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How does water move throughout plants?

How does water move throughout plants?


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I haven't yet found a decent explanation for how water moves throughout plants. It does seem to travel more efficiently upward than out or down. Why is that? How does it travel through the plant?


Most water moves up through the xylem by capillary action. Imagine dipping a pipette into a small pool of water; the water would rush up into the pipette. Or, imagine dipping the edge of a paper towel in water. The water "runs" up the paper towel. This is capillary action.

As water evaporates out of the leaves and such in higher regions of the plant, a capillary force pulls up more water. If for instance, you were to dry the top of your saturated paper towel, more water would be pulled up from the pool below to wet that top section.

As for a molecular explanation, Wikipedia has a good explanation of Cohesion-tension theory.


How does water move throughout plants? - Biology

Unit Seven. Plant Life

Most of the carbohydrates manufactured in plant leaves and other green parts are moved through the phloem to other parts of the plant. This process, known as translocation, makes suitable carbohydrate building blocks available at the plant’s actively growing regions. The carbohydrates are converted into transportable molecules, such as sucrose, and moved through the plant.

The pathway that sugars and other substances travel within the plant has been demonstrated precisely by using radioactive isotopes and aphids, a group of insects that suck the sap of plants. Aphids thrust their piercing mouthparts into the phloem cells of leaves and stems to obtain the abundant sugars there. When the aphids are cut off of the leaf, the liquid continues to flow from the detached mouthparts protruding from the plant tissue and is thus available in pure form for analysis. The liquid in the phloem contains 10% to 25% dissolved solid matter, almost all of which is usually sucrose. The harvesting of sap from maple trees uses a similar process. The starch, stored over the winter, is converted into sap that is carried up throughout the plant. A hole is drilled in the tree and the sugar-rich fluid is drained from the tree using tubing and collected in buckets. The sap is then processed into maple syrup.

Using aphids to obtain the critical samples and radioactive tracers to mark them, researchers have learned that movement of substances in the phloem can be remarkably fast— rates of 50 to 100 centimeters per hour have been measured. This translocation movement is a passive process that does not require the expenditure of energy by the plant. The mass flow of materials transported in the phloem occurs because of water pressure, which develops as a result of osmosis. The Key Biological Process illustration on the right walks you through the process of translocation. Sucrose produced as a result of photosynthesis is actively “loaded” into the sieve tubes (or sieve cells) of the vascular bundles (panel 1). This loading increases the solute concentration of the sieve tubes, so water passes into them by osmosis (panel 2). An area where the sucrose is made is called a source an area where sucrose is delivered from the sieve tubes is called a sink. Sinks include the roots and other regions of the plant that are not photosynthetic, such as young leaves and fruits. Water flowing into the phloem forces the sugary substance in the phloem to flow down the plant (panel 3). The sucrose is unloaded and stored in sink areas (panel 4). There the solute concentration of the sieve tubes is decreased as the sucrose is removed. As a result of these processes, water moves through the sieve tubes from the areas where sucrose is being added into those areas where it is being withdrawn, and the sucrose moves passively with the water. This is called the pressure-flow hypothesis.

Key Learning Outcome 33.7. Carbohydrates move through the plant by the passive osmotic process of translocation.

Before reading this chapter, you may have wondered how water gets to the top of a tree, 10 stories above its roots. A column of water that tall weighs an awful lot. If you were to make a tube of drinking straws that tall and fill it with water, you would not be able to lift it. The answer to this puzzle was first proposed by biologist Otto Renner in Germany in 1911. He suggested that dry air moving across the tree's leaves captured water molecules by evaporation, and that this water was replaced with other water molecules coming in from the roots. Renner's idea, which was essentially correct, forms the core of the cohesion-adhesion-tension theory described in this chapter. Essential to the theory is that there is an unbroken water column from leaves to roots, a "pipe” from top to bottom through which the water can move freely.

There are two candidates for the role of water pipe, each a long series of narrow vessels that runs the length of the stem of a tree. As you have learned earlier, these two vessel systems are called xylem and phloem. In principle, either xylem or phloem could provide the plumbing through which water moves up a tree trunk or other stem. Which is it?

An elegant experiment demonstrates which of these vessel systems carries water up a tree stem. The bottom end of a stem was placed in water containing the radioactive potassium isotope 42 K. A piece of wax paper was carefully inserted between the xylem and the phloem in a 23-cm section of the stem to prevent any lateral transport of water between xylem and phloem.

