9.3-PC'

Within in~vidual cells, water is transported mainly by osmosis. In an entiremulticellular organism, water must often be transported over much longerdistances than across a single cell. Some trees, such as the one in Figure 9.10,transport water as far as 100 m or more from the root tips to the highest leaves.In this section, you will investigate the transport of water and nutrients in theplant vascular system. You will also identify the cells, tissues, and organs thatmake up this system.

Xylem Vessels and Phloem VesselsVascular plants have a system of vessels that transport water, minerals, and sugarsthroughout the plant. Like the circulatory system that carries blood throughoutyour body, the plant vascular system is made up of a series of interconnectedtubes that extend throughout the plant. Xylem and phloem are the specializedtissues that make up this system of transport. You have seen xylem and phloemin leaves. These tissues are also found in the roots and stems of many plants.

The xylem tissue transports water and dissolved minerals from the soil to theleaves. In a mature plant, most xylem cells are dead. They form hollow tubesconsisting of only the cell walls. The cells are linked end to end, forming longcontinuous tubes called xylem vessels. Xylem vessels extend from near the tipsof the roots up into the rest of the plant. WIthout the water transported by thexylem vessels, the plant would die, since water is used in photosynthesis. In somenon-woody plants, the water transported by the xylem vessels also prevents wilting.

Some of the sugars produced during photosynthesis are transportedthroughout the plant by cells of phloem tissue. As with xylem, phloem iscomposed of cylindrical cells joined end to end to form phloem vessels.Unlike xylem, however, phloem cells are living cells. Their cell walls are porous,allowing them to exchange materials with neighbouring cells. Sugary sap flowsdown the phloem vessels by passing through these pores. Figure 9.11 illustrateshow xylem vessels and phloem vessels are grouped together in bundles inroots and stems of plants.

A B~ Cross sections through A the root of a plant and B the stem of a plant show thatstrands of xylem and phloem are grouped together in bundles.

Chapter 9 From Cell to Organism: Focus on Plants. MHR 331

over long distances? How doindividual plant cells functionin these processes?

Figure 9.12 gives a more detailed look at the parts of xylemand phloem tissue. The long hollow cells within xylem vesselsare called tracheids or vessel elements. Some types of plantshave only tracheids, while others have both tracheids and vesselelements. Both of these structures begin as living cells, whichgrow end-to-end in a young plant. When these cells mature, thecontent of their cells die, leaving only the cell walls. Fluids passfrom one tracheid or vessel element to the next within the xylem,to move water through the plant. Small pits, thin areas in theend walls, cpnnect the tracheids. Perforations in the end wallsconnect vessel elements. Phloem cells consist of sieve tubes andcompanion cells arranged end-to-end. These cells connect inlong tubes, separated by sieve plates, to make the phloem vessels.

pit

tracheid

]vesselelement

xylem

Water Uptake in Roots

Water and minerals enter a plant from me roots. At me cote ofme root are me xylem and phloem, encircled by several omerlayers of cells. Epidermal tissue covers me root. At its tip, meepidermal cells are permeable to water, and mis location iswhere most water uptake occurs. Water enters me cells of meroot epidermis by osmosis. The surface area for absorbing waterand dissolved minerals from me soil is increased by hundreds ofroot hairs (see Figure 9.13). Each tiny root hair is an outgrowthof a single epidermal cell. Water continues diffusing mroughme root tissue until it reaches me xylem vessels.

sieve-tubeJelement phloem

companion cell

parenchyma cells

sieve plate

~ Shown here are illustrations of xylemvessels and phloem vessels superimposed onelectron micrographs of these structures. Xylemtransports water and dissolved minerals from theroots to the leaves. Phloem carries sugar-rich sapfrom the leaves throughout the plant.

~ Water and dissolved minerals enter the roots byosmosis through the root hairs. On the left is a scanning electronmicrograph of a radish seed root hair. As shown above, water andminerals move from the root hairs into or between cells fartherin the root. At the innermost layer of these cells, substancesare prevented from moving between the cells because ofa waterproof seal around each cell.

