Chapter 38 Study Guide
Transport in Plants
Overview: Pathways for Survival
•	The algal ancestors of plants obtained water, minerals and CO2 from the  __________   in which they were completely immersed.
•	For vascular plants, the evolutionary journey onto land involved the differentiation of the plant body into  __________  , which absorb water and minerals from the soil, and  __________  , which absorb light and atmospheric CO2 for photosynthesis.
•	This morphological solution created a new problem: the need to  __________   materials between roots and shoots.
•	__________  transports water and minerals from the  __________   to the  __________  .
•	__________   transports  __________   from the site of production to the regions that need them for growth and metabolism.
A. An Overview of Transport Mechanisms in Plants
•	Transport in plants occurs on three levels:
1.	The uptake and loss of water and solutes by individual  __________  , such as root hairs.
2.	Short-distance transport of substances from cell to cell at the level of  __________   or  __________  , such as the loading of sugar from photosynthetic leaf cells into the sieve tubes of phloem.
3.	Long-distance transport of sap within xylem and phloem at the level of the  __________   plant.
1. Transport at the cellular level depends on the  __________   __________   of membranes.
•	The selective permeability of a plant cell’s  __________   __________   controls the movement of solutes between the cell and the extracellular solution.
•	Molecules tend to move down their concentration gradient. Diffusion across a membrane is called  __________   transport and occurs without the direct expenditure of metabolic energy by the cell.
•	__________   transport is the pumping of solutes across membranes against their electrochemical gradients, and requires expenditure of energy by the cell.
•	The cell must expend metabolic energy, usually in the form of  __________  , to transport solutes “uphill.”
•	Transport  __________   embedded in the membrane can speed movement across the membrane.
•	Some transport proteins  __________   selectively to a solute on one side of the membrane and __________   it on the opposite side.
•	Others act as  __________   __________  , providing a selective passageway across the membrane.
•	For example, the membranes of most plant cells have potassium channels that allow potassium ions (K+) to pass, but not similar ions, such as sodium (Na+).
•	Some channels are  __________  , opening or closing in response to certain environmental or biochemical stimuli.
2. Proton pumps play a central role in transport across plant membranes.
•	The most important active transport protein in the plasma membrane of plant cells is the  __________   __________  .
•	It hydrolyzes ATP and uses the released energy to pump hydrogen ions (H+) out of the cell.
•	This creates a proton gradient because the H+ concentration is higher  __________   the cell than  __________  .
•	It also creates a  __________   __________   or voltage, a separation of opposite charges across a membrane.
•	Both the concentration gradient and the membrane potential are forms of  __________   (stored) energy that can be harnessed to perform cellular  __________  .
•	This potential energy is used to drive the transport of many different solutes.
•	For example, the membrane potential generated by proton pumps contributes to the uptake of potassium ions (K+) by root cells.
•	The proton gradient also functions in  __________  , in which the downhill passage of one solute (H+) is coupled with the uphill passage of another, such as NO3− or sucrose.
•	The role of proton pumps in transport is a specific application of the general mechanism called  __________  , a unifying process in cellular energetics.
•	In chemiosmosis, a transmembrane  __________   gradient links energy-releasing processes to energy-consuming processes.
•	The  __________   __________   that couple H+ diffusion to ATP synthesis during cellular respiration and photosynthesis function somewhat like proton pumps.
•	However, proton pumps normally run in reverse, using ATP energy to pump H+  __________   its gradient.
3. Differences in water potential drive water transport in plant cells.
•	The survival of plant cells depends on their ability to  __________   water uptake and loss.
•	The net uptake or loss of water by a cell occurs by  __________  , the passive transport of water across a membrane.
•	In the case of a plant cell, the direction of water movement depends on solute  __________   and physical  __________  .
•	The combined effects of solute concentration and pressure are called  __________   potential, represented by the Greek letter “psi.”
•	Water will move across a membrane from the solution with the  __________   water potential to the solution with the  __________   water potential.
•	For example, if a plant cell is immersed in a solution with a higher water potential than the cell, osmotic uptake of water will cause the cell to  __________  .
•	By moving, water can perform  __________  , such as expanding the cell.
•	Therefore the potential in water potential refers to the potential  __________   that can be released to do work when water moves from a region with higher psi to lower psi.
•	Plant biologists measure psi in units called megapascals (MPa), where one MPa is equal to about 10 atmospheres of pressure.