After enough time had elapsed to allow water movement up the stem, the 23-cm section of the stem was removed, cut into six segments, and the amounts of 42 K measured both in the xylem and in the phloem of each segment, as well as in the stem immediately above and below the 23-cm section. The amount of radioactivity recorded provides a direct measure of the amount of water that has moved up from the bottom of the stem through either the xylem or phloem.

The results are presented in the graph above.

1. Applying Concepts. What is the dependent variable?

a. In the portion of the stem below where the 23-cm section was removed, do xylem and phloem both contain radioactivity? How about in the portion above where the 23-cm section was removed?

b. In the central portion of the 23-cm segment of the stem (segments 2, 3, 4, 5), do xylem and phloem both contain radioactivity?

a. In the 23-cm section, is more 42 K found in xylem or phloem? What might you conclude from this?

b. Above and below the 23-cm section, is more 42 K found in xylem or phloem? How would you account for this? [Hint: These sections did not contain the wax paper barrier that prevents lateral transport between xylem and phloem.]

c. Within the 23-cm section, the phloem in segments 1 and 6 contains more 42 K than interior segments. What best accounts for this?

d. Is it fair to infer that water could move through either xylem or phloem vessel systems? Drawing Conclusions Does water move up a stem through phloem or xylem? Explain. Further Analysis Devise an experiment along similar lines to test which vessel system is responsible for transporting sugars produced by photosynthesis in a plant's leaves to the cells in its roots.

4. Drawing Conclusions. Does water move up a stem through phloem or xylem? Explain.

5. Further Analysis. Devise an experiment along similar lines to test which vessel system is responsible for transporting sugars produced by photosynthesis in a plant's leaves to the cells in its roots.

2. In vascular plants, phloem tissue primarily

b. transports carbohydrates.

3. The ground tissue that carries out most of the metabolic and storage functions is

4. In roots, growth of lateral branches begins

5. In stems, the tissue responsible for secondary growth is the

6. One difference between monocot and dicot plant stems is the

a. absence of buds in monocots.

b. organization of vascular tissue.

c. presence of guard cells.

7. In vascular plant leaves, gases enter and leave the plant through pores called

8. Which of the following is not a process that directly assists in water movement from the roots to the leaves?

9. The passive process of moving carbohydrates throughout a plant is called


Transport in Plants

Transport is the movement of things from one place to other. It happens all the time. For example, you might transport the stinky bag of trash in your kitchen to the curb for garbage pickup. Or you might be transported from the bus stop to school or work. Transport happens inside our bodies, too. Our heart is connected to a superhighway network of veins and blood vessels that make up our circulatory system, which is responsible for transporting nutrients from the burger you ate throughout your body from your nose to your toes.

TRANSPORT IN PLANTS?

What about transport in plants, how does a Redwood, one of the tallest trees in the world, move water from the soil to the needles on its tallest branches over 300 ft in the air? (That’s over 30 stories high!) Or how does a carrot transport the sugars made in its green, leafy tops below the surface of the soil to grow a sweet, orange taproot? Well, certain types of plants (vascular plants) have a system for transporting water, minerals, and nutrients (food!) throughout their bodies it’s called the vascular system. Think of it as the plant’s plumbing, which is made up of cells that are stacked on top of one another to form long tubes from the tip of the root to the top of the plant. To learn more about it, let’s study the stem.

STEM OVERVIEW

Ah, the stem, the part of the plant that connects the leaves to the roots! But, not all stems are similar! For example, cactus stems are swollen and store water. Some stems twist and have grasping tendrils like the pea plants growing up a garden trellis or lianas in the tropics.

Other stems are covered in thorns, providing lyrical inspiration for 80s power ballads and making the stem less palatable to herbivores. Stems give a plant structural support so they can grow upright and position their built in solar panels (leaves) towards the sun, but stems are also flexible allowing them to bend in the wind and not snap. Despite the shape or modification, inside every stem of a vascular plant is a bundle of tubes, and this my friends is where transport happens in the plant.

STEM VISUALIZATION

To understand transport in plants, let’s start with a little stem anatomy. Imagine that you’re holding a handful of drinking straws and chopsticks with a rubber band around them.

This bundle is your imaginary plant stem.