332 MHR . Unit 3 Cycling of Matter in Living Systems

Although water readily diffuses across the cell membranes, minerals do not.Root cells use facilitated diffusion or, if working against a concentration gradient,active transport to move minerals across their membranes.

The solution of water and minerals that accumulates in the root xylem iscalled xylem sap. Xylem vessels carry the xylem sap upward from the roots,through the stem, and into the leaves. As the xylem tissue enters the leaves, thevessels branch into the many veins often visible on the leaf surface. At the endof each xylem vessel, water and minerals are absorbed by the cells for the leaf.In the next Find Out Activity you will investigate the properties that allowfluid in the xylem to flow against gravity.

Find Out

IModelling Water Transport:Pushing and Pulling

You have learned how root cells transport water andminerals from cell to cell using diffusion and activetransport. Once xylem sap enters the xylem vessels,how does the fluid flow upward against gravity?

3. While you are pullingthe water column upfrom above, have yourlab partner use scissorsto snip a small hole inthe side of the plastictubing. Continuepulling the water up.Note what happensto the water column.

4. Touch the tip of a microcapillary tube tothe surface of the water. What happens?

Safety Precautions

Imm~n. Never eat or drink anything in the laboratory.

. Handle the glass microcapillary tubes carefully.

They break easily and, if broken, could injureyou or a classmate.

. Follow your teacher's instructions for disposing

of the glass microcapillary tubes.

Materials

100 mL beakertap waterfood colouring

plastic syringe

20 cm clear plastic tubingscissorsglass microcapillary tube

Procedure - ~:~!iU:I:l" !T[ ~ I~ !;::;~) i~,iJ,

2. Using a syringe and a piece of plastic tubing,experiment with different ways of moving thewater against gravity. Try methods that pushthe water from below as well as methods thatpull the water from above.

What Did You Find Out? . rill~~"JI~r'!IJ: ~nlll iiTiir~.i!~ii::iii.i!l~;

1. Which method of water transport movedthe water column the highest?

2. What limited how high the water columncould go?

3. How did cutting a hole in the plastic tubingaffect your ability to move the water upward?Explain your observations.

4. Cohesion is the property of water that caused itto rise in a continuous column when you pulledit from above. Based on your observations,write your own definition for cohesion.

5. Adhesion is the property of water that causedit to rise in the microcapillary tube. Based onyour observations, write your own definitionfor adhesion in your science notebook.

Chapter 9 From Cell to Organism: Focus on Plants. MHR 333

1. With your lab partner, set up the model system.Fill a beaker with water and add 2-3 drops offood c%uring.

Properties of WaterHow can xylem sap rise in such a long, continuous column? The force holdingthe column together is the same force that allows you to pull water upwardwith a syringe or drink through a straw. The shape of water molecules, andthe weak electrical forces between them, cause water molecules to be attractedto each other. Cohesion, the tendency of water molecules to stick to otherwater molecules, transmits the upward pull from the tip of the leaves to thetip of the roots.

What would happen if the column of water within a xylem vessel were tobreak? The effect would be similar to removing a link from a chain. If you pulledthe chain up from one end, only the chain links above the point of the breakcould be lifted. In a xylem vessel, a break in the water column:such as a bubble,blocks the rising xylem sap. Bubbles can form in the xylem sap as a result offreezing in winter. Only the water molecules above the point of the break can bepulled upward. (You may have witnessed the same phenomenon in the previousFind Out Activity, when you cut a hole in the plastic tubing.) -

Another property of water, adhesion, helps water fight the force of gravity.Adhesion is the tendency of water molecules to stick, or adhere, to certainsurfaces. Just as water molecules are attracted to each other, they are also attractedto the molecules of other substances, such as the glass of a microcapillary tube,or the cellulose wall of a xylem vessel. The clinging of xylem sap to the xylemwalls helps to prevent the sap from falling back down to the roots.

Root Pressure PushesIf you did the previous Find Out Activity, you saw that water can move upwardover long distances by two main mechanisms: pushing and pulling. In the roots,one force that pushes fluid upward is turgor pressure inside the root xylem.