•	An atmosphere is the pressure exerted at sea level by an imaginary column of air—about 1 kg of pressure per square centimeter.
•	A car tire is usually inflated to a pressure of about 0.2 MPa; water pressure in home plumbing is about 0.25 MPa.
•	In contrast, plant cells exist at approximately 1 MPa.
•	Both pressure and solute concentration affect water  __________  .
•	The combined effects of pressure and solute concentrations on water potential are incorporated into the following equation, where psip is the pressure potential and psis is the solute potential (or osmotic potential).
psi = psip + psis
•	__________   potential is the physical pressure on a solution and can be positive or negative.
•	The water in the dead vessel element cells of xylem may be under  __________   pressure of less than −2 MPa.
•	Water in living cells is usually under  __________   pressure. The cell contents press the plasma membrane against the cell wall, producing  __________   pressure.
•	The solute potential (or  __________   potential) of a solution is proportional to the number of dissolved solute molecules.
•	By definition, the solute potential of pure water is __________  .
•	The addition of solutes lowers the water potential because the solutes bind water molecules, which have less freedom to move than they do in pure water.
•	Any solution at atmospheric pressure has a  __________   water potential.
•	For instance, a 0.1-molar (M) solution of any solute has a water potential of −0.23 MPa.
•	If a 0.1 M solution is separated from pure water by a selectively permeable membrane, water will move by osmosis into the solution.
•	Water will move from the region of higher psi (0 MPa) to the region of lower psi (−0.23 MPa).
•	Water potential affects the uptake and loss of water in plant cells.
•	In a  __________   cell, psip = 0 and the cell is limp.
•	If this cell is placed in a solution with a higher solute concentration (and, therefore, a lower psi), water will  __________   the cell by osmosis.
•	Eventually, the cell will  __________   by shrinking and pulling away from its wall.
•	If a flaccid cell is placed in pure water (psi = 0), the cell will have lower water potential than pure water due to the presence of solutes, and water will  __________   the cell by osmosis.
•	As the cell begins to swell, it will push against the cell  __________  , producing  __________   pressure.
•	The partially elastic wall will push back until this __________   is great enough to offset the tendency for water to enter the cell because of solutes.
•	When psip and psis are equal in magnitude (but opposite in sign), psi = 0, and the cell has reached a dynamic  __________   with the environment, with no further  __________   movement of water in or out.
•	A walled cell with a greater solute concentration than its surroundings will be  __________  , or firm.
•	Healthy plants are turgid most of the time, and their turgor contributes to  __________   in nonwoody parts of the plant.
•	You can see the effects of turgor loss in  __________  , the drooping of leaves and stems as plant cells become flaccid.
4. Aquaporins affect the rate of water transport across  __________  .
•	Both plant and animal membranes have specific transport proteins, __________  , which facilitate the passive movement of water across a membrane.
•	Aquaporins do not affect the water potential gradient or the direction of water flow, but rather increase the  __________   at which water diffuses down its water potential gradient.
•	Evidence is accumulating that the rate of water movement through aquaporins is regulated by changes in  __________   messengers such as calcium ions (Ca2+).
•	This raises the possibility that the cell can regulate its rate of water uptake or loss when its water potential is  __________   from that of its environment.
5. Vacuolated plant cells have three major  __________  .
•	While the thick cell wall helps maintain cell  __________  , it is the cell  __________  , not the cell wall, which regulates the traffic of material into and out of the  __________  .
•	This membrane is a barrier between two major compartments: the cell  __________   and the  __________  .
•	Most mature plants have a third major compartment, the __________  , a large organelle that can occupy as much as 90% of the protoplast’s volume.
•	The membrane that bounds the vacuole, the  __________  , regulates molecular traffic between the cytosol and the contents of the vacuole, called the cell sap.
•	Proton pumps in the tonoplast expel H+ from the cytosol into the  __________  .
•	The resulting  __________   gradient is used to move other ions across the tonoplast by  __________  .
•	In most plant tissues, two of the three cellular compartments are continuous from cell to cell.
•	__________   connect the cytosolic compartments of neighboring cells.
•	This cytoplasmic continuum, the  __________  , forms a continuous pathway for transport of certain molecules between cells.
•	The walls of adjacent plant cells are also in contact, forming a second continuous compartment, the  __________  .
