9.2

To supply oxygen to our body cells, we breathe. Airenters our lungs and oxygen diffuses into our blood andis distributed throughout our body. Plants, however,lack lungs and blood to take in gases from the air. How,then, do they exchange gases with their environment?

The stomata in the outer tissues of the plant's leavesallow gases to diffuse in and out of the leaf. Inside theleaf, between the upper and lower leaf surfaces, thereare spaces between some of the cells. Gases move in andout of these intercellular spaces. Here, carbon dioxide,oxygen, and water vapour move by passive transportbetween the plant cells and the surrounding air.

leaf hair

Water andminerals enterleaf throughxylem. \

palisadetissue cells

air space

leaf vein

.spongytissue cells

~Sugar exits

phloem.

~- / "\ '~ guard cell

CO2 enters leaf O2 and H2Oh stomata. exit leaf

through stomata.

~ Gases diffusethrough the cell membranes of

the spongy tissue cells. Carbon

dioxide enters the leaf through

stomata and diffuses into the

spongy tissue cells. Oxygen

diffuses out of the spongy tissue

cells and is released from the leaf

through the stomata. Water, which

enters the leaf through the network

of xylem, also exits through the

stomata as vapour.

~

Something in the Air throug

The air we breathe is a mixture of oxygen, carbon dioxide, water vapour,nitrogen, and other gases. However, the ratio of these gases is different in theair we inhale and the air we exhale. To break down glucose and release its energy,our body cells consume oxygen and produce carbon dioxide waste. Therefore,the air we exhale has lower levels of oxygen and higher levels of carbon dioxidethan the air we inhale.

Plant cells, like animal cells, consume some oxygen and produce carbondioxide and water during cellular respiration. During photosynthesis, however,plants also consume carbon dioxide and water and produce oxygen. In fact,when plants photosynthesize, they consume far more carbon dioxide than theyproduce due to cellular respiration. Far greater volumes of gases are exchangedduring photosynthesis than during any other cellular process in plants.

~ A lenticel. Lenticelsusually appear as small whitespots on the woody stems oftree trunks.

Leaves and LenticelsThe most important gas-exchange organ in plants is the leaf. Howdoes carbon dioxide enter a leaf? As Figure 9.5 shows, leaves containair spaces that are connected to the external environment by stomata.

Air diffuses through the stomata and into the leaf. It circulates in thespaces between the spongy and palisade tissue cells. Carbon dioxidediffuses down its concentration gradient, dissolving into the wateryfilm around the cells and diffusing into the cells themselves. There, thechloroplasts use the carbon dioxide in photosynthesis. Oxygen produced i

during photosynthesis passes out of the cells and into the air spaces. L-The oxygen then diffuses through the stomata and out of the plant.

In the roots and stem, some gas exchange can also occur in cells near thesurface. However, in woody plants, such as trees, layers of dead cork cellsand waxy substances prevent direct gas exchange between the externalenvironment and living cells below the wood. Lens-shaped openings, calledlenticels, perforate the bark of these plants (see Figure 9.6). Air can diffusethrough the lenticels. Lenticels enable cells within the roots and stem toexchange oxygen and carbon dioxide with the environment.

Chapter 9 From Cell to Organism: Focus on Plants. MHR 325

QuestionHow can you detect gas exchange in plants?

HypothesisFormulate a hypothesis about how light affects the amount of carbon dioxideconsumed by plants.

Procedure0 Read the entire procedure. Prepare a data table

for collecting and recording all the informationyou will need during this experiment.

Safety Precautions

___~n. Bromothymol blue is poisonous if ingested and can

irritate your skin. Always wear safety goggles, a labapron, and gloves when handling this solution.

. Avoid spilling bromothymol blue on your clothes, asit stains.

. Follow your teacher's instructions for handling anddisposing of bromothymol blue.