This is called root pressure. As root cells bring minerals into the xylemthrough active transport, the mineral concentration in the

xylem sap increases. This increases the tendency of waterto diffuse into the root xylem by osmosis. Water flows

in, building root pressure in the xylem vessels. Thispressure forces fluid up the xylem. Adhesion of the

xylem sap to the xylem vessel walls helps the fluidclimb upward. Figure 9.14 shows xylem sap oozingfrom a plant's cut stem. In the next Find OutActivity you can set up a model system to studyroot pressure.

~ This tomato plant has been severed near the

334 MHR . Unit 3 Cycling of Matter in Living Systems

bottom of its stem. Xylem sap is oozing from the cut. Whateffect does this injury have on the transport of substancesthrough the xylem?

Find OutUp with Root PressureCan root pressure really provide enough force topush a column of water upward against gravity?In this activity, you will answer this question byusing a dialysis bag to model a root xylem vessel.

Safety Precautions

Im~n. Handle the glass tubing gently. If the tubing

breaks, you could easily cut your hands.

. Dispose of any broken tubing according

to your teacher's directions.

3. Immerse the bag in a beaker filled with tapwater. Use the metal stand to support theglass tube so that it is held upright.

4. Observe the fluid level in the glass tubingover several minutes.

What Did You Find Out? . r:1"".JUI~'j"i~ ~~cf~i ILii:;Jli'~i~iil]JJ

Materials

string10 cm dialysis tubing

concentrated sugar solution

5 cm glass tubing (5 mm diameter)

250 mL beaker

tap water

metal stand with a small clamp

Procedure - ~~:; iiic(1U'ljj ~~l~~ iic:;~~(ii~~!.I~l

2. By what process did water flow acrossthe dialysis membrane?

3. How successful was root pressure in yourmodel system in moving water against gravity?

2. Place a piece of glass tubing in the open endof the fluid-filled dialysis bag. Use string to tiethe open end of the bag around the glasstube. Tie it tightly to prevent leaks.

4. Compare the fluid movement you observedin this activity with the flow of xylem sap.

Transpiration PullsRoot pressure can move water upward a few metres at most. Therefore, rootpressure can account for the flow of xylem sap only in some small plants. Forthe roots of a tall tree to raise a column of water 100 m, the root pressure wouldhave to be more than 10 times greater than atmospheric pressure. Such rootpressures have never been demonstrated. Indeed, many tall trees show nomeasurable root pressure at all. How, then, dqes xylem sap make the rest ofthe journey upward to the leaves?

Xylem sap can flow upward fasterthan 15 m/h. At this rate, how longwould it take for water to travelfrom the roots to the leaves ina tree 80 m tall with roots20 m deep?

Chapter 9 From Cell to Organism: Focus on Plants. MHR 335

1. What happened to the fluid inside the

bag? Why?

1. Work together with a lab partner to use stringto tie closed (tightly) one end of the dialysistubing. Fill the tubing with concentratedsugar solution.

The remaining work of water transport is accomplished by pulling fromabove. Transpiration from the leaves generates this pulling force, or tension.Therefore, although too much water loss through stomata can put a plant atrisk of drying out, transpiration is very beneficial to plants.

As water vapour exits the leaf tissue, the air within the leaf becomes slighdydrier. This causes water to diffuse out of the leaf cells and into the fluid betweenthe cells, where solutes are more concentrated. As a result, more water evaporatesfrom the surface of the cells. Cohesive forces between the water moleculescause more water to be pulled up the xylem vessels in the leaf, replacing thewater that has evaporated. In turn, more water travels along the xylem vesselstoward the leaf cells. Ultimately, this transpirational pull is exerted throughoutthe xylem vessels and to the plant's water source - the soil. Like an unbrokenchain, the entire column of fluid is pulled upward. Figure 9.15 demonstratesthe roles of transpiration, cohesion, and adhesion in this pulling-upward process.

The Sun causes water to evaporate.The energy for xylem transport comes from the Sun.

xylem in leaf vein

water moleculesoutside airstoma

~

Transpiration (evaporation) of water from leaves createstension that pulls the water column in xylem from the roots.