The rubber band, the drinking straws, and the chopsticks represent the three types of tissues found in vascular plant stems. The rubber band symbolizes the dermal tissue that covers the outside of the plant stem, and like our skin it acts as a protective layer. Ideally the rubber band would completely cover your makeshift stem bundle, so you’ll just have to use your imagination. The chopsticks fill in the space between the rubber band and the drinking straws and represent what is called ground tissue. Ground tissue is made up of cells that provide structural support to the stem. The drinking straws represent the third tissue type, the vascular tissue. Depending on the type of plant, the drinking straws might be arranged in the stem in a very organized way or scattered throughout haphazardly. Regardless of their arrangement each straw has a job to do either transport water and minerals or transport sugars.

XYLEM: DRINK UP!

In our example, the straws that transport water and minerals up from the roots to the leaves are called xylem (zy-lem). Now imagine that each straw is actually a certain type of cell stacked one on top of the other creating a tube. Depending on the type of plant, xylem tissue can be made up of one or two different types of cells. Plants like ferns and conifers have xylem “straws” that are made of slender cells called tracheids. At maturity these cells die, leaving behind a rigid cell wall scaffolding tube to conduct water and minerals. Flowering plants have an additional type of xylem tissue called a vessel element. Like tracheids, vessel elements are dead at maturity, but unlike tracheids, vessel elements are much wider – more like a smoothie straw! This means that they can transport more water at a faster rate. Just think of how much faster you can slurp a soda with a wider straw! Just because vessel elements are wider, doesn’t necessarily mean that they’re better. Vessel elements are prone to getting little air bubbles caught in them, and once an air pocket occurs, the party is over and it is very difficult to move water up the stem.

PHLOEM: IT’S ALIVE!

Back to our imaginary plant stem, the remaining straws transport food made in the leaves to the rest of the plant and are called phloem (flo-um). Phloem tissue is also made up of two types of cells that are less rigid and much more lively than their water carrying compatriots (no really, they don’t die at maturity like xylem cells do). One cell type does the heavy phlo-ing, while the other is the wingman. Here’s how it works: sieve tube elements are masters of flow. They stack one on top of the other separated by perforated plates creating the tube-like structure we’re familiar with. Sieve tube elements clear almost everything out of their cells that could slow the flow including organelles and even their nucleus! Anything that’s leftover gets squeezed up against the cell wall like pushing all the chairs to the side of a room so you can break dance in the middle. The sieve tube elements are busy, but they couldn’t do it alone. Directly connected to the sieve tube elements through holes in their cell walls are their faithful buddies the companion cells. These cells have all the necessary cellular machinery to keep themselves and their adjacent sieve tube element alive and kickin’. And while companion cells don’t conduct food along the stem of the plant, they do play an integral role in loading food into and out of the sieve tube elements.

PRIMARY AND SECONDARY GROWTH: IT TAKES TWO TO TANGO

But don’t forget, plant stems can grow in two directions. Our imaginary plant stem helps us to visualize what the inner workings of a soft, green herbaceous stem – similar to what a dandelion stem might look like.

The dandelion stem will grow in length until it’s taller than the grass around it in your lawn – making it an easy target for the lawnmower. We call the increase in stem length primary growth. How does a stem actually get longer? Do the individual cells along the stem just keep getting bigger and bigger? Nope! (But individual cells and their cell walls will elongate to a certain size.) Primary growth originates in the apical meristems or places of rapid cell division, which are located at the top of the growing plant and at the tips of the roots. New cells are made in the apical meristems, so plant length increases by adding these new cells to the end of the stem, just like if you were using wooden blocks to build a tower. Each block you add to the top increases the height of the structure.

But what about stem growth in a tree? How does the trunk of a tree grow to be so much thicker than a dandelion stem? A tree seedling stem will start off green and flexible but over time, the tree will grow larger, become woody, more massive, and will need structural support to keep itself from falling over. The tree does this by increasing the width of the stem, which is called secondary growth.