•	The vacuole is not  __________   with neighboring cells.
6. Both the symplast and the apoplast function in transport within tissues and organs.
•	__________ - __________    transport in plants, the movement of water and solutes from one location to another within plant tissues and organs, is called lateral transport because its usual direction is along the radial axis of plant organs, rather than up or down the length of the plant.
•	Three routes are available for lateral transport.
•	In one route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell by the same mechanism.
•	This  __________   route requires repeated crossings of plasma membranes.
•	The second route, via the symplast, requires only one crossing of a plasma membrane.
•	After entering one cell, solutes and water move from cell to cell via  __________  .
•	The third route is along the apoplast, the extracellular pathway consisting of cell  __________   and extracellular  __________  .
•	Water and solutes can move from one location to another within a root or other organ through the continuum of cell walls without ever entering a  __________  .
7. Bulk flow functions in __________  - __________   transport.
•	Diffusion in a solution is fairly efficient for transport over distances of  __________   dimensions (less than 100 microns).
•	However, diffusion is much too slow for long-distance transport within a plant, such as the movement of water and minerals from  __________   to  __________  .
•	Water and solutes move through xylem vessels and sieve tubes by  __________   __________  , the movement of a fluid driven by pressure.
•	In phloem, __________   pressure generated at one end of a sieve tube forces sap to the opposite end of the tube.
•	In xylem, it is actually  __________   (negative pressure) that drives long-distance transport.
•	__________  , the evaporation of water from a leaf, reduces pressure in the leaf xylem.
•	This creates a tension that  __________   xylem sap upward from the roots.
•	Rate of flow through a capillary depends on a capillary’s internal diameter.
•	To maximize bulk flow, the sieve-tube members are almost entirely devoid of internal  __________  .
•	Vessel elements and tracheids are  __________  at maturity.
•	The porous plates that connect contiguous sieve-tube members and the perforated end walls of xylem vessel elements also enhance bulk flow.
B. Absorption of Water and Minerals by  __________  
•	Water and mineral salts from soil enter the plant through the  __________   of roots, cross the root  __________  , pass into the  __________   __________  , and then flow up xylem vessels to the  __________   system.
1.	The uptake of soil solution by the hydrophilic epidermal walls of root hairs provides access to the  __________  , and water and minerals can soak into the cortex along this route.
2.	Minerals and water that cross the plasma membranes of root hairs enter the __________  .
3.	Some water and minerals are transported into cells of the epidermis and cortex and then move inward via the  __________  .
4.	Materials flowing along the apoplastic route are blocked by the waxy  __________   strip at the endodermis. Some minerals detour around the Casparian strip by crossing the  __________   __________   of an endodermal cell to pass into the  __________   cylinder.
5.	Endodermal and parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The water and minerals enter the dead cells of xylem vessels and are transported  __________   into the shoots.
1. Root hairs, mycorrhizae, and a large surface area of cortical cells enhance water and mineral absorption.
•	Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root __________   are located.
•	Root hairs, extensions of  __________  , account for much of the  __________   __________   of roots.
•	The soil solution flows into the hydrophilic walls of epidermal cells and passes freely along the  __________   into the root cortex, exposing all the parenchyma cells to soil solution and increasing membrane surface area.
•	As the soil solution moves along the apoplast into the roots, cells of the epidermis and cortex take up water and certain solutes into the  __________  .
•	Selective  __________   __________   of the plasma membrane and tonoplast enable root cells to extract essential minerals from the dilute soil solution and concentrate them hundreds of times higher than in the soil solution.
•	This selective process enables the cell to extract K+, an essential mineral nutrient, and exclude most Na+.
•	Most plants form partnerships with  __________   fungi to absorb water and minerals from soil.
•	“Infected” roots form  __________  , symbiotic structures consisting of the plant’s roots united with the fungal hyphae.
•	Hyphae  __________   water and selected minerals, transferring much of these to the host plants.
•	The mycorrhizae create an enormous  __________   __________   for absorption and enable older regions of the roots to supply water and minerals to the plant.
2. The endodermis functions as a selective sentry between the root cortex and vascular tissue.
•	Water and minerals in the root cortex cannot be transported to the rest of the plant until they enter the  __________   of the vascular cylinder.
•	The  __________  , the innermost layer of cells in the root cortex, surrounds the vascular cylinder and functions as a final checkpoint for the selective passage of minerals from the cortex into the vascular tissue.
