Julianne Zedalis; John Eggebrecht

Learning Objectives

In this section, you will explore the following questions:

What is water potential, and how is it influenced by solutes, pressures, gravity, and the matric potential?
How do water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants?
How are photosynthates transported in plants?

Connection for AP^® Courses

Information in this section applies to concepts we explored in previous chapters by connecting them to the transport of water and solutes through a plant, showing ways that plants take up and transport materials. These concepts include the processes of photosynthesis and cellular respiration, the chemical and physical properties of water, and the coevolution of plants with mutualistic bacteria and fungi. The vascular system of terrestrial plants allows the efficient absorption and delivery of water through the cells that comprise xylem, whereas phloem delivers sugars produced in photosynthesis to all parts of the plant, including the roots for storage. The physical separation of xylem and phloem permits plants to move different nutrients simultaneously from roots to shoots and vice versa. Nearly all plants use related mechanisms of osmoregulation, and we will focus on the transport of water and other nutrients.

You likely remember the concept of water potential (Ψ) from our exploration of diffusion and osmosis in the chapter where we discuss the structure and function of plasma membranes. Water potential is a measure of the differences in potential energy between a water sample with solutes and pure water. Water moves via osmosis from an area of higher water potential (more water molecules, less solute) to an area of lower water potential (less water, more solutes). The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and other factors (matrix effects). Water potential and transpiration influence how water is transported through the xylem.

Carbohydrates synthesized in photosynthesis, primarily sucrose, move from sources to sinks through the plant’s phloem. Sucrose produced in the Calvin cycle is loaded into the sieve-tube elements of the phloem, and the increased solute concentration causes water to move by osmosis from the xylem into the phloem.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 4 of the AP^® Biology Curriculum Framework. The AP^® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP^® Biology course, an inquiry-based laboratory experience, instructional activities, and AP^® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 2	Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.A	Growth, reproduction and maintenance of living systems require free energy and matter.
Essential Knowledge	2.A.3 Molecules and atoms from the environment are necessary to build new molecules; the movement of water in a plant depends on the properties of water.
Science Practice	4.1: The student can justify the selection of the kind of data needed to answer a particular scientific question.
Learning Objective	2.8 The student is able to justify the selection of data regarding the types of molecules that an animal, plant or bacterium will take up as necessary building blocks and excrete as waste products.
Big Idea 2	Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.A	Growth, reproduction and maintenance of living systems require free energy and matter.
Essential Knowledge	2.A.3 Molecules and atoms from the environment are necessary to build new molecules; the movement of water in a plant depends on the properties of water.
Science Practice	1.1: The student can create representations and models of natural or man-made phenomena and systems in the domain.
Science Practice	1.4: The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Learning Objective	2.9 The student is able to represent graphically or model quantitatively (or qualitatively) the exchange of molecules between an organism and its environment, and the subsequent use of these molecules to building new molecules that facilitate dynamic homeostasis, growth and reproduction.
Big Idea 4	Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A	Interactions within biological systems lead to complex properties.
Essential Knowledge	4.A.4 Interactions and coordination between organs and organ systems provide essential biological activities.
Science Practice	3.3: The student can evaluate scientific questions.
Learning Objective	4.8 The student is able to evaluate scientific questions concerning organisms that exhibit complex properties due to the interaction of their constituent parts.
Big Idea 4	Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A	Interactions within biological systems lead to complex properties.
Essential Knowledge	4.A.4 Interactions and coordination between organs and organ systems provide essential biological activities.
Science Practice	3.3: The student can evaluate scientific questions.
Learning Objective	4.9 The student is able to predict the effects of a change in the component(s) of a biological system on the functionality of an organism(s).
Big Idea 4	Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A	Interactions within biological systems lead to complex properties.
Essential Knowledge	4.A.4 Interactions and coordination between organs and organ systems provide essential biological activities.
Science Practice	1.3: The student can refine representations and models of natural or man-made phenomena and systems in the domain.
Science Practice	6.4: The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective	4.10 The student is able to refine representations and models to illustrate biocomplexity due to interactions of the constituent parts.

The Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.40][APLO 4.12][APLO 2.1][APLO 2.8][APLO 2.9][APLO 2.41][APLO 1.2][APLO 1.22][APLO 1.25][APLO 2.19][APLO 2.32]

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.

Water Potential

Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure 23.31a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 23.31b). Plants achieve this because of water potential.

Photo (a) shows the brown trunk of a tall sequoia tree in a forest. Photo (b) shows a grey tree trunk growing between a road and a sidewalk. The roots have started to lift up and crack the concrete slabs of the sidewalk.

Figure 23.31 With heights nearing 116 meters, (a) coastal redwoods (Sequoia sempervirens) are the tallest trees in the world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. (credit a: modification of work by Bernt Rostad; credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.)

Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ_w^{pure H2O}) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψ_w^{pure H2O}.

