Measuring the Photosynthesis Rate

Page 1

Figure 1

Most plant life is dependent on photosynthesis to survive. This process uses the energy from light to turn carbon dioxide (CO2) and water into carbohydrates. In this process molecular oxygen is generated. The light is absorbed by pigments in photosynthetic organisms such as chlorophyll in green plants. Under optimal conditions of light intensity, carbon dioxide concentrations and temperature, photosynthesis rates depend on the surface area or mass of the plant exposed to light. In this experiment we follow the photosynthesis rate in the aquatic plant Elodea ernstiae using Pressure Sensors to measure the rate of oxygen release.

einstein™Tablet with MiLAB or Android /IOS Tablet with MiLAB and einstein™LabMate Two 50 ml glass test tubes Two stoppers with a hole for syringe extenders Two syringe extenders* Two three-way valves* Perspex stand to support tubes and sensors One liter flat water bottle (glass or plastic) or tissue culture bottle (heat filter) Two Pressure Sensors (150-1150 mbar) 2 g of fresh Elodea Plastic knife Bright light source (e.g. 150 W Halogen lamp) 0.5% bicarbonate solution *contained in einstein™ Pressure Kit


Assemble the equipment as shown in Figure 1. a. Fill each glass test tube with 0.5% bicarbonate solution. Leave a small volume of air between the solution surface and the stopper. b. Slice the Elodea branch into segments that fit the test tube. c. Place the Elodea segments into one test tube. The other test tube will serve as the experimental control. d. Seal the test tubes tightly with the stoppers. e. Insert syringe extenders into the stoppers (Figure 2).

Figure 2

f. g. h. i.

Attach three-way valve to the other end of the syringe extenders. Connect Pressure Sensors to the valves. Position the light source 25 cm from the glass test tubes (see Figure 1). Fill the flat plastic bottle with water and place it between the light source and the test tubes. The water absorbs the heat that radiates from the light source.


Launch MiLAB (


3. 4.

Connect the Pressure Sensors to ports on the einstein™ Tablet or einstein™ LabMate. In the Current Setup Summary window choose Full Setup and use the table below to set up the experiment. Make sure that only the Pressure Sensors are selected under Measurements.

Program the sensors to log data according to the following setup: Pressure (150 – 1150 mbar) Rate:

Every 1 sec


5000 sec

Checking the experimental setup: Before starting the experiment, make sure that the test tubes are tightly sealed. For more details see Sealing. Performing the experiment 1.


Make sure that the experiment begins with both test tubes at atmospheric pressure. Turn the three-way valves to position A (see Sealing), and then return to position B. The pressure in both test tubes should now equal atmospheric pressure. Turn on the light source.


Tap Run (


Monitor the photosynthesis rate until the pressure in the test tube with the Elodea reaches about 1100 mbar.


Tap Stop (


Save your data by tapping Save (

) to begin recording data.

) to stop collecting data. ).

For more information on working with graphs see: Working with Graphs in MiLAB 1. 2.

To calculate the Transpiration rate you’ll need to create a difference graph. Save the graph

3. 4.

Select the graph of Input 1 (the control flask) then select the lowest point of Input 2. Select Functions a. Select Subtraction from the Functions dropdown list. b. In the Functions drop down menu select Subtract. c. In the G1 drop down menu select Pressure -1. In the G2 drop down menu select Pressure -2. d. In the Name edit box enter a name (e.g. Difference). 5. Apply a linear fit to the difference graph: a. Use the First cursor and the Second cursor by tapping on the screen. Then, select the desired range. )To remove the cursor, select and quickly flick it off the screen in any direction) b. Select Linear fit. The fit equation will be displayed below the x-axis. c. The slope of the fit line is the net reaction rate. An example of the graph, obtained in this experiment is shown below:

Pressure (mbar)

Pressure in the experimental system

Pressure in the control system

Figure 3

Pressure (mbar)

In the following graph, the result of the subtraction and the linear fit to this data are shown:

Figure 4

1. 2. 3. 4.


How is the pressure generated in the experiment related to photosynthesis? Why is a control system necessary in this experiment? Two types of controls can be set: one containing bicarbonate solution only, the other containing bicarbonate solution plus boiled Elodea segments. What is the difference between these two controls? How might an increase in the test tubes’ temperature during the experiment affect the photosynthesis rate?

Photosynthesis rates depend on several factors: mass of Elodea, concentration of bicarbonate, and light intensity. How does each of these factors affect the reaction rate? Using the system described in this experiment, design new experiments to measure the effect of each of these factors. Try repeating the experiment with a double mass of Elodea, stronger light intensity, etc.