Background:

The process of photosynthesis is the chemical pathway by which all plants and some protists make food from carbon dioxide, water and sunlight. The entire photosynthetic pathway is a complex series of enzyme transformations that take place in chloroplasts. During the transformation, hydrogen from water is added to molecules of carbon dioxide to make carbohydrates:

The overall effect is that carbon dioxide is combined with hydrogen to produce carbohydrate molecules – mainly sugars, starch, and cellulose.

The rate of photosynthesis depends on several conditions including which colors of light are available to be absorbed by the pigments in a plant leaf. The principal pigment in advanced plants is chlorophyll a. Chlorophyll b, carotenes, and xanthophylls play a secondary role. They transfer the energy they absorb to chlorophyll a for use in photosynthesis. The different pigments absorb different colors of light.

When a plant is exposed to light, it undergoes photosynthesis and aerobic cellular respiration at the same time. When the plant is not exposed to light, it undergoes aerobic cellular respiration (and uses oxygen), but it does not undergo photosynthesis.

In this lab you will use a low pressure sensor to measure the change in pressure in a test tube containing an aquatic plant, Elodea, that is exposed to white light, and then measure the change in pressure when the plant is exposed to green light, and no light.

When photosynthesis occurs in the Elodea, the released oxygen increases the pressure in the test tube. When aerobic cellular respiration occurs, oxygen is consumed and the pressure in the test tube decreases slightly. (Carbon dioxide released during respiration is easily dissolved in water.). When both photosynthesis and aerobic cellular respiration occur, the pressure increases, but not as much as it would due to photosynthesis alone.

How does the rate of photosynthesis change when a plant is exposed to different light conditions?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: Be careful when cutting with sharp objects. Lamp will be HOT.

*Note - keep the sensor dry in this experiment - it is not designed to get wet.

• With a help more drops of glycerin, put a narrow connector into the other end of the tube, and then push the narrow connector into the top of the rubber stopper:

• Place the lamp next to the beaker. The tube, beaker, and lamp should be as close together as possible. Do not turn on the lamp yet.

Step 4:

Data collection:

White Light

Turn on the lamp. Click the Start button ( ) to begin collecting data. Data collection should stop automatically after 10 minutes - but if it does not, press the Stop ( ) button. Turn off the lamp. CAREFULLY disconnect the tubing from the Low Pressure Sensor to release the pressure in the tubing, and then re-connect the tubing.

Step 5:

No Light

Cover the test tube with the aluminum foil, then repeat step 4.

Step 6:

Green Light

Remove the aluminum foil, then add a few drops of green food colouring to the water in the beaker (enough to give it a medium green colour), then repeat step 4.

Step 7:

No Light - green pre-treatment

Repeat step 4 once again, but DO NOT turn on the lamp and wrap the test tube with aluminum foil again.

Step 8:

Clean up all your experimental apparatus as instructed by your teacher.

Analyzing and Interpreting:

Step 9:

Use the graph display to determine the minimum and maximum pressure for each run of data. Click the 'Statistics' menu button ( ). Minimum and Maximum are selected by default. The values for min and max are shown in the legend in the graph:

Step 10:

Record all of the following in a suitable data table. If you wish, download and print out this pdf data table:

• Record your values for the minimum and maximum pressure for each run.
• Calculate the difference between the minimum and maximum pressure for Run #1 and record this as Net Change – Photosynthesis (White Light).
• Calculate the difference between the minimum and maximum pressure for Run #2 and record this as Net Change – Aerobic Cellular Respiration (White Light).
• Add the net change during photosynthesis to the net change during aerobic cellular respiration. Record the sum as Gross Change – Photosynthesis (White Light).
• Calculate the Rate of Photosynthesis –White Light (per hour) by multiplying Gross Change – Photosynthesis (White Light) by 6.
• Calculate the difference between the minimum and maximum pressure for Run #3 and record this as Net Change – Photosynthesis (Green Light).
• Calculate the difference between the minimum and maximum pressure and record this as Net Change – Aerobic Cellular Respiration (Green Light).
• Add the net change during photosynthesis to the net change during aerobic cellular respiration. Record the sum as Gross Change – Photosynthesis (Green Light).
• Calculate the Rate of Photosynthesis –Green Light (per hour) by multiplying Gross Change – Photosynthesis (Green Light) by 6.

1.	Does photosynthesis use green light? Explain.
2.	Why didn't the pressure increase in the tube under no light conditions?

Forming Conclusions:

3.	Based on the data you have collected, write a summary statement for the following question: How does the rate of photosynthesis change when a plant is exposed to different light conditions?

Extension:

4.	Research whether or not red and blue light will promote photosynthesis, then a design and carry out an experiment to test your research.