of transpiration:

Assists in mineral transfer from roots to shoots.
Evaporative cooling reduces risk of leaf temperatures becoming too high for enzymes to function.

If transpiration exceeds delivery of water by xylem, plants wilt.

Plants can adjust to reduce risk of wilting.
Regulating the size of stomatal openings also reduces transpiration.

B. How Stomata Open and Close

Guard cells = Cells that flank stomata and control stomatal diameter by changing shape.

When turgid, guard cells "buckle" due to radially-arranged microfibrils and stomata open; when flaccid, guard cells sag and stomatal openings close.

The change in turgor pressure that regulates stomatal opening results from reversible uptake and loss of K+ by guard cells. (See Campbell, Figure 32.8)

Uptake of K+ decreases guard cell water potential so H2O is taken up, cells become turgid, and stoma opens. The tonoplast plays a role as most of the K+ and water are stored in the vacuole.
The increase in positive charge is countered by the uptake of chloride (CI-), export of H+ ions released from organic acids, and the negative charges acquired by organic acids as they lose their protons.
Closing of the stomata results when K+ exits the guard cells and creates an osmotic loss of water.

Evidence from studies using patch clamping techniques indicates that K+ fluxes across the guard cell membrane are likely coupled to membrane potentials created by proton pumps.

Stomatal opening correlates with active transport of H+ out of the guard cell.
The resulting membrane potential drives K+ into the cell through specific membrane channels.

By integrating internal and external environmental cues, guard cells open and close, balancing the requirements for photosynthesis with the need to conserve water from

Transpirational loss.

Stomata open at dawn in response to three cues:

=> Light induces guard cells to take up K+ by:

¯Activating a blue-light receptor which stimulates proton pumps in the plasma membrane.

¯ Driving photosynthesis in guard cell chloroplasts, making A TP available for the ATP-driven proton pumps.

=> Decrease of CO2 in leaf air spaces due to photosynthesis in the mesophyll.

=> An internal clock of guard cells will make them open even if kept in dark (a

circadian rhythm approximates a 24 hour cycle ).

Guard cells may close stomata during the daytime if:

=> There is a water deficiency resulting in flaccid guard cells.

=> Mesophyll production of abscisic acid (a hormone) in response to water deficiency signals guard cells to close.

=> High temperature increases CO2 in leaf air spaces due to increased respiration, closing guard cells.

C. Evolutionary Adaptations that Reduce Transpiration

Xerophytes, plants adapted to arid climates, have some of the following evolutionary adaptations that reduce transpiration:

Small, thick leaves (reduced surface area: volume, so less H2O loss).
A thick cuticle.
Stomata are in depressions on the underside of leaves to protect from water loss due to drying winds.
Some shed leaves in the driest time of the year.
Cacti and others store water in stems during the wet season.

Plants of family Crassulaceae use CAM (crassulacean acid metabolism) to assimilate CO2.

At night, mesophyll cells assimilate CO2 into organic acids.
During the day, the acids are broken down, releasing CO2 which is used to synthesize sugars by the conventional C3 pathway.
Thus, stomata can close during the day, the plant conserves water, and there is still an ample supply of CO2 for photosynthesis.

V. A bulk-flow mechanism translocates phloem sap from sugar sources to sugar sinks

Translocation = The transport of the products of photosynthesis by phloem to the rest of the plant.

In angiosperms, sieve-tube members are the specialized cells of phloem that function in translocation.

=> Sieve-tube members are arranged end-to-end forming tong sieve tubes.

=> Porous cross walls called sieve plates are in between the members and allow phloem to move freely along the sieve tubes.

Phloem sap contains primarily sucrose, but also minerals, amino acids and hormones.

A. Source-to-Sink Transport

Phloem sap movement is not unidirectional; it moves through the sieve tubes from a source (production area) to a sink (use or storage area).

Source = Organ where sugar is produced by photosynthesis or by the breakdown of starch (usually leaves).

Sink = Organ that consumes or stores sugar (growing parts of plants, fruits, non-green stems and trunks, and others).

Sugar flows from source to sink.

Source and sink depend on season. A tuber is the sink when stockpiling in the summer, but is the source in the spring.
Minerals may also be transported to sinks.
The sink is usuatty supptied by the nearest source.
Direction of flow within a phtoem etement can change, depending on locations of the source and sink.

B. Phloem Loading and Unloading

Sugar produced at a source must be loaded into sieve-tube members before it can be translocated to a sink.

In some plant species, the sugar may move through the s)'mplast from mesophyll cells to the sieve members.

In other species, the sugar uses a combination of symptastic and apoptastic routes. (See Campbell, Figure 32.9)
Some plants have transfer cells. These are modified companion cells which have numerous ingrowths of their walls. These structures increase the cells' surface area and enhances solute transfer between apoplast and symptast.
In plants such as corn, active transport accumulates sucrose in sieve-tube members to two to three the concentration in mesophyll cells.

=> Proton pumps power this transport by creating a H+ gradient-

=> A membrane protein uses the potential energy stored in the gradient to drive the cotransport of sucrose by coupling sugar transport to the diffusion of H+ back

into the celt.

Sucrose is unloaded at the sink end of sieve tubes.

In some plants, sucrose is unloaded from the phloem by active transport.
In other species diffusion moves the sucrose from the phloem into the cells of the sink.
Both symplastic and apoplastic routes may be involved.

C. Pressure Flow (Bulk Flow) of Phloem Sap

Phloem sap flows up to I meter per hour, too fast for just diffusion or cytoplasmic streaming.

The flow is by a bulk flow (pressure-flow) mechanism; buildup of pressure at the source and release of pressure at the sink causes source-to-sink flow. (See Campbell, Figure

32.10)

At the source end, phloem loading causes high solute concentrations.

=> Water potential decreases, so water flows into tubes creating hydrostatic

pressure.

=> Hydrostatic pressure is greatest at the source end of the tube.

At the sink end, the water potential is lower outside the tube due to the unloading of sugar; osmotic loss of water releases hydrostatic pressure.
Xylem vessels recycle water from the sink to the source.

Aphids have been used to study flow in phloem. (See Campbell, Figure 32.11)

The aphid stylet punctures phloem and the aphid is "force-fed" by the pressure.
The aphid is severed from its stylet and flow is measured through the stylet.
The flow exerts greater pressure and has a higher sugar concentration the closer the source.