Transport in Plants: Transport in Plants


35 Transport in Plants: 35 Transport in Plants 35.1 How Do Plant Cells Take Up Water and Solutes? 35.2 How Are Water and Minerals Transported in the Xylem? 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? 35.4 How Are Substances Translocated in the Phloem?


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Terrestrial plants obtain water and mineral nutrients from the soil. Water is needed for photosynthesis; it is essential for transporting solutes upward and downward, for cooling the plant, and for internal pressure that helps support the plant. Plants lose large quantities of water to evaporation, which must be replaced.


Slide4: Figure 35.1 The Pathways of Water and Solutes in the Plant


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Osmosis: movement of water through a membrane in accordance with the laws of diffusion. Osmosis is passive: no input of energy is required.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Solute potential (osmotic potential): The greater the solute concentration of a solution, the more negative the solute potential, and the greater the tendency for water to move into it from another solution of lower solute concentration.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? For osmosis to occur, two solutions must be separated by a selectively permeable membrane; permeable to water, but not to the solute. Plants have rigid cell walls. As water enters a cell due to its negative solute potential, entry of more water is resisted by an opposing pressure potential (turgor pressure).


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Water enters plant cells until the pressure potential exactly balances the solute potential. At this point the cell is turgid: it has significant positive pressure potential.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? The overall tendency of a solution to take up water from pure water, across a membrane, is called water potential (ψ). Water potential is the sum of its negative solute potential and positive pressure potential. ψ = ψs + ψp


Slide10: Figure 35.2 Water Potential, Solute Potential, and Pressure Potential


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Water always moves across a selectively permeable membrane toward a region of lower (more negative) water potential. Solute potential, pressure potential, and water potential can be measured in megapascals (MPa).


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Osmosis is extremely important to plants. Physical structure is maintained by the positive pressure potential. If this is lost, the plant wilts. Over long distances in xylem and phloem, flow of water and dissolved solutes is driven by a gradient of pressure potential.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Bulk flow: movement of a solution due to difference in pressure potential. Bulk flow in xylem is between regions of different negative pressure potential (tension). Bulk flow in phloem is between regions of different positive pressure potential (turgidity).


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Aquaporins - membrane channel proteins that water can pass through rapidly. Abundance in plasma membrane and tonoplast (vacuole membrane) depends on cell’s need to obtain or retain water. Rate of water movement can be regulated but direction of movement can not.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Mineral ions cannot pass membranes without transport proteins. Molecules and ions move with their concentration gradients as permitted by membrane characteristics. Concentration of most ions in the soil solution is lower than in the plant; uptake must be active transport, requiring energy.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Electric charge differences are also important. Combination of electrical and concentration gradients is called an electrochemical gradient. Uptake against this gradient requires ATP.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Plants have a proton pump that moves protons out of a cell against a gradient. Accumulation of H+ outside the cell results in an electric gradient and a concentration gradient of protons. Inside of cell is now more negative than outside, cations such as K+ can move in by facilitated diffusion.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? The proton gradient can be harnessed to drive active transport of anions into the cell against a gradient; a symport couples movement of H+ and Cl–. The proton pump and other transport activities results in the interior of the cell being very negative; they build up a membrane potential of about –120 mV.


Slide19: Figure 35.3 The Proton Pump in Active Transport of K+ and Cl–


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Where water is moving by bulk flow, dissolved minerals are carried along. Water and minerals also move by diffusion, and minerals move by active transport (e.g., at root hairs). Ions must cross other membranes to reach the vessels and tracheids.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Movement of ions across membranes can result in movement of water. Water moves into a root because the root has a more negative water potential than the soil. Water moves from the cortex into the stele because the stele has a more negative water potential than the cortex.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Water and minerals can move into the stele by two pathways: The apoplast: cell walls and intercellular spaces form a continuous meshwork that water can move through, without crossing any membranes. Water movement is unregulated until it reaches the Casparian strips of the endodermis.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Symplast: water passes through cells via the plasmodesmata. Selectively permeable membranes of root hair cells control access to the symplast.


Slide24: Figure 35.4 Apoplast and Symplast (Part 1)


Slide25: Figure 35.4 Apoplast and Symplast (Part 2)


Slide26: Figure 35.4 Apoplast and Symplast (Part 3)


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? Casparian strips are regions of suberin- impregnated endodermal cell walls that form a water-repelling belt around each cell. They separate the apoplast of the cortex from the apoplast of the stele. Water and ions can enter the stele only through the endodermal cells.


