L27 Chapter 36
Transport in Plants
Transport in Plants
• Uptake and Movement of Water and Solutes
• Transport of Water and Minerals in the Xylem
• Transpiration and the Stomata
• Translocation of Substances in the Phloem
Uptake and Movement of Water and Solutes
• Plants need water for photosynthesis, transporting solutes, temperature control, and developing the internal pressure that supports the plant body.
• Plants usually obtain water and minerals from the soil via the roots, which in turn obtain carbohydrates and other important materials from the leaves.
• Water enters the plant through osmosis, but the uptake of minerals requires transport proteins.
Uptake and Movement of Water and Solutes
• Osmosis is the movement of water through a __________ in accordance with the laws of diffusion.
• Osmotic potential, or solute potential, determines the direction of water movement across a membrane.
• Dissolved solutes have the effect of lowering the concentration of water.
• The greater the solute concentration, the more negative the solute potential, and the greater the tendency of water to move into it from a lower solute concentration.
Uptake and Movement of Water and Solutes
• For osmosis to occur, a membrane must be permeable to water but not to the solutes.
• Plant cells have a __________ cell wall.
• As water enters the cell, the plasma membrane presses against the cell wall, restricting expansion.
• The opposing force exerted by the rigid cell wall as water enters is called the pressure potential, or __________ pressure.
• Water enters a plant cell until the pressure potential exactly balances the solute potential. The cell is then ____________________
Uptake and Movement of Water and Solutes
• The tendency of a solution to take up water from pure water, across a membrane, is called water __________, represented by .
• Water potential is the sum of the negative solute potential (s) and the (usually positive) pressure potential (p).
s + p
• Solute potential, pressure potential, and water potential are measured in megapascals (Mpa).
Uptake and Movement of Water and Solutes
• Water always moves across a semipermeable membrane toward the region of more negative (lower) water potential.
• The structure of many plants is maintained by the pressure potential in their cells. Loss of pressure potential causes wilting.
• __________ flow is the term used for the movement of fluids in plants due to differences in pressure potential.
Uptake and Movement of Water and Solutes
• Aquaporins are membrane channel proteins through which water moves.
• They influence the permeability of the membranes, which in turn influences rates of movement but not the direction.
• Water movement through aquaporins is passive.
Uptake and Movement of Water and Solutes
• Mineral ions generally require transport proteins in order to cross membranes.
• When the concentration of ions is greater in the soil than in the plant, the plant can take up ions by facilitated diffusion, a passive process.
• If the concentration of ions is lower in the soil than in the plant, however, ion uptake requires energy.
Uptake and Movement of Water and Solutes
• __________ gradients also affect the cell’s ability to take up ions.
• The combination of concentration and electrical gradients is called an electrochemical gradient.
• Uptake against an electrochemical gradient is active transport.
Uptake and Movement of Water and Solutes
• Plants have no sodium–potassium pump.
• Plants use a proton pump, which uses ATP to move protons out of root cells, often against a proton gradient.
• The H+ causes the region outside the membrane to become __________ charged and a proton gradient is established.
• The positive charge outside the cell enhances the movement of positively charged ions (such as K+) into the more negatively charged cell interior through membrane channels.
• These ions move into the more negatively charged interior of the cell by facilitated diffusion.
Uptake and Movement of Water and Solutes
• A __________ protein couples the diffusion of H+ back into the cell (along its electrochemical gradient) to the transport of Cl– into the cell (against its electrochemical gradient). This is secondary active transport.
• A difference in charge develops across the membrane, called the membrane potential. The inside of the plant cell is negative relative to the outside.
• Plant cells maintain a membrane potential of at least –120 millivolts (mV).
Uptake and Movement of Water and Solutes
• Where __________ flow of water is occurring, dissolved minerals are carried along.
• When movement is less, minerals move by diffusion.
• Minerals must be actively transported across certain membranes.
• The cells at the surface of the root hairs actively transport ions.
• Water moves into the cells of the root because the root cells have more negative water potential than the soil solution.
Uptake and Movement of Water and Solutes
• Water and solutes get to the dermal and ground tissues and into the stele via the apoplast and symplast.
• The cell walls and __________ spaces are the apoplast.
• The apoplast is a __________ network in which water and minerals can flow without crossing membranes.
• This is unregulated movement.
• The apoplast allows movement up to the endodermis, the innermost layer of the root cortex.
