Download Teacher Demo/Student Activity: Transpiration

SNC2D Tissues, Organs, and Systems of Living Things Teacher Demo/Student Activity: Transpiration Topics Timing specialized plant tissues
preparation: 3 min
demonstration: 10 min
Specific Expectations SNC2D A.1.1 formulate scientific question about observed relationships, ideas, problems, and/or issues,
make predictions, and/or formulate hypotheses to focus inquiries or research
A1.5 conduct inquiries, controlling some variables, adapting or extending procedures as
required, and using standard equipment and materials safely, accurately and effectively, to
collect observations and data
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
B3.3 explain the links between specialized cells, tissues, organs, and systems in plants and
animals (e.g., muscle cells and nerve cells form the tissue found in the heart, which is a
component of the circulatory system; granum and thylakoid structures act as solar collectors
in the chloroplast to produce carbohydrates for plant growth)
Introduction Xylem and phloem tissues make up the “circulatory” system of a plant. The xylem tissues
transport water from the roots to all of the other living tissues, while the phloem tissues transport
sugars from the green tissues where photosynthesis occurs to non-green tissues such as flowers,
seeds, and roots. Transpiration is the loss of water through open stomata in the leaf and the large
leaf veins. This demonstration allows students to see the xylem tissues and observe the direction
of the water flow. The pale colour of Napa cabbage makes it ideal for observation.
Materials Napa cabbage
blue or red food colouring
water
two 500 mL beakers
razor blade (single edged)
desk lamp with incandescent light bulb
Safety Considerations •
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Handle razors with care. Always cut away from your hands and body.
Dispose of used razors in a proper hard-sided sharps container with a lid.
Procedure Wear appropriate PPE: safety goggles
1.
Dilute 8–10 drops of the food colouring into approximately 200 mL of water in each of the
500 mL beakers
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Use a fresh razor blade to make a horizontal cut to remove two of the leaves from the Napa
cabbage near the base.
Quickly place the cut end of each leaf into the food colouring solution, one in each beaker.
Place only one of the leaves (in a beaker) under the lamp. Turn the lamp on. Place the other
leaf away from a light source for the duration of the activity.
Allow the heat and light to promote transpiration revealing the veins of the leaves and
therefore the location of the xylem tissues.
Predict/Explain
Cut more leaves from the Napa cabbage and allow students to examine them. Have
students describe what they see and relate this to their theoretical knowledge of different
anatomical features of the leaf. Ask students to predict when and where the water flows
most easily. Have students brainstorm ideas about when it would be most advantageous for
water to flow through the leaves and when it might not.
Observe
Reveal the two leaves, the one that has been under the light and the one that was not.
Allow students to observe each leaf.
If this is set up as an inquiry for students to try, they can either check on the leaves
throughout the period or visit later to see if food colouring is coming through.
Explain
Have students revisit their brainstorming, and assess which of their ideas about water flow
were supported by their observations.
Disposal The cabbage leaves can be composted or placed in the municipal compost/organics container or
regular trash. Pour the remaining water down the sink. Follow disposal procedures that are
consistent with school board protocol and appropriate for your municipality.
What happens? The veins of the leaf that has been exposed to the light will turn an intense blue (or red) colour as
water is drawn up in response to the loss of water through the stomata (due to transpiration). The
colour will be most obvious closer to the water/food colouring solution.
How does it work? Stomata are minute structures on plants consisting of two curved cells, called guard cells,
surrounding a small pore-like opening called a stoma. Stomata are typically found on the
epidermis of leaves and other parts of above-ground plants. Stomata open to allow gas exchange
in leaves and close to prevent it. Gases (specifically water vapour, oxygen, and carbon dioxide)
need to be exchanged for photosynthesis to occur. Stomata open in response to light exposure.
The differential in water vapour concentration inside the leaf and the heat provided by the lamp
cause the water to leave the cells by transpiration. Strong forces of cohesion between water
molecules pull them up the length of the tube created by end-to-end xylem tissues. Water is
therefore drawn into the xylem tissues at the bottom of the leaf to replace the water lost through
transpiration.
Teaching Hints/Suggestions 1.
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Apply petroleum jelly to a small section of the top of the leaf that will be placed under the
lamp. The blue food colouring will not enter the area of the leaf covered by petroleum jelly
because transpiration is prevented in these areas and therefore water will not evaporate
from the leaf. Consequently, water is not drawn up in response to the loss of water from the
stomata.
If microscopes are available this is a great opportunity to make cross sections of the dyed
leaves and allow the students to observe vascular tissues close up.
Students can create a flow chart to summarize the flow of water through the leaf, starting at
the dyed water, and ending as the water vapour travels out of the stomata.
A KWL (What I Know/What I Want to Know/What I Learned) chart could be used in this
activity, to have students summarize their prior knowledge, and connect it to their
observations.
Next Steps Ethanol extraction of chlorophyll from leaf mesophyll tissues and staining with iodine can show
that leaf tissues are specialized for energy collection and sugar production.
CAUTION: Ethanol is a flammable and combustible liquid. Use only the smallest quantity of
ethanol possible and ensure that there are no sources of ignition in the room. If ethanol is used to
extract chlorophyll it should always be heated in a water bath well away from flames and other
heat sources.
Iodine stains skin and clothing. Wear safety goggles, a lab apron, and protective gloves when
handling iodine solution.
Additional Resources 1.
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A description of the process of transpiration in plant leaves and how it is involved in the
water cycle is at: http://ga.water.usgs.gov/edu/watercycletranspiration.html
This animation illustrates how water moves through plants by the process of transpiration:
http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&to
picorder=2&maxto=8&minto=1
A sample KWL chart is available here:
http://www.eduplace.com/graphicorganizer/pdf/kwl.pdf