. Wash your hands at the end of the investigation. . Working with a lab partner, place four test tubes- .. -- -

Apparatussafety goggles4 glass test tubes with stopperstest tube rackpermanent marker2 medicine droppersrubber gloves

. Use a medicine dropper to add 20 drops of

Materials100-200 mL tap water50 mL bottle bromothymol blue solution2-4 sprigs of Cabomba20-50 mL carbonated water

326 MHR . Unit 3 Cycling of Matter in Living System!

in a test tube rack. Using a permanent marker,label them 1 through 4. Fill each test tube withtap water.

bromothymol blue to each test tube. Bromothymolblue is an indicator solution that changes colourin the presence of carbon dioxide. High carbondioxide concentrations produce a yellow colour.Low carbon dioxide concentrations producea blue colour.

As plants exchange gases with the environment, they alter the composition of the air aroundthem. One way to measure a plant's gas exchange with its environment is to determine howmuch carbon dioxide levels change in the surroundings. In this investigation, you willindirectly observe carbon dioxide consumption by the aquatic plant Cabomba.

0 Use a different medicine dropper to add 10 dropsof carbonated water to test tubes 2 through 4.

I I I I20 dropsbromothvmol blue

I I I10 dropscarbonated water

1 Cabomba

..

test tube

" Add a sprig of Cabomba to test tubes3 and 4 and stopper all of the tubes.

1-

. Record the colour of the water ineach tube.J . Place test tubes 1 ilirough 3 in brightsunlight, and place test tube 4 in adark location.

r0 Based on your hypothesis, predict

how each treatment condition inyour data table will affect the colourof the solution.

. Observe test tubes 1 through 3periodically for 60 min. In yournotebook, record the colours of thesolutions in the test tubes duringthis period. After 60 min, observethe solution in test tube 4 and noteits colour.

~ Observe all four test tubes againthe following day. Record your results.

Chapter 9 From Cell to Organism: Focus on Plants. lvlliR 327

?The evaporation of water has acooling effect. That's why people

I perspire and dogs pant on hotsummer days. Plants can useevaporative cooling to createtheir own "air conditioning."Transpiration can cool a leafas much as 10 to 15°C belowthe surrounding air temperature.Although the plant may lose a lotof water, evaporative cooling canprevent heat damage.

Gas Exchange Is Tied to Water LossAs you know from seeing your breath on a cold day, the air you exhale containswater vapour. Plants, too, lose water during gas exchange. Palisade and.spongytissue cells are coated with a thin layer of water. The water evaporates, saturatingthe air spaces within the leaf with water vapour. As air diffuses out of thestomata, some of that water is lost. The evaporation of water from leaves iscalled transpiration. Transpiration can cause a plant to lose as much as 99percent of the water absorbed by the roots. Figure 9.7 shows a large cloudof water vapour produced by transpiration.

~ The combined effect of evaporation from the leaves of thousands of trees producedthis visible cloud of water vapour, which rose up from the forest in the early morning hours.

Plants that lose so much water through transpiration might seem to bein danger of drying out and dying. What keeps these plants from rapidlyexhausting their water reserves? As you read earlier, guard cells can changetheir shape to cause the stomata to open or close. The size of the stomatacontrols the amount of gas exchange and transpiration. When the stomataare open, carbon dioxide can enter the leaf and oxygen and water vapourexit. Therefore, high rates of photosynthesis are possible when the stomataare open. When the stomata are closed, gas exchange and water exchangeare reduced. Less photosynthesis occurs when the stomata are closed.

328 MHR . Unit 3 Cyclinf{ of Matter in Livinf{ Systems

The openi.ng and closing of the stomata are determined by the amount ofwater in the guard cells. Water moves into and out of the guard cells by osmosis.As water moves into guard cells, the water pressure inside the cells increasesand causes the cells to swell. The high water pressure, called turgor pressure,pushes the elastic cell membrane against the rigid cell wall. The swollen guardcells change shape, opening the stoma. Water vapour then passes O\Jt of theleaf through transpiration. Transpiration causes water to be lost from the plant'scells. As the amount of water in the guard cells decreases, the cells deflate andchange shape again, as shown in Figure 9.8. This action closes the stoma.

?During droughts, dry, non-turgidguard cells keep the stomataclosed and reduce further waterloss. However, as a result, manyplants photosynthesize lessbecause they get less carbondioxide from the air. This is onereason crop yields are lowerduring droughts.