•	Minerals already in the symplast continue through the  __________   of the endodermal cells and pass into the vascular cylinder.
•	These minerals were already screened by the selective membrane they crossed to enter the symplast.
•	Those minerals that reach the endodermis via the  __________   are blocked by the  __________  strip in the walls of each endodermal cell.
•	This strip is a belt of suberin, a waxy material that is impervious to  __________   and dissolved  __________  .
•	To enter the vascular cylinder, minerals must cross the  __________   __________   of the endodermal cell and enter the vascular cylinder via the symplast.
•	The  __________  , with its Casparian strip, ensures that no minerals reach the vascular tissue of the root without crossing a  __________   __________   plasma membrane.
•	The endodermis acts as a sentry on the  __________  - __________   cylinder border.
•	The last segment in the soil-to-xylem pathway is the passage of water and minerals into the  __________   and  __________   __________   of the xylem.
•	Because these cells lack protoplast, the lumen and the cell walls are part of the  __________  .
•	Endodermal cells and parenchyma cells within the vascular cylinder discharge minerals into their  __________  .
•	Both diffusion and active transport are involved in the transfer of solutes from the  __________   to  __________  , finally entering the tracheids and xylem vessels.
C. Transport of Xylem Sap
•	Xylem sap flows  __________   to veins that branch throughout each leaf, providing each with water.
•	Plants lose an astonishing amount of water by  __________  , the loss of water vapor from leaves and other aerial parts of the plant.
•	A single corn plant transpires 125 L of water during its growing season.
•	The flow of water transported up from the  __________   replaces the water lost in  __________   and also carries minerals to the shoot system.
1. The ascent of xylem sap depends mainly on transpiration and the physical properties of water.
•	Xylem sap rises against  __________   to reach heights of more than 100 m in the tallest trees.
•	At night, when transpiration is very low or zero, the root cells continue to  __________   energy while pumping mineral ions into the xylem.
•	The accumulation of minerals in the vascular cylinder  __________   water potential there, generating a positive pressure, called  __________   pressure, which forces fluid up the xylem.
•	Root pressure causes  __________  , the exudation of water droplets that can be seen in the morning on the tips of grass blades or the leaf margins of some small, herbaceous dicots.
•	In most plants, root pressure is not the major mechanism driving the ascent of xylem sap.
•	At most, root pressure can force water  __________   only a few meters, and many plants generate no root pressure at all.
•	For the most part, xylem sap is not pushed from below by root pressure but is  __________   upward by the leaves themselves.
•	__________   provides the pull, and the  __________   and  __________   of water due to hydrogen bonding transmits the upward pull along the entire length of the xylem to the roots.
•	The mechanism of transpiration depends on the generation of  __________   pressure (tension) in the leaf due to the unique physical properties of water.
•	As water transpires from the leaf, water coating the  __________   cells replaces water lost from the air spaces.
•	As water evaporates, the remaining film of liquid water retreats into the pores of the cell walls, attracted by adhesion to the  __________   walls.
•	Cohesive forces in water resist an  __________   in the surface area of the film.
•	Adhesion to the wall and surface tension cause the surface of the water film to form a  __________  , “pulling on” the water by __________   and  __________   forces.
•	The water film at the surface of leaf cells has a  __________   pressure, a pressure less than atmospheric pressure.
•	The more  __________   the meniscus, the more negative the pressure of the water film.
•	This tension is the pulling force that draws water  __________   of the leaf xylem, through the mesophyll, and toward the cells and surface film bordering the  __________   __________  .
•	The tension generated by adhesion and surface tension  __________   the water potential, drawing water from an area of __________   water potential to an area of  __________   water potential.
•	Mesophyll cells  __________   water to the surface film lining the air spaces, which in turn  __________   water by transpiration.
•	The water lost via the  __________   is replaced by water pulled out of the leaf  __________  .
•	The transpirational pull on xylem sap is transmitted all the way from the __________   to the  __________   tips and even into the soil  __________  .
•	Cohesion of water due to  __________   bonding makes it possible to pull a column of sap from above without the water molecules separating.
•	Helping to fight gravity is the strong adhesion of water molecules to the  __________   walls of the xylem cells.
•	The very small  __________   of the tracheids and vessel elements exposes a large proportion of the water to the hydrophilic walls.
