AP Biology Investigation 11: a Study of Transpiration

AP Biology Investigation 11: a Study of Transpiration

AP Biology Investigation 11: A Study of Transpiration

Introduction: Transpiration is the evaporation of water from plants. It occurs chiefly in the leaves while their stomata (tiny openings in the undersurface of a leaf) are open for the passage of CO2 and O2 during photosynthesis. Air that is not fully saturated with water vapor (100% relative humidity) will dry the surfaces of cells with which it comes in contact. So the photosynthesizing leaf loses substantial amount of water by evaporation. This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem of the roots and stem. Transpiration accounts for approximately 10% of all evaporating water on this planet. As leaves transpire, the outward flow of water lowers the pressure in the leaf, creating a vacuum that pulls water upward. Remember, water moves from high water potential to low water potential. This force is responsible for most of the water flow in plants, including lifting water to the tops of trees.

Essential Question: How do environmental factors affect transpiration rates in plants?

Purpose: In this investigation, you will compare the rates of transpiration for several plant species under varying environmental conditions. You will investigate the effect of environmental factors (heat, light, and wind) on the transpiration rate.


  • Describe the process of transpiration in vascular plants
  • Investigate the effects of various environmental factors on the transpiration rate in plants

Hypothesis: Under which environmental condition do you think the most transpiration will take place?

Part One: Procedure for Ambient Plant Conditions:

1) Obtain a plant. Take the plant out of the plastic/peat cup.

2) Take a small plastic bag and place the root pack inside the small plastic bag. Grab a piece of masking tape and wrap the tape tightly around the plastic bag right above the root pack. Make sure to have the shoot system of the plant exposed.

3) Take the plant (roots wrapped in plastic) and mass the total plant. Record the initial mass in a data table.

4) Place the plant in an area of the classroom that receives normal “ambient” conditions. This will serve as our control.

5) Mass the plant every day as we come into class. Record data.

Part Two: Procedure for Environmental Conditions Affecting the Plant:

6) As a group, pick an environmental factor that may affect transpiration rates of a plant. Such factors may be wind, humidity, or light availability. Let me know what equipment you need.

7) Repeat the procedures during Part One, incorporating the manipulated variable. Make sure to control as many of the variables as possible. **If you chose humidity, you need to remove the plastic bag before you mass the set-up.**


Construct a table recording your masses of the ambient plant and the plant that was exposed to an environmental factor. Mass should be collected for four days. After the final mass has been taken, calculate the percent change of the mass of the plant (to eliminate variables). % change = (ending mass – begin mass)/begin mass X 100.

Also gather the data of two other groups that chose different environmental factors. Record their data as well.


Construct a graph to show your data and your peers. Identify the independent and dependent variable. Label all axes and give your graph a title.


Using the data, construct a CER table to make a scientific claim about which environmental factor increased transpiration rates.

Extension Analysis Questions: Answer the questions below using complete sentences.

1) Describe how water moves through a plant, starting with the soil and ending with the atmosphere. Make sure to mention water potential, cohesion, adhesion, and stomata.

2) Suppose you coated the leaves of a plant with petroleum jelly. How would the plant's rate of transpiration be affected?

3) Of what value to a plant is the ability to lose water through transpiration?

4) How could you make this experiment more accurate? Explain.