35.1 How Do Plant Cells Take Up Water and Solutes?: 35.1 How Do Plant Cells Take Up Water and Solutes? In the stele, minerals enter the apoplast by active transport, water potential in the cell walls becomes more negative, and water moves into the apoplast by osmosis. Water and minerals end up in the xylem, forming xylem sap.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Xylem transport: a maple tree in summer looses 220 liters of water per hour. Xylem must transport 220 liters per hour to prevent wilting. The tallest trees exceed 110 meters. Xylem must transport water to great heights. Several models for xylem transport have been proposed.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? First proposal was pumping action by living cells. Ruled out in 1893 by an experiment in which cut trees were placed in a poison solution. Solution rose through trunk to leaves (which died), then stopped rising.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Experiment established three points: Living, pumping cells were not involved. Leaves were crucial: solution continued to rise until leaves were dead. Movement was not caused by the roots.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Some hypothesized that xylem transport is based on root pressure. Higher solute concentration and more negative water potential in roots than in soil solution; water enters stele and from there has no where to go but up.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Guttation is evidence of root pressure; water is forced out through openings in leaves. Root pressure also causes sap to ooze from cut stumps. But it can’t account for ascent of sap in trees.


Slide34: Figure 35.5 Guttation


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? If root pressure was pushing sap up the xylem, there would be a positive pressure potential in the xylem at all times. But xylem sap in most trees is under tension; negative pressure potential.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? An alternative to pushing is pulling. Leaves pull the xylem sap upwards. Evaporative water loss from the leaves creates a pulling force or tension on the water in the apoplast of leaves. Hydrogen bonding between water molecules makes sap cohesive enough to withstand the tension and rise by bulk flow.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Concentration of water vapor in the atmosphere is lower than in the leaf. Water vapor diffuses from leaf through the stomata: transpiration. Within the leaf, water evaporates from walls of mesophyll cells, film of water on cells shrinks, creating more surface tension (negative pressure potential). Draws more water into cell walls to replace what was lost.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Resulting tension in mesophyll cells draws water from nearest vein. Removal of water from veins results in tension on the entire column of water in the xylem, so that water is drawn up.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Ability of water to be drawn up through tiny tubes is due to cohesion: water molecules stick together because of hydrogen-bonding. The narrower the tube, the greater the tension the water column can withstand. Water also adheres to the xylem walls.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? This transpiration–cohesion–tension mechanism requires no energy from the plant. Water moves passively toward a region of more negative water potential. Dry air has the most negative water potential, and soil solution has the least. Mineral ions in xylem sap rise passively with the water.


Slide41: Figure 35.6 The Transpiration–Cohesion–Tension Mechanism


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Transpiration also helps cool plants. As water evaporates from mesophyll cells, heat is taken up from the cells, and leaf temperature drops. Important for plants in hot environments.


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? A demonstration of the negative pressure potential, or tension, in xylem sap was done by measuring tension with a pressure chamber. Also determined that tension disappeared at night in some plants, when transpiration stopped.


Slide44: Figure 35.7 A Pressure Chamber


35.2 How Are Water and Minerals Transported in the Xylem?: 35.2 How Are Water and Minerals Transported in the Xylem? Xylem sap does not rise at night, when there is no transpiration. During the day, rate of ascension depends on temperature, light intensity, and wind velocity, which all affect transpiration. Rate of flow may also depend on concentration of K+—seems to change size of pits.


Slide46: Figure 35.8 Potassium Ions Speed Transport in the Xylem (Part 1)


Slide47: Figure 35.8 Potassium Ions Speed Transport in the Xylem (Part 2)


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? Leaf and stem epidermis has a waxy cuticle to minimize water loss, but it also prevents gas exchange. Stomata, or pores in the leaf epidermis, allow CO2 to enter by diffusion. Guard cells control opening and closing.


Slide49: Figure 35.9 Stomata (A)


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? Most plants open stomata when light intensity is enough for moderate rate of photosynthesis. At night, stomata remained closed; CO2 not needed, and no water is lost. During the day stomata close if water is being lost too rapidly.


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? Cues for stomatal opening include light, and concentration of CO2 in intercellular spaces in the leaf: low levels favor opening of stomata. If plant is under water stress or water potential of mesophyll cells is too negative, they release the hormone abscisic acid—acts on guard cells and causes them to close.