The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:

Ψ_{system} = Ψ_{total} = Ψ_{s} + Ψ_{p} + Ψ_{g} + Ψ_{m}

where Ψ_s, Ψ_p, Ψ_g, and Ψ_m refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψ^soil), root water (Ψ^root), stem water (Ψ^stem), leaf water (Ψ^leaf) or the water in the atmosphere (Ψ^atmosphere): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψ^soil must be > Ψ^root > Ψ^stem > Ψ^leaf > Ψ^atmosphere.

Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψ_system, by manipulating the individual components (especially Ψ_s), a plant can control water movement.

Solute Potential

Solute potential (Ψ_s), also called osmotic potential, is related to the solute concentration (in molarity). That relationship is given by the van 't Hoff equation: Ψ_s = –MiRT; where M is the molar concentration of the solute, i is the van 't Hoff factor (the ratio of the amount of particles in the solution to amount of formula units dissolved), R is the ideal gas constant, and T is temperature in Kelvin degrees. The solute potential is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψ_w) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψ_s decreases with increasing solute concentration. Because Ψ_s is one of the four components of Ψ_system or Ψ_total, a decrease in Ψ_s will cause a decrease in Ψ_total. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content (Figure 23.32). Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.

Plant cells can metabolically manipulate Ψ_s (and by extension, Ψ_total) by adding or removing solute molecules. Therefore, plants have control over Ψ_total via their ability to exert metabolic control over Ψ_s.

Visual Connection

Illustration shows a U-shaped tube holding pure water. A semipermeable membrane, which allows water but not solutes to pass, separates the two sides of the tube. The water level on each side of the tube is the same. Beneath this tube are three more tubes, also divided by semipermeable membranes. In the first tube, solute has been added to the right side. Adding solute to the right side lowers psi-s, causing water to move to the right side of the tube. As a result, the water level is higher on the right side. The second tube has pure water on both sides of the membrane. Positive pressure is applied to the left side. Applying positive pressure to the left side causes psi-p to increase. As a results, water moves to the right so that the water level is higher on the right than on the left. The third tube also has pure water, but this time negative pressure is applied to the left side. Applying negative pressure lowers psi-p, causing water to move to the left side of the tube. As a result, the water level is higher on the left.

Figure 23.32 In this example with a semipermeable membrane between two aqueous systems, water will move from a region of higher to lower water potential until equilibrium is reached. Solutes (Ψ_s), pressure (Ψ_p), and gravity (Ψ_g) influence total water potential for each side of the tube (Ψ_total ^{right or left}), and therefore, the difference between Ψ_total on each side (ΔΨ). (Ψ_m , the potential due to interaction of water with solid substrates, is ignored in this example because glass is not especially hydrophilic). Water moves in response to the difference in water potential between two systems (the left and right sides of the tube).

Positive water potential is applied on the left side of a tube by increasing Ψp so that the water level rises on the right side. The equation for water potential is: Ψsystem = Ψtotal = Ψs + Ψp + Ψg + Ψm where Ψs, Ψp, Ψg, and Ψm refer to the solute, pressure, gravity, and matric potentials, respectively. Could you equalize the water level on each side of the tube by adding solute?

Yes, water level can be equalized by adding solute to the right side of the tube so that water moves toward the left until the water levels are equal.
No, water level cannot be equalized on both sides of the tubes by adding solutes with no other action.
Yes, water level can be equalized by adding solute to the left side of the tube so that water moves toward the left until the water levels are equal.
No, water level cannot be equalized by adding solutes because solutes are always pulled down by gravity, thereby not letting water equalize.

Pressure Potential

Pressure potential (Ψ_p), also called turgor potential, may be positive or negative (Figure 23.32). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψ_p (compression) increases Ψ_total, and a negative Ψ_p (tension) decreases Ψ_total. Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψ_p of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in^-2 MPa^-1 = 210 lb/in^-2). As a comparison, most automobile tires are kept at a pressure of 30–34 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure 23.33). Water is lost from the leaves via transpiration (approaching Ψ_p = 0 MPa at the wilting point) and restored by uptake via the roots.

A plant can manipulate Ψ_p via its ability to manipulate Ψ_s and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψ_s will decline, Ψ_total will decline, the ΔΨ between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψ_p will increase. Ψ_p is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψ_p and Ψ_total of the leaf and increasing ΔΨ between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.

Left photo shows a wilted plant with wilted leaves. Right photo shows a healthy plant.

Figure 23.33 When (a) total water potential (Ψ_total) is lower outside the cells than inside, water moves out of the cells and the plant wilts. When (b) the total water potential is higher outside the plant cells than inside, water moves into the cells, resulting in turgor pressure (Ψ_p) and keeping the plant erect. (credit: modification of work by Victor M. Vicente Selvas)

Gravity Potential

Gravity potential (Ψ_g) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (Ψ_total). The taller the plant, the taller the water column, and the more influential Ψ_g becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m^-1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψ_g.

Matric Potential

Matric potential (Ψ_m) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψ_m is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψ_m, the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψ_m is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψ_m cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.

Movement of Water and Minerals in the Xylem

Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ.

Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent.

Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells (Figure 23.34), thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

Visual Connection

Illustration shows a pine tree. A blowup of the root indicates that negative water potential draws water from the soil into the root hairs, then into the root xylem. A blowup of the trunk indicates that cohesion and adhesion draws water up the xylem. A blowup of a leaf shows that transpiration draws water from the leaf through the stoma. Next to the tree is an arrow showing water potential, which is low at the roots and high in the leaves. The water potential varies from ~–0.2 MPA in the root cells to ~–0.6 MPa in the stem and from ~–1.5 MPa in the highest leaves, to ~–100 MPa in the atmosphere.

Figure 23.34 The cohesion–tension theory of sap ascent is shown. Evaporation from the mesophyll cells produces a negative water potential gradient that causes water to move upwards from the roots through the xylem.

Which statement is false?

Negative water potential draws water into the root hairs, cohesion and adhesion draw water up the xylem, and transpiration draws water from the leaf.
Negative water potential draws water into the root hairs, cohesion and adhesion draw water up the phloem, and transpiration draws water from the leaf.
Water potential decreases from the roots to the top of the plant.
Water enters the plant through the root hairs and exits through stomata.

Transpiration—the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.

Control of Transpiration

The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.

Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.

Plants have evolved over time to adapt to their local environment and reduce transpiration(Figure 23.35). Desert plants (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.

Photo (a) shows a cactus with flat, oval, prickly leaves and a red cylindrical fruit on top; (b) is an orchid with a purple and white flower and glossy leaves; (c) shows a field of plants with long stems, many leaves and a bushy head of small golden flowers; (d) is a water lily in a pond. The water lily has round, flat leaves and a pink and white flower.

Figure 23.35 Plants are suited to their local environment. (a) Xerophytes, like this prickly pear cactus (Opuntia sp.) and (b) epiphytes such as this tropical Aeschynanthus perrottetii have adapted to very limited water resources. The leaves of a prickly pear are modified into spines, which lowers the surface-to-volume ratio and reduces water loss. Photosynthesis takes place in the stem, which also stores water. (b) A. perottetii leaves have a waxy cuticle that prevents water loss. (c) Goldenrod (Solidago sp.) is a mesophyte, well suited for moderate environments. (d) Hydrophytes, like this fragrant water lily (Nymphaea odorata), are adapted to thrive in aquatic environments. (credit a: modification of work by Jon Sullivan; credit b: modification of work by L. Shyamal/Wikimedia Commons; credit c: modification of work by Huw Williams; credit d: modification of work by Jason Hollinger)

Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.

Transportation of Photosynthates in the Phloem

Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.

Structures that produce photosynthates for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.

The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.

Translocation: Transport from Source to Sink

Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H⁺ symporter.

Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs (Figure 23.36).

Illustration shows phloem, a column-like structure that is composed of stacks of cylindrical cells called sieve-tube elements. Each cell is separated by a sieve-tube plate. The sieve-tube plate has holes in it, like a slice of Swiss cheese. Lateral sieve areas on the side of the column allow different phloem tubes to interact.

Figure 23.36 Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells.

Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψ_s, which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink (Figure 23.37). Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.

Illustration shows the transpiration of water up the tubes of the xylem from a root sink cell. At the same time, sucrose is translocated down the phloem to the root sink cell from a leaf source cell. The sucrose concentration is high in the source cell, and gradually decreases from the source to the root.

Figure 23.37 Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration causes water to return to the leaves through the xylem vessels.

Science Practice Connection for AP® Courses

Activity

Based on water’s molecular properties, create a visual diagram/model to illustrate how water travels up a 300-foot California redwood tree through xylem.

Lab Investigation

AP^® Biology Investigative Labs: Inquiry-Based, Investigation 11: Transpiration. Design and conduct a series of experiments to investigate the effects of environmental variables on transpiration rates.

Think About It

Desert travelers claim that cactus juice tastes sweeter during the day than at night. Based on your understanding of photosynthesis, transpiration, and the regulation of stomata by guard cells in response to environmental conditions, is there any validity to this claim?

Teacher Support

The activity is an application of AP^® Learning Objective 2.9 and Science Practices 1.1 and 1.4 because students are creating a representation to model how the cohesive and adhesive properties of water help move water from the environment to the leaves of a tree where water is necessary for photosynthesis.
The lab investigation is an application of AP^® Learning Objective 2.8 and Science Practice 4.1 and Learning Objective 2.9 and Science Practices 1.1 and 1.4 because students will collect data to determine the effect(s) of environmental variables on the uptake of water and nutrients necessary for photosynthesis via transpiration.
Think About It: Cactus juice would taste sweeter during the day than at night because photosynthesis is only partially completed at night, ending with the storing of sour-tasting malic acid in vacuoles. During the day, the malic acid is recovered and converted into CO2 and pyruvate, which enters the Calvin cycle, allowing the completion of photosynthesis and the production of sugars. These sugars would allow cactus juice to taste sweeter during the day than at night.
The Think About It is an application of AP^® Learning Objective 4.8 and Science Practice 3.3 because students are evaluating a question about environmental variables and transpiration rates.

23.5 Transport of Water and Solutes in Plants