•	The upward pull on the cohesive sap creates  __________   within the xylem.
•	This tension can actually cause a measurable  __________   in the diameter of a tree on a warm day.
•	Transpiration puts the xylem under tension all the way down to the root tips, __________   the water potential in the root xylem and pulling water from the soil.
•	Transpirational pull extends down to the roots only through an unbroken chain of water molecules.
•	__________  , the formation of water vapor pockets in the xylem vessel, breaks the chain.
•	This occurs when xylem sap  __________   in water.
•	Small plants use root  __________   to refill xylem vessels in spring.
•	Root pressure cannot push water to the top of a tree. In trees, a xylem vessel with a water vapor pocket can never function as a water capillary again.
•	The transpirational stream can detour around the water vapor pocket, and  __________   growth adds a new layer of xylem vessels each year.
•	Only the youngest, __________   secondary xylem vessels in trees transport  __________  . The older xylem vessels no longer function in water transport but do provide  __________   for the tree.
2. Xylem sap ascends by solar-powered bulk flow: a review.
•	Long-distance transport of water from roots to leaves occurs by  __________   flow.
•	The movement of fluid is driven by a water  __________   difference at opposite ends of a conduit, the xylem vessels or chains of tracheids.
•	The water potential difference is generated at the leaf end by __________   pull, which lowers water potential (increases tension) at the “upstream” end of the xylem.
•	On a smaller scale, gradients of water potential drive the  __________   movement of water from cell to cell within root and leaf tissue.
•	Differences in both  __________   concentration and  __________   pressure contribute to this microscopic transport.
•	In contrast, bulk flow, the mechanism for long-distance transport up xylem vessels, depends only on  __________  .
•	In contrast to osmosis, bulk flow moves the whole solution, __________   plus  __________   and any other solutes dissolved in the water.
•	The plant expends none of its own metabolic  __________   to lift xylem sap up to the leaves by bulk flow.
•	The absorption of sunlight drives transpiration by causing water to  __________   from the moist walls of mesophyll cells and by lowering the water  __________   in the air spaces within a leaf.
•	Thus, the ascent of xylem sap is ultimately  __________   powered.
D. The Control of Transpiration
1. __________   __________   mediate the photosynthesis-transpiration compromise.
•	Most leaves have broad surface areas and high ratios of surface  __________   to  __________  .
•	These features are morphological adaptations to enhance the absorption of  __________   for photosynthesis.
•	They also increase  __________   loss through stomata.
•	To make food, a plant must spread its leaves to the sun and obtain CO2 from air.
•	__________   __________   diffuses into and  __________   diffuses out of the leaf via the stomata.
•	Within the leaf, CO2 enters a honeycomb of air spaces formed by the irregularly shaped parenchyma cells.
•	This  __________   surface may be 10 to 30 times greater than the  __________   leaf surface.
•	This structural feature increases exposure to  __________   but also increases the surface area for  __________  .
•	A leaf may transpire more than its weight in water each day.
•	Water flows in  __________  vessels may reach 75 cm/min.
•	Transpiration also results in  __________   __________  , which can lower the temperature of a leaf by as much as 10–15°C relative to the surrounding air.
•	About 90% of the water that a plant loses escapes through  __________  , though these pores account for only 1–2% of the external leaf surface.
•	The amount of water lost by a leaf depends on the  __________   of stomata and the average  __________   of their apertures.
•	The stomatal  __________   of a leaf is under both genetic and environmental control.
•	Desert plants have  __________   stomatal densities than do marsh plants.
•	__________   light intensities and  __________  carbon dioxide levels during plant development tend to increase stomatal density in many plant species.
•	A recent British survey found that stomatal density of many woodland species has decreased since 1927. This is consistent with the dramatic  __________   in CO2 levels due to burning of fossil fuels.
•	This prevents the leaf from reaching temperatures that could  __________   enzymes.
•	Each stoma is flanked by a pair of  __________   __________   that are suspended by other epidermal cells over an air chamber, leading to the internal air space.
•	Guard cells control the diameter of the stoma by changing  __________  , thereby widening or narrowing the gap between the two cells.
•	When guard cells take in water by  __________  , they become more __________  , and because of the orientation of cellulose __________  , the guard cells buckle __________  .
•	This increases the  __________  between cells.
•	When guard cells  __________   water and become  __________  , they become less bowed, and the space between them __________  .