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? Opening and closing of stomata is controlled by K+ in the guard cells. Blue light is absorbed by pigments in guard cells and activates a proton pump. Resulting gradient drives K+ into guard cell, making its water potential more negative. Water enters cell by osmosis. Increased pressure potential causes guard cells to change shape, and a gap appears between them.


Slide53: Figure 35.9 Stomata (B)


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? The process is reversed when active transport of protons stops. K+ diffuse out of guard cells passively, and water follows by osmosis. Pressure potential decreases, and cells sag, closing the gap between them.


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? Demonstration of how much K+ moves in and out of guard cells was done by using an electron probe microanalyzer.


Slide56: Figure 35.10 Measuring Potassium Ion Concentration in Guard Cells (Part 1)


Slide57: Figure 35.10 Measuring Potassium Ion Concentration in Guard Cells (Part 2)


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? Farmers and gardeners would like to reduce the amount of transpiration from crops, to reduce need for irrigation. An antitranspirant: a compound to reduce transpiration without limiting CO2 uptake. Abscisic acid is too expensive for large-scale trials.


35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2?: 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO2? Transgenic plants with a mutant allele for the era gene are very sensitive to abscisic acid and thus resistant to wilting in droughts. Some compounds form a polymer film around leaves, and seal the stomata. They are mostly used for transplants of nursery stock.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Movement of carbohydrates and other solutes through the phloem is translocation. Substances are translocated from sources to sinks. Sources (e.g., leaves, produce more sugars than they require). A sink consumes sugars for growth or storage.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? In a classic experiment, a tree was girdled; a ring of bark containing the phloem was removed. Organic solutes collect in the phloem above the girdle, causing it to swell. Eventually the bark, then roots below, and whole tree die because sugars are not being translocated downwards.


Slide62: Figure 35.11 Girdling Blocks Translocation in the Phloem


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Characteristics of translocation: Stops if phloem is killed. Proceeds in both directions simultaneously. Inhibited by compounds that inhibit respiration and limit ATP supply.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Plant physiologists needed to sample pure phloem sap from individual sieve tube elements. Aphids feed on plants by drilling into sieve tubes and inserting their stylet. Pressure in the sieve tube forces sap through stylet and into aphid’s digestive tract.


Slide65: Figure 35.12 Aphids Collect Sap


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Plant physiologists use aphids by cutting the body away from the stylet—phloem sap continues to flow for hours and can be collected and analyzed. Using radioactive tracers, they can infer how long it takes for translocation to occur. These and other experiments led to development of the pressure flow model.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Two steps in translocation require energy: Transport of solutes from sources into sieve tubes: loading. Removal of solutes at sinks: unloading.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Where loading is occurring, solute concentration in those sieve tube elements is greater than in surrounding cells. Water enters by osmosis, which makes a greater pressure potential, and water plus solutes is pushed toward the sink.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? At the sink, the solutes are unloaded by active transport, maintaining the pressure gradient.


Slide70: Figure 35.13 The Pressure Flow Model


Slide71: Table 35.1 Mechanisms of Sap Flow in Plant Vascular Tissues


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? For the pressure flow model to be valid, two requirements must be met: Sieve plates must be unobstructed so that bulk flow is possible. There must be effective methods for loading and unloading solutes.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Early electron microscope studies of sieve tubes indicated the plates were blocked by fibrous proteins. These proteins were a response to damage when phloem was prepared for study. When tissues were undamaged, sieve plates were shown to be open.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Secondary active transport loads sucrose into companion cells and sieve tubes by a sucrose–proton symport. The apoplast must have a high concentration of protons—supplied by the proton pump (active transport).


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? At sinks, sucrose is actively transported out of sieve tubes and into the surrounding tissues. This maintains the pressure gradient, and promotes buildup of sugars and starches in storage areas, such as developing fruits and seeds.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Many substances move through the symplast via the plasmodesmata, including at loading and unloading sites. In sink tissues, plasmodesmata are abundant and allow passage of large molecules.


35.4 How Are Substances Translocated in the Phloem?: 35.4 How Are Substances Translocated in the Phloem? Plants (and viruses) produce “movement proteins” that change the permeability of plasmodesmata and allow large molecules to pass. Biologists study these proteins in hopes of modifying plasmodesmata—for example, to divert more photosynthetic output to seeds to increase crop yields.