Stems get wider at two places: the vascular cambium and the cork cambium. The vascular and cork cambium are also places in the stem where cells are dividing rapidly – the difference is where they are located. Cork cambium is a circular band of dividing cells found just beneath the outer covering of the stem. Its job is to make cork, or the outer most layer of bark that you see on trees. The vascular cambium is also a circular band of dividing cells, but it is located deeper into the stem between the two types of vascular tissue we talked about earlier: xylem and the phloem. The vascular cambium is a jack-of-all-trades. Cells in the vascular cambium divide and if the new cells are located toward the outside of the stem they become phloem, and if they are located toward the inside of the stem the cells become xylem. The vascular cambium will continue to divide creating new layers of cells in two different directions on either side of itself, and over time the stem will become thicker.

WATER AND NUTRIENT TRANSPORT: WHAT MOVES YOU?

So now we know what parts of the stem are responsible for transporting water (xylem) and nutrients (phloem), but we don’t know yet how they move or what drives their movement. Keep in mind that one requires energy and one does not.

Let’s start with water. The movement of water in a plant is like a one-way street, it is unidirectional and it travels along this route: soil -> roots -> stem -> leaves -> air. The movement of water throughout a plant is driven by the loss of water through it’s leaves, or transpiration. The water molecules that move through the xylem are connected in a continuous “stream”. They are able to do this because 1) water molecules really like each other (a property called cohesion) and 2) they also like to stick to other substances (a property called adhesion), and these two properties allow water to move up the xylem “straw” we visualized earlier. As water evaporates into the atmosphere from the surface of the leaf, it “tugs” the adjacent water molecules inside the leaf, which “tugs” on the water molecules in the stem, which “tugs” the water molecules from the roots, which “tugs” water molecules into the roots from the soil. So water evaporating from the leaf initiates the “tug” or pull of water through the stem. But, the important thing to remember is that this movement of water is passive, meaning that it doesn’t require any energy to transport water through the plant.

Now let’s move onto the sweet stuff, phloem. The movement of sugars in a plant is much different than the movement of water. First of all, phloem can move both up and down a plant, which comes in handy when a plant needs energy down below to grow new roots, or when a tasty apple is developing on a high branch. The sugars are made in the leaves as a product of photosynthesis. To get the food made in the leaves to other parts of the growing plant requires energy. So, with the help of some water from the xylem, sugars are actively loaded into the phloem where the sugars were made (which is called the source) and actively offload where they are needed (which is called the sink). Ever seen a dumb waiter in an older home? Phloem loading and unloading works sort of the same way. Someone in the kitchen can open the door and put a plate of food inside the mini elevator, then with the help of some energy and a pulley system, the tray of food is taken up the elevator shaft to another floor where someone opens the door and retrieves it. In plants the movement of nutrients through the phloem is driven by where the sugar is most needed for the growth of the plant.


23.5 Transport of Water and Solutes in Plants

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.

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 (Ψw 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 Ψw 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:

where Ψs, Ψp, Ψg, and Ψm 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 Ψsystem, by manipulating the individual components (especially Ψs), a plant can control water movement.

Solute Potential

Solute potential (Ψs), also called osmotic potential, is related to the solute concentration (in molarity). That relationship is given by the van 't Hoff equation: Ψs = –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 Ψw) 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, Ψs decreases with increasing solute concentration. Because Ψs is one of the four components of Ψsystem or Ψtotal, a decrease in Ψs will cause a decrease in Ψtotal. 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 Ψs (and by extension, Ψtotal) by adding or removing solute molecules. Therefore, plants have control over Ψtotal via their ability to exert metabolic control over Ψs.

Visual Connection

  1. 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.
  2. No, water level cannot be equalized on both sides of the tubes by adding solutes with no other action.
  3. 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.
  4. 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 (Ψp), 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 Ψp (compression) increases Ψtotal, and a negative Ψp (tension) decreases Ψtotal. 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 Ψp 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 Ψp = 0 MPa at the wilting point) and restored by uptake via the roots.

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

Gravity Potential

Gravity potential (Ψg) 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 (Ψtotal). The taller the plant, the taller the water column, and the more influential Ψg 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 Ψg.

Matric Potential

Matric potential (Ψm) 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. Ψm 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 Ψm, 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. Ψm 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. Ψm 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.


Capillary Action and Water

Plants and trees couldn't thrive without capillary action. Capillary action helps bring water up into the roots. With the help of adhesion and cohesion, water can work it's way all the way up to the branches and leaves. Read on to learn more about how this movement of water takes place.