•	Changes in  __________   pressure that open and close stomata result primarily from the reversible uptake and loss of __________   ions (K+) by guard cells.
•	Stomata open when guard cells actively  __________   K+ into the vacuole.
•	This decreases water  __________   in guard cells, leading to an  __________   of water by osmosis and increasing cell  __________  .
•	Stomatal  __________   results from an exodus of K+ from guard cells, leading to osmotic  __________   of water.
•	Regulation of aquaporins may also be involved in the swelling and shrinking of guard cells by varying the permeability of the membranes to water.
•	The K+ fluxes across the guard cell membranes are coupled to the generation of membrane potentials by  __________   pumps.
•	Stomatal opening correlates with  __________   transport of H+ out of guard cells.
•	The resulting voltage (membrane  __________  l) drives K+ into the cell through specific membrane channels.
•	In general, stomata are  __________   during the day and  __________   at night to minimize water loss when it is too dark for photosynthesis.
•	At least three cues contribute to stomatal opening at dawn.
•	First, blue-light receptors in the guard cells stimulate the activity of ATP-powered  __________   pumps in the plasma membrane, promoting the uptake of K+.
•	A second stimulus is depletion of  __________   within air spaces of the leaf as photosynthesis begins.
•	A third cue in stomatal opening is an internal “clock” located in the guard cells.
•	Even in the dark, stomata will continue their daily rhythm of opening and closing due to the presence of internal clocks that regulate cyclic processes.
•	The opening and closing cycle of the stomata is an example of a  __________   __________  , cycles that have intervals of approximately 24 hours.
•	Various environmental stresses can cause stomata to close during the day.
•	When the plant is suffering a water  __________  , guard cells may lose  __________   and close stomata.
•	__________   acid, a hormone produced by the mesophyll cells in response to water deficiency, signals guard cells to close stomata.
•	While reducing further wilting, this also slows photosynthesis.
2. Xerophytes have evolutionary adaptations that reduce  __________  .
•	Plants adapted to arid climates, called  __________  , have various leaf modifications that reduce the rate of transpiration.
•	Many xerophytes have small, thick leaves, reducing leaf  __________   __________   relative to leaf volume.
•	A  __________  cuticle gives some of these leaves a leathery consistency.
•	During the driest months, some desert plants  __________   their leaves, while others (such as cacti) subsist on water  __________   in fleshy stems during the rainy season.
•	In most plants, the stomata are concentrated on the  __________   (shady) leaf surface.
•	In xerophytes, they are often located in  __________   (“crypts”) that shelter the pores from the dry wind.
•	__________   (“hairs”) also help minimize transpiration by breaking up the flow of air, keeping humidity higher in the crypt than in the surrounding atmosphere.
•	An elegant adaptation to arid habitats is found in ice plants, in succulent species of the family Crassulaceae, and in representatives of many other families.
•	These assimilate  __________   by an alternative photosynthetic pathway, crassulacean acid metabolism (__________  ).
•	Mesophyll cells in CAM plants store CO2 in organic  __________   during the night and release the CO2 from these organic acids during the day.
•	This CO2 is used to synthesize sugars by the conventional  __________   photosynthetic pathway, allowing the stomata to remain  __________   during the day when transpiration is greatest.
E. Translocation of Phloem Sap
•	The  __________   transports the organic products of  __________   throughout the plant via a process called __________  .
•	In angiosperms, the specialized cells of the phloem that function in translocation are the  __________   __________  .
•	These are arranged end to end to form long  __________   __________   with porous cross-walls between cells along the tube.
•	Phloem sap is an aqueous solution in which  __________  , primarily the disaccharide  __________  , is the most common solute.
•	Sucrose concentration in sap can be as high as 30% by weight.
•	Sap may also contain minerals, amino acids, and hormones.
1. Phloem translocates its sap from sugar  __________   to sugar  __________  .
•	In contrast to the  __________   flow of xylem sap from roots to leaves, the direction that phloem sap travels can  __________  .
•	Sieve tubes always carry food from a sugar source to a sugar sink.
•	A sugar  __________   is a plant organ (especially mature leaves) in which sugar is being produced by either photosynthesis or the breakdown of starch.
•	A sugar __________   is an organ (such as growing roots, shoots, or fruit) that is a net consumer or store of sugar.
•	Mature  __________   are the primary sugar sources.
