AP Biology
Name___________________
Transpiration Lab
Objectives: In this laboratory, students will:
-Understand how water moves from roots to leaves in terms of the physical and
chemical properties of water and the forces provided by differences in water potential.
-Understand the role of transpiration in the transport of water within a plant.
-Understand the structures used by plants to transport water and regulate water
movement.
-Test the effects of environmental variables on rates of transpiration using a controlled
experiment.
Introduction: The amount of water needed daily by plants for growth and maintenance of
tissues is small in comparison to the amount that is lost through the process of transpiration.
Transpiration is the evaporation of water from plant surfaces. If the water that is lost from
aerial plant parts is not replaced by water transported up from the plant roots, the plant will
wilt and eventually die.
Water is transported up from plant roots in the xylem. Other structures involved in this process
are the stomata of leaves, each of which is surrounded by two guard cells. The stomata of a
leaf open into the air spaces that surround the mesophyll (middle) cells of the leaf. Water
evaporates from the leaf surface, a process called transpiration. As water leaves the leaf, it
pulls additional water molecules to the surface due to hydrogen bonding between the
molecules. Since the opening of stomata, which allows transpiration to occur, is also required
for the entry of CO2 used in photosynthesis, a balance must be maintained between the two
processes by regulating the opening and closing of stomata on the leaf surface.
Many environmental conditions, including those conditions that influence the opening and
closing of stomata, will affect the rate of transpiration. Increases in temperature, light
intensity, and the presence of dry air currents can increase the transpiration rates. A humid
environment usually slows the rate of transpiration. In this exercise, you will measure the rate
of transpiration over time with potted plants.
Procedure:
1. Moisten the root ball of the plants you are given completely. Let the excess water drain
out.
2. Wrap the entire root ball in a plastic bag, snug it up to the stem with string, and mark
with your group name.
3. Weigh the mass of each plant with an electronic balance and record on data table.
4. Place one plant in each of the various conditions: Classroom, Lighted Environment, Dark
Environment, Near a Fan Blowing Air, Misted Aquarium.
5. Weigh each successive day for one week.
6. Note: If your plant blooms, be sure any leaves or blooms that fall off are put back in the
center of the plant to be weighed each day. Otherwise, their mass will be represented in
your data as water loss.
7. Hint: Write the mass of the plant and bag on the plastic bag each day so data will not get
“lost” during the week.
8. Calculate the amount of surface area for each plant by calculating it for one leaf in cm 2
for one leaf and extrapolating to the entire plant by counting the number of leaves.
9. To determine % water loss each day, subtract the mass from the Day 1 mass. Then,
divide this by the total mass from Day 1 X 100.
Hypothesis:_________________________________________________________________
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Data Table: Mass of Plants, grams
Name of Plant Used in Experiment: _____________________________________
Environmental Day 1
Condition:
Classroom
Dark
Light
Blowing Air
Humidity
% Water loss/day:
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Data Table 2: Surface Area of Plants
Environmental
Condition
Classroom
Surface Area of 1
Leaf: cm2
Total Number of
Leaves in Plant
Total Surface Area
Dark
Light
Blowing Air
Humidity
Data Table 3: Cumulative Water Loss in cm2
Environmental
Condition
Classroom
Total Water Loss in 7 Days
Water Loss in g/cm2
Dark
Light
Blowing Air
Humidity
Analysis Questions:
1. Which treatment best serves as a control for this experiment? Why?
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2. Explain how each of the experimental conditions causes an increase or decrease in
transpiration compared to the control.
Treatment: _________________
Effects:
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Treatment:__________________
Effects:
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Treatment: ___________________
Effects:
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Treatment: ____________________
Effects:
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3. What are disadvantages to a plant keeping its stomates closed? Explain
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4. What are some adaptations that enable plants to reduce water loss from their leaves?
Include both structural and physiological adaptations.
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