Pause&- Reflect

In plant cells, the cell wall limitsthe amount of water that can flowinto the cell by osmosis. Waterstops diffusing into the cell whenthe cell's turgor pressure reachesan upper limit. Compare howplant and animal cells respondto the continued inflow of waterby osmosis.

~ In the centre of this micrograph, you can see an open stoma surrounded by turgid(swollen) guard cells. On the right, is a closed stoma. How does the stomata being open duringthe day and closed at night affect photosynthesis and water loss in plants?

In most plants, the stomata open during the dayand close at night. However, in plants adapted toextremely dry conditions, the stomata only openat night. In desert plants, carbon dioxide is storedin a different chemical form until it can be used inphotosynthesis during daylight hours.

Although stomata can help prevent excess waterloss, a plant can still dry out if its water source (soil,for example) is depleted. The plant's leaves droopand wither, and the stem softens and bends as shownin Figure 9.9. What happens to the plant's cells thatcause it to wilt? Like the guard cells, other cellsthroughout the plant have reduced turgor pressure asa result of water loss. If they are supplied with morewater, the limp cells have their turgor pressurerestored, which renews their shape and rigidity.You can learn more about turgor pressure in thenext Find Out Activity.

~ Turgor pressure can act as a plant's "skeleton." Cellswith high turgor pressure have firm, rigid shapes. Cells in thestems of non-woody plants need high turgor pressure to holdthe plant upright.

Chapter 9 From Cell to Organism: Focus on Plants. MHR 329

Find Out

place a cover slip on the slide.

3. Prepare another onion specimen ~ n steps 1

and 2, but use three drops of salt w er instead.

4. Allow your specimens to sit for 5 in or moreand then examine them under the microscope.Use low power first, before moving to higher

magnifications. Draw what you see.

Open and ShutIn this activity, you will test the effect of saltwater on the turgor pressure in guard cells.

Safety Precautions E4I. Be careful when using sharp objects such

as scissors and tweezers.. Handle microscope slides and cover slips

carefully so they do not break and cut you.

Materials

scissorstweezerstap water

green onion stalk2 microscope slides2 medicine droppers

2 cover slipssalt water

microscope

2. Did the activity provide a good model systemfor studying the effect of drought on guardcells? What were its limitations?

Procedure - !~~rjij):ilTr 1$ "JT)! ;~:;;~)i~r.r£

2. Use a medicine dropper to add about three

drops of tap water to the specimen. Carefully

3. Which other green onion cells, if any,

appeared to be affected by the salt water?What do you infer happened to these cells?

Section 9.2 SummaryThe stomata allow carbon dioxide, oxygen, and water vapour to diffusethrough the leaf. Plant cells and animal cells consume oxygen and producecarbon dioxide and water during cellular respiration. During photosynthesis:plants consume carbon dioxide and produce oxygen. Water evaporation fromleaves is called transpiration. Stomata can change their size to regulate gasexchange and prevent excessive water loss. The water pressure, or turgorpressure, in the guard cells surrounding the stomata regulates the stomata.If transpiration is high, pressure in the guard cells is reduced, and thestomata close to minimize gas exchange and conserve water.

The lower epidermis of a leaf mayhave between 10 000 and 100 000stomata per cm2, How manystomata would be present on aleaf with a surface area of 10 cm2and an average of 50 000 stomataper cm2? How many guard cellswould there be?

Check Your Understanding1. Which gases are produced and consumed during photosynthesis in plants:

2. The cells of some plant stems are surrounded by a protective layer ofcork or waxy material. Describe how the cells of stems exchange gasesacross this barrier.

3. Apply How could you demonstrate which leaf structure regulates theamount of transpiration that occurs in a plant?

4. Thinking Critically Why do plants require oxygen?

5. Apply Design an experiment to test the effect of a range of watertemperatures on the amount of photosynthesis in an aquatic plant.

330 MHR . rnit 3 Cycling of Matter in Living Systems

1. Describe the shape of the guard cells in tapwater and in salt water. Explain what happenedto make the guard cells look different.

1. Cut the stalk of a green onion. Use tweezers topeel off a thin section of the epidermis. Placethe specimen in the centre of a slide.