•	Growing  __________  , __________  , __________  , and  __________   are sugar sinks.
•	A  __________   organ, such as a tuber or a bulb, may be either a source or a sink, depending on the  __________  .
•	When the storage organ is stockpiling carbohydrates during the summer, it is a sugar  __________  .
•	After breaking dormancy in the early spring, the storage organ becomes a  __________   as its starch is broken down to sugar, which is carried away in the phloem to the growing buds of the shoot system.
•	Other solutes, such as minerals, are also transported to sinks along with sugar.
•	A sugar sink usually receives its sugar from the sources nearest to it.
•	The upper leaves on a branch may send sugar to the  __________   shoot tip, whereas the lower leaves of the same branch export sugar to  __________  .
•	One sieve tube in a vascular bundle may carry phloem sap in one direction while sap in a different tube in the same bundle may flow in the opposite direction.
•	The direction of transport in each sieve tube depends only on the locations of the  and  __________   connected by that tube.
•	Sugar from mesophyll cells or other sources must be  __________   into sieve-tube members before it can be  __________   to sugar sinks.
•	In some species, sugar moves from mesophyll cells to sieve-tube members via the  __________  .
•	In other species, sucrose reaches sieve-tube members by a combination of  __________   and  __________   pathways.
•	For example, in corn leaves, sucrose diffuses through the symplast from mesophyll cells into small veins.
•	Much of this sugar moves out of the cells into the apoplast in the vicinity of sieve-tube members and companion cells.
•	Companion cells pass the sugar they accumulate into the sieve-tube members via  __________  .
•	In some plants, companion cells (__________   cells) have numerous ingrowths in their walls to increase the cell’s surface area and enhance the transfer of solutes between __________   and  __________  .
•	In corn and many other plants, sieve-tube members accumulate sucrose at concentrations two to three times higher than those in mesophyll cells.
•	This requires  __________   transport to load the phloem.
•	__________   pumps generate an H+ gradient, which drives sucrose across the __________   via a  __________   protein that couples sucrose transport to the __________   of H+ back into the cell.
•	Downstream, at the  __________   end of the sieve tube, phloem unloads its sucrose.
•	The mechanism of phloem unloading is highly variable and depends on plant species and type of organ.
•	Regardless of mechanism, because the concentration of free sugar in the sink is  __________   than in the phloem, sugar molecules  __________   from the phloem into the sink tissues.
•	Water follows by __________  .
2. __________   __________   is the mechanism of translocation in angiosperms.
•	Phloem sap flows from source to sink at rates as great as 1 m/hr, faster than can be accounted for by either diffusion or cytoplasmic streaming.
•	Phloem sap moves by bulk flow driven by  __________   pressure.
•	Higher levels of sugar at the source  __________   the water potential and causes water to  __________  into the tube.
•	Removal of sugar at the sink __________   the water potential and causes water to flow  __________   of the tube.
•	The difference in  __________   pressure drives phloem sap from the __________   to the  __________  .
•	Pressure flow in a sieve tube drives the bulk flow of phloem sap.
1.	Loading of  __________   into the sieve tube at the source reduces the water potential inside the sieve-tube members and causes the uptake of  __________  .
2.	This absorption of water generates hydrostatic  __________   that forces the sap to  __________   along the tube.
3.	The pressure is relieved by  __________   of sugar and  __________   of water from the tube at the sink.
4.	For leaf-to-root translocation, __________  recycles water from sink to source
•	The pressure flow model explains why phloem sap always flows from  __________   to  __________  .
•	Researchers have devised several experiments to test this model, including an innovative experiment that exploits natural phloem probes: __________   that feed on phloem sap.
•	The closer the aphid’s stylet is to a sugar source, the faster the sap will flow and the greater its sugar concentration.
•	In our study of how sugar moves in plants, we have seen examples of plant transport on three levels.
1.	At the cellular level across membranes, sucrose accumulates in phloem cells by  __________   __________  .
2.	At the short-distance level within organs, sucrose migrates from mesophyll to phloem via the  __________   and  __________  .
3.	At the long-distance level between organs, __________   __________   within sieve tubes transports phloem sap from sugar  __________   to sugar  __________  .
•	Interestingly, the transport of sugar from the leaf, not photosynthesis, limits plant yields.
•	Genetic engineering of higher-yielding crop plants may depend on a better understanding of factors that limit bulk flow of sugars.