Capillary Action

Capillary Action . in Action! Without capillary action, the water level in all tubes would be the same. Smaller diameter tubes have more relative surface area inside the tube, allowing capillary action to pull water up higher than in the larger diameter tubes

Even if you've never heard of capillary action, it is still important in your life. Capillary action is important for moving water (and all of the things that are dissolved in it) around. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension.

Capillary action occurs because water is sticky, thanks to the forces of cohesion (water molecules like to stay close together) and adhesion (water molecules are attracted and stick to other substances). Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact. Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules. The height to which capillary action will take water in a uniform circular tube (picture to right) is limited by surface tension and, of course, gravity.

Not only does water tend to stick together in a drop, it sticks to glass, cloth, organic tissues, soil, and, luckily, to the fibers in a paper towel. Dip a paper towel into a glass of water and the water will "climb" onto the paper towel. In fact, it will keep going up the towel until the pull of gravity is too much for it to overcome.

Capillary action is all around us every day

We know that no one will ever spill a bottle of Cherry Berry Go drink on the Mona Lisa, but if it happened, capillary action and paper towels would be there to help clean up the mess.

  • If you dip a paper towel in water, you will see it "magically" climb up the towel, appearing to ignore gravity. You are seeing capillary action in action, and "climbing up" is about right - the water molecules climb up the towel and drag other water molecules along. (Obviously, Mona Lisa is a big fan of capillary action!)
  • Plants and trees couldn't thrive without capillary action. Plants put down roots into the soil which are capable of carrying water from the soil up into the plant. Water, which contains dissolved nutrients, gets inside the roots and starts climbing up the plant tissue. Capillary action helps bring water up into the roots. But capillary action can only "pull" water up a small distance, after which it cannot overcome gravity. To get water up to all the branches and leaves, the forces of adhesion and cohesion go to work in the plant's xylem to move water to the furthest leaf.
  • Capillary action is also essential for the drainage of constantly produced tear fluid from the eye. Two tiny-diameter tubes, the lacrimal ducts, are present in the inner corner of the eyelid these ducts secrete tears into the eye. (Source: Wikipedia)
  • Maybe you've used a fountain pen . or maybe your parents or grandparents did. The ink moves from a reservoir in the body of the pen down to the tip and into the paper (which is composed of tiny paper fibers and air spaces between them), and not just turning into a blob. Of course gravity is responsible for the ink moving "downhill" to the pen tip, but capillary action is needed to keep the ink flowing onto the paper.

The proof is in the pudding . I mean, in the celery

You can see capillary action in action (although slowly) by doing an experiment where you place the bottom of a celery stalk in a glass of water with food coloring and watch for the movement of the color to the top leaves of the celery. You might want to use a piece of celery that has begun to whither, as it is in need of a quick drink. It can take a few days, but, as these pictures show, the colored water is "drawn" upward, against the pull of gravity. This effect happens because, in plants, water molecules move through narrow tubes that are called capillaries (or xylem).

Do you think you know a lot about water properties?
Take our interactive water-properties true/false quiz and test your water knowledge.


Matric Potential

Matric potential (Ψm) 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. Ψm 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 Ψm, 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. Ψm 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. Ψm cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.


Overview of Translocation: Transport from Source to Sink

Sugars move (translocate) from source to sink, but how? The most commonly accepted hypothesis to explain the movement of sugars in phloem is the pressure flow model for phloem transport. This hypothesis accounts for several observations:

  1. Phloem is under pressure
  2. Translocation stops if the phloem tissue is killed
  3. Translocation proceeds in both directions simultaneously (but not within the same tube)
  4. Translocation is inhibited by compounds that stop production of ATP in the sugar source

In very general terms, the pressure flow model works like this: a high concentration of sugar at the source creates a low solute potential (Ψs), which draws water into the phloem from the adjacent xylem. This creates a high pressure potential (Ψp), or high turgor pressure, in the phloem. The high turgor pressure drives movement of phloem sap by “bulk flow” from source to sink, where the sugars are rapidly removed from the phloem at the sink. Removal of the sugar increases the Ψs, which causes water to leave the phloem and return to the xylem, decreasing Ψp.

This video provides a concise overview of sugar sources, sinks, and the pressure flow hypothesis:


How Does Water Move Through Plants?

The xylem helps in the movement of water from the root to the leaves. Two types of cells in the xylem, tracheids and vessels, form tubes that allow water to move up the plant. Tracheids are found in all vascular plants, but vessels are only found in flowering plants.