Uptake and Movement of Water and Solutes
• The symplast is the portion of the plant body enclosed by membranes—the continuous cytoplasm of the living cells.
• Cells are connected to each other via plasmodesmata.
• The selectively permeable plasma membranes control access to the symplast.
Uptake and Movement of Water and Solutes
• The endodermis cell walls have Casparian strips—waxy, suberin-containing structures that form a hydrophobic belt sealing the cell and preventing movement of water and ions between the cells.
• The Casparian strips thus separate the apoplast of the cortex from the apoplast of the stele.
• Water and ions can enter the stele only by way of the __________—by entering and passing through the endodermal cytoplasm.
Uptake and Movement of Water and Solutes
• Once past the endodermal barrier, water and minerals leave the symplast and enter the apoplast of the stele.
• Parenchyma cells in the pericycle or xylem help minerals move back to the apoplast.
• Some of these cells, called transfer cells, use ATP to move ions from their cytoplasm (the symplast) to their cell walls (the apoplast).
• Increasing the ion concentration in the apoplast lowers its water potential, so water then moves into the apoplast by osmosis.
• Active transport of ions moves the ions directly, and water follows passively.
Transport of Water and Minerals in the Xylem
• In the __________, water and minerals constitute the __________ sap.
• Many experiments have been done to determine how xylem sap moves through plants.
Transport of Water and Minerals in the Xylem
• Eduard Strasburger cut trees at the base and placed the cut ends into a bucket of water and poison.
• Transport continued until the poison reached the leaves, at which point it stopped.
• His experiment established three important points:
“Pumping cells” are not responsible for transport.
The leaves play a crucial role in transport.
The roots are not the cause of transport.
Transport of Water and Minerals in the Xylem
• Plant physiologists once thought that root pressure was the basis for transport of xylem sap.
• Root pressure is due to the higher solute concentration and more negative water potential in the xylem sap than in the soil.
• Guttation, in which water is forced out through openings in the leaves, is an example of root pressure.
• Root pressure is at most 0.1–0.2 MPa, however, which is inadequate to account for the ascent of fluids to tree heights.
• Transport is observed also when negative pressure occurs in xylem.
Transport of Water and Minerals in the Xylem
• The transpiration–cohesion–tension mechanism:
• The concentration of water vapor is higher inside the leaf than outside, so water diffuses out of the leaf through the stomata. This process is called transpiration.
• This creates a tension in the mesophyll that draws water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells.
• The removal of water from the veins, in turn, establishes tension on the entire volume of water in the xylem, so the column is drawn up from the roots.
Transport of Water and Minerals in the Xylem
• Hydrogen bonding between water molecules results in __________, the tendency of water molecules to stick to one another.
• The narrower the tube, the greater the tension the water column can stand.
• The water column is also maintained by adhesion of water molecules to the walls of the tube.
Transport of Water and Minerals in the Xylem
• The key elements in water transport in xylem:
Transpiration
Tension
Cohesion
• The transpiration–cohesion–tension mechanism does not require energy.
• At each step, water moves passively toward a region with a more negative water potential.
Transport of Water and Minerals in the Xylem
• __________ ions in the xylem sap rise passively with the solution.
• Transpiration also contributes to the plant’s temperature regulation, cooling plants in hot environments.
Transport of Water and Minerals in the Xylem
• If the transpiration–cohesion–tension mechanism is correct, the column of sap must be under tension (have a negative pressure potential).
• This was demonstrated by Per Scholander, who measured tension in stems with a pressure bomb.
Transport of Water and Minerals in the Xylem
• Scholander found that in all plant species in which xylem sap was ascending, he could measure tension.
• The tension is absent at night when transpiration ceases.
• In developing vines, xylem sap is under no tension until leaves form.
• During the day the rate of sap ascension varies according to factors such as the concentration of K+ in the sap, temperature, light intensity, and wind velocity.
Transpiration and the Stomata
• Leaf and stem epidermis has a waxy cuticle that is impermeable to water, but also to CO2.
• __________, or pores, in the epidermis allow CO2 to enter by diffusion.
• Guard cells control the opening and closing of the stomata.
• Most plants open their stomata only when the light is intense enough to maintain photosynthesis.
• Stomata also close if too much water is being lost.
Transpiration and the Stomata
• Opening closing and of the stomata are regulated by controlling K+ concentrations in the guard cells.