~cohesionI between waterI molecules

::::::::::==;,,> cell wall

adhesion of watermolecules to cell wall

Water column is held togetber by cohesion;adhesion keeps water column in place.

",,' ::::>

root hair~ ~:: particles

xylemwater molecule

~ ,I

Water from soil enters xylem in root;tension in water column extends from leaves to root.

Tension created by transpiration at the leaves creates a forcethat pulls the water in the xylem in the roots and stems upward to theleaves. Leaves absorb the energy in sunlight, which causes waterevaporation through stomata.

336 MHR . Unit 3 Cycling of Matter in Living Systems

Transpiration inDifferent Plant TypesIn this investigation, you will design your ownexperiment to compare the amount of transpirationin different types of plants.

QuestionIs mere a difference in me amount of transpirationfrom different plant types?

HypothesisChoose four to six different types of plants to usein your experiment. With your group members,formulate a hypothesis about how the type of plantaffects the amount of transpiration in plants.

Safety Precautions. Never eat or drink anything in the laboratory.

. Wash your hands with soap and water afteryou have completed the investigation.

Materials

small potted plantswaterclear plastic bags

(large enough tofit over plants)

For tips on designing scientificinvestigations, turn to Skill Focus 5.

Procedure0 With your group, decide how you will test your

hypothesis. Identify the manipulated variableand the responding variable. Be sure to includea control in your experimental design.. Based on your hypothesis, predict what theoutcome of your experiment will be.

e Write a step-by-step outline for your experi-mental procedure. Describe exactly how youwill use the materials listed.

0 Prepare a data sheet for recording your dataand notes.

" Perfonn your experiment.

rh"ntPr Q Frnm rpll tn Oruoni.mo FnMl. nn PJ"nt. 8 MHR 337

I DidYouKnow?, Sugar Transport in PhloemAfter water and minerals have been delivered to the leaves, the plant is readyto carry out photosynthesis. The sugars produced by the palisade tissue cellsand spongy tissue cells provide energy for the whole plant. Before the sugarscan be used by cells in other parts of the plant, they must make the trek fromthe leaves to the stem, roots, and growing shoots and fruits.

The phloem vessels transport the sugars and other substances throughout theplant. Sugar, minerals, and other nutrients are pumped into the leaf phloem byactive transport. As the sugar concentration increases within the phloem cells,water follows the sugar by osmosis. The cells swell with the increase in turgorpressure. The sugar, nutrient, and water mixture, called phloem sap, flowsdown the concentration gradient. The fluid pressure forces the phloem sapthrough the pores in the phloem cell walls and into neighbouring cells, wherepressure is lower. I

~~

A Sucrose enters thephloem in the leaf, increasingthe concentration of thesucrose solution in thephloem. This causes waterto move by osmosis intothe phloem. In turn, themovement of water leadsto an increase in pressurein the phloem.

B Sucrose is removed from thephloem by the tissues of the plantstem and root. This causes theconcentration of sucrose solutionto decrease, and therefore watermoves out of the phloem byosmosis. Pressure in the phloemdecreases.

0

sugar water

A simplifiedillustration of how sugaris transported in a typicalplant. Dots represent theconcentration of sucrose,while the diagonal linesrepresent pressure.

C The pressure gradientbetween the leaf and rootcauses a flow of solutionthrough the phloem fromthe leaf to the root.

As the phloem sap travels down to the roots, some of it leaves the phloemand nourishes the surrounding tissue. The nutrients are taken up by growingshoots and roots, as well as by fruits and other organs that store energy. Thus,in these tissues, the pressure within the phloem remains low. Because thepressure in the roots is almost always lower than in the leaves, phloem usuallyflows downward. Figure 9.16 illustrates the transport of sugar throughouta plant from leaf to root.

338 MHR . Unit 3 Cycling of Matter in Living Systems

Some scientists use insects calledaphids as research assistants tostudy phloem! Aphids drink thewater and nutrients in phloem.They tap the phloem with a long,I needle-like mouthpart, called astylet. The stylet pierces a singlephloem cell, and the pressure inthe phloem force-feeds the aphid.Researchers can anaesthetize anaphid and cut off its stylet. Theaphid's body is removed, and thestylet is left in place, where itcontinues tapping the phloem.The scientists can then collectand analyze the phloem thatoozes through the stylet.