Water moves from the soil to the roots by osmosis and causes a positive pressure. This pressure pushes the water upward to the leaves. Root pressure is highest in the morning. The evaporation of water from the leaves to the atmosphere causes a negative pressure in the xylem, which pulls water up from the roots. This mechanism is called transpirational pull. The vessels transporting the water are small in diameter in order to prevent the water column from breaking.

Water molecules tend to form hydrogen bonds with each other. This intermolecular attraction helps water flow upward against the gravitational force. When water evaporates from the leaf, it pulls another water molecule into it. Water is required for photosynthesis, the process by which leaves produce their own food. The vascular system made up of the xylem helps carry the water and some nutrients to the leaves where it is utilized for photosynthesis.


30.5 Transport of Water and Solutes in Plants

By the end of this section, you will be able to do the following:

  • Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric potential
  • Describe how water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants
  • Explain how photosynthates are transported in plants

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 30.31a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 30.31b). Plants achieve this because of water potential.

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 (Ψw 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 Ψw 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:

where Ψs, Ψp, Ψg, and Ψm 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 Ψsystem, by manipulating the individual components (especially Ψs), a plant can control water movement.

Solute Potential

Solute potential (Ψs), also called osmotic potential, is related to the solute concentration (in molarity). That relationship is given by the van 't Hoff equation: Ψs= –Mi RT 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 Ψw) 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 Ψs is one of the four components of Ψsystem or Ψtotal, a decrease in Ψs will cause a decrease in Ψtotal. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content (Figure 30.32). 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 Ψs (and by extension, Ψtotal) by adding or removing solute molecules. Therefore, plants have control over Ψtotal via their ability to exert metabolic control over Ψs.

Visual Connection

Positive water potential is placed on the left side of the tube by increasing Ψp such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?

Pressure Potential

Pressure potential (Ψp), also called turgor potential, may be positive or negative (Figure 30.32). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψp (compression) increases Ψtotal, and a negative Ψp (tension) decreases Ψtotal. 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 Ψp 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 30.33). Water is lost from the leaves via transpiration (approaching Ψp = 0 MPa at the wilting point) and restored by uptake via the roots.

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

Gravity Potential

Gravity potential (Ψg) 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 (Ψtotal). The taller the plant, the taller the water column, and the more influential Ψg 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 Ψg.

Matric Potential

Matric potential (Ψm) 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. Ψm 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 Ψm, 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. Ψm 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. Ψm 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 30.34), 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 nonfunctional.

Visual Connection

Which of the following statements is false?

  1. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf.
  2. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the phloem. Transpiration draws water from the leaf.
  3. Water potential decreases from the roots to the top of the plant.
  4. Water enters the plants through root hairs and exits through stoma.

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 30.35). Desert plant (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.

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 30.36).

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 Ψs, 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 30.37). 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.


Transpiring trees

The adhesion is quite important in getting the water to the top of a tall tree, but this is only part of it. The other part of the puzzle is called transpiration.

Transpiration is the process where water is evaporated into the air by the leaves of a plant. In a way, this is sort of like the way we perspire or sweat. This can amount to an enormous amount of water that is evaporated off by a tree. For instance, an oak tree can transpire many tons of water every day.

At the top of a tree, though, the transpiration causes a partial vacuum in the tiny tubes and tissues inside the tree that transport water. Because of the adhesion of the water, the fluid moves up to fill the vacuum. This is the same principle as sucking a drink through a straw. The adhesion is so strong, in fact, that it allows the tree to suck up water from over 300 feet below!

If a person thinks about it, this is astonishing. Put in another way, at the pressure of our atmosphere at sea level, water is incapable of rising above 30 feet. However, because the tiny water tubes in a tree are so small in diameter, plus the strength of the cohesion of water, plus the vacuum caused by transpiration, the tree generates a pressure of over 1,000 gravities.

The process is amazingly simple, yet it shows tremendous design. It also took mankind a long time to figure out how it was done. One of the amazing things is that the best pumps designed by man can’t push water straight up 300 feet without increasing the water pressure at the bottom. Trees have no pumps and function with water that is at the same pressure as the water that is around it.

The water isn’t pushed up. That is the secret. Instead, it is pulled up.


Watch the video: Water Transport In Plants (January 2023).