• Blue light activates a proton pump to actively pump protons out of the guard cells. The proton gradient drives accumulation of K+ inside the cells.
• Increasing K+ concentration makes the water potential of guard cells more negative, and water enters by osmosis.
• The guard cells respond by changing their shape and allowing a gap to form between them.
Transpiration and the Stomata
• The guard cells close when the process is reversed; when active transport of protons ceases. K+ diffuses out of the cell, and water follows.
• This occurs in the absence of blue light or when abscisic acid is present.
• Abscisic acid is produced by the mesophyll cells on hot, sunny, windy days so that guard cells will close the __________ to prevent water loss.
Transpiration and the Stomata
• An antitranspirant is a compound, such as abscisic acid, that will reduce water loss by keeping stomata closed.
• Another way to reduce water loss is to produce transgenic plants that are more sensitive to their own abscissic acid and are drought resistant.
• Polymeric films can be applied to plants to form a barrier to evaporation, but can only be used for a short time, such as during transplantation.
Translocation of Substances in the Phloem
• Sugars, amino acids, some minerals, and other solutes are transported in __________ and move from sources to sinks.
• A source is an organ such as a mature leaf or a starch-storing root that produces more sugars than it requires.
• A sink is an organ that consumes sugars, such as a root, flower, or developing fruit.
• These solutes are transported in phloem, not xylem, as shown by Malpighi by girdling a tree.
Translocation of Substances in the Phloem
• Translocation (movement of organic solutes) stops if the phloem is killed.
• Translocation often proceeds in both directions— both up and down the stem simultaneously.
• Translocation is inhibited by compounds that inhibit respiration and the production of __________.
Translocation of Substances in the Phloem
• Plant physiologists have used aphids to collect sieve tube sap from individual sieve tube elements.
• An aphids inserts a specialized feeding tube, or stylet, into the stem until it reaches a sieve tube.
• Sieve tube sap flows into the aphid. The aphid is then frozen and cut away from its stylet, which remains in the sieve tube.
• Sap continues to flow out the sieve tube and can be collected and analyzed by the physiologist.
Translocation of Substances in the Phloem
• There are two steps in translocation that require energy:
__________ is the active transport of sucrose and other solutes into the sieve tubes at a source.
__________ is the active transport of solutes out of the sieve tubes at a sink.
Translocation of Substances in the Phloem
• Sieve tube cells at the source have a greater sucrose concentration that surrounding cells, so water enters by osmosis. This causes greater pressure potential at the source, so that the sap moves by bulk flow towards the sink.
• At the sink, sucrose is unloaded by active transport, maintaining the solute and water potential gradients.
• This is called the pressure flow model.
Translocation of Substances in the Phloem
• If the pressure flow model is valid, two requirements must be met:
The sieve plates must be unobstructed.
There must be effective methods for loading and unloading the solute molecules.
• The first condition has been shown by microscopic study of phloem tissue.
• Mechanisms for loading and unloading the solutes exist in all plants.
Translocation of Substances in the Phloem
• Sugars and other solutes produced in the mesophyll cells leave the cells and enter the apoplast.
• The solutes are then actively transported to companion cells and phloem tubes, thus reentering the symplast.
• The passage of solutes to the __________ and back to the __________ allows for selectivity of solutes to be transported.
Translocation of Substances in the Phloem
• Secondary active transport loads the sucrose into companion cells and sieve tubes.
• Sucrose is carried across the membrane by sucrose–proton symport. For this symport to work, the apoplast must have a high concentration of protons.
• These protons are supplied by a primary active transport — the proton pump.
Translocation of Substances in the Phloem
• Many substances move from cell to cell within the symplast through plasmodesmata.
• The plasmodesmata participate in the loading and unloading of sieve tubes.
• Solutes enter companion cells by active transport and move into the sieve tubes through plasmodesmata.
• At sinks, plasmodesmata connect __________ tubes, companion cells, and the cells that will receive the solutes. Plasmodesmata in sink tissues are abundant and allow large molecules to pass.
Animation 36.1 The Pressure Flow Model (Part 1)
Animation 36.1 The Pressure Flow Model (Part 2)
Video 36.1 Germination of soybean plants
Video 36.2 Recovery in a wilted plant, Coleus
Video 36.3 Cell walls and stomatal complexes in Tradescantia
Video 36.4 Stomatal complexes forming