Think About ItWhat happens to a plant if the flow in vasculartissue is interrupted? In 1686, an Italian scientistnamed Marcello Malpighi (1628-1694) asked thisquestion. He investigated it by removing a ringof phloem tissue from a tree.

What to Do0 Read the following account of Malpighi's

experiment. Then answer the Analyzequestions below.

In trees, phloem forms the layer of living tissuejust beneath the bark. Xylem lies beneath thephloem. In his experiment, Malpighi peeledaway a ring of bark and the-outer layer ofliving tissue from a tree.

Shortly after this treatment, a swellingappeared in the bark of the tree immediatelyabove the stripped ring. Sweet-tasting fluidoozed out from this swelling.

Although it appeared at first that Malpighi'smanipulations had not seriously damaged thetree, the tree died a few weeks later.

Trees damaged by removing rings of phloem tissue

Chapter 9 From Cell to Organism: Focus on Plants. MHR 339

I Acro:;'.Canada. , Dr. Malcolm King comes from a long line of

people whose life's work involved healing.His grandfather was a traditionalAboriginal healer who learned aboutmedicines from his own mother. Dr. Kingis carrying on the tradition. Much of hiswork involves exploring herbal medicine,

c' specifically the use of traditional AboriginalU - '---" peoples' remedies to treat respiratory

illnesses, his area of specialization. Dr. King researches diseasessuch as asthma, bronchitis, and cystic fibrosis, and his work hasled him to patent two therapies for chronic respiratory disease.

A winner of many awards for his scientific research and work withAboriginal students, Dr. King is currently a professor of the pulmonarydivision in the department of medicine at the University of Alberta.

Dr. King and his students have been using extracts from ratroot and varieties of licorice root in experiments to find out whatworks best in assisting the lungs to clear themselves of mucusand infection. In his model system, licorice root worked especiallywell in helping to clear mucus; Dr. King believes this is due to thecomplex sugars that give licorice its distinctive sweetness.

Section 9.3 SummaryLong continuous tubes, xylem vessels and phloem vessels, transport fluidsthroughout the plant. Xylem tissue transports water and dissolved mineralsfrom the soil to the leaves. Xylem vessels are made of long, hollow, dead cellscalled tracheids or vessel elements. Phloem vessels transport sugars producedduring photosynthesis throughout the plant. Phloem cells are living cells.

Water first enters plant roots by osmosis. Hundreds of root hairs increasethe surface area over which osmosis occurs. Minerals are moved into the root byfacilitated diffusion or active transport. Fluid can flow through the xylem againstgravity, because of the water properties of cohesion and adhesion. Upward fluidmovement through plants is also facilitated by the pushing pressure createdby turgor pressure in the root xylem and the pulling force of transpirationthrough the leaves.

Pause&- Reflect

The processes of diffusionand active transport both playimportant roles in water transportin plants. List several examplesin which diffusion and activetransport allow for the movementof water as it travels from the soiland through the plant. Name thepart of the plant where diffusionor active transport occurs, anddescribe how the processtransports water.

Check Your Understanding1. Describe the structure of the cells that make up phloem tissue.

2. %11 a daisy plant transpire more in a humid environment or in a dryenvironment? Explain your reasoning.

3. Explain why root pressure alone cannot transport water from the rootsto the leaves of most stems.

4. Apply If the stem of a plant is bent or snapped, the part of the plantabove the bend will usually die, even if it is propped up with a support.

Explain why.

5. Is transpiration part of the gas exchange system in plants, the waterexchange system in plants, or both? Explain your answer.

6. Thinking Critically Maple syrup is a sweet treat harvested from mapletrees. Which part of the tree produces the syrup and from which part ofthe tree is the sap tapped? Explain your reasoning.

7. Thinking Critically Root cells actively transport minerals from the soil.In doing so, they promote water transport in the xylem. Explain whymineral transport influences water transport.

340 MHR . Unit 3 Cycling of Matter in Living: Systems