*Lab is not needed to complete this assignment* You are simply making notes based on facts as in what will happen in the situations. Answer all questions and make notes for all statements. Please also fill in the attached document!

Photosynthesis

Background

Figure 1. The mouse, Mus musculus

Can a Mouse Survive in a Jar?

Oxygen was discovered over 200 years ago when Joseph Priestley experimented with mice in jars. He closed a mouse in an airtight jar and, after a short time, the mouse collapsed. Priestley then closed a plant in an airtight jar and it survived for weeks. So he decided to combine the two. The mouse in the jar with the plant was able to survive long past the mouse alone in the jar.

Photosynthesis was not explained during Priestley’s lifetime, so he never found out that the plant in the jar generated oxygen through photosynthesis and that is why the mouse was able to survive. Comparatively, the mouse that collapsed had used up all the oxygen in the jar.

Photosynthesis

Humans, like other animals, require food to generate energy. Plants, in contrast, produce their own food. They use the process of photosynthesis to use carbon dioxide (CO2) and sunlight from the environment in order to generate molecules of sugar. Sugar is further converted to chemical energy that plants need to sustain their existence.

Photosynthesis has been used by organisms for millions of years. The first photosynthetic organisms were the ancestors of modern-day cyanobacteria. Photosynthesis takes sunshine, CO2 from the air, and some hydrogen atoms from water to produce two very important molecules — glucose (C6H12O6) and oxygen (O2).

The basic formula for photosynthesis is:

6CO2+6H2O?C6H12O6+6O2

carbon dioxide + water ? glucose + oxygen

Photosynthetic organisms are the foundation of every ecosystem because they take limited inputs and produce physical matter. In this role, they are referred to as producers. Organisms that consume producers are accordingly called consumers.

Photosynthesis takes place inside chloroplasts, special organelles located in the cells of plants and other photosynthesizing organisms. Chloroplasts are green because they utilize the pigment chlorophyll. The primary light-absorbing organs of plants are the leaves. Although chloroplasts are located in cells throughout a plant, chloroplast density is by far the highest in the leaves. Between 440,000 and 790,000 chloroplasts can be found per square millimeter in the leaf of a plant.

One of the byproducts of photosynthesis is oxygen, an essential molecule vital to the existence of humans and animals on Earth. Earth’s atmosphere is about 22% oxygen and most of the remainder is nitrogen. Humans and animals rely on plants as the source of their oxygen.

Rate of Photosynthesis

The rate of photosynthesis is affected by a number of factors:

• Light intensity
• Temperature
• Availability of water
• Availability of nutrients

There is a maximum rate of photosynthesis that is constrained by the limits of these factors. For example, there is a value for light intensity above which the rate of photosynthesis can no longer increase. Similarly, increasing the temperature from 10 °C to 20 °C will increase the rate of photosynthesis, because enzymes in the plant will be closer to their optimal working temperatures, and molecules in the cells will move faster owing to increased kinetic energy. However, if the temperature is raised above a certain level, the rate of photosynthesis will drop as plant enzymes are denatured.

Net Exchange of Gases

It is important to remember that while carrying out photosynthesis in the chloroplasts, the plant is also carrying out cellular respiration, which releases CO2. In this lab, you will measure the net rate of gas exchange for the combined processes of photosynthesis and cellular respiration.

The CO2 for photosynthesis is supplied in this experiment by sodium bicarbonate dissolved in water. CO2 is much more soluble in water than oxygen is. More CO2 is used up by photosynthesis than is released by respiration. Therefore, it is expected that the net change in CO2 will be negative. This means that more CO2 will go into the plant than will be removed from the water.

On the other hand, oxygen is not very soluble in water, but is produced during photosynthesis. Therefore, it is expected that its net change will be positive. This means that the plant produces more oxygen during photosynthesis than it consumes in respiration.

Overall, the gas being produced and measured is oxygen.

A plant’s rate of respiration can be determined by measuring the rate of oxygen uptake during periods of darkness, when no photosynthesis takes place. Again, oxygen’s insolubility in water helps with this measurement. As the plant respires, oxygen is removed from the gas in the system. At the same time, CO2 is released but remains dissolved in the water. The total change in the volume of the solution is negligible.

About This Lab

In this lab, you will measure the rate of photosynthesis in the aquatic plant Elodea under various conditions. You will measure the rate of photosynthesis by observing gas production. You will explore the gas exchange of the plant with its environment in both light and dark conditions and observe if photosynthesis and respiration take place in parallel when light is present.

You will modify the light source intensity to test how the rate of photosynthesis changes depending on the intensity of light falling on the leaves of the plant. You will also test the effect of temperature on the rate of photosynthesis.

Experiments

Open the simulation by clicking on the virtual lab icon below. The simulation will launch in a new window.

You may need to move or resize the window in order to view both the Procedure and the simulation at the same time.

Follow the instructions in the Procedure to complete each part of the simulation. When instructed to record your observations, record data, or complete calculations, record them for your own records in order to use them later to complete the post-lab assignment.

Procedures

Experiment 1: Measuring the Rate of Photosynthesis

Part 1: Set-up

1. Take a plant light box from the Instruments shelf and place it on the workbench.
2. Take a 250 mL Erlenmeyer flask from the Containers shelf and place it onto the workbench.
3. Take a branch of Elodea from the Materials shelf and add it to the Erlenmeyer flask.
4. Add 100 mL of 0.1 M sodium bicarbonate solution from the Materials shelf to the Erlenmeyer flask. Make sure the solution covers the branch, as Elodea is a submergent aquatic plant and acquires carbon dioxide from the water.
5. Record the plant name and the solution the plant is in to reference later.

1. Place the Erlenmeyer flask into the plant light box.
2. Set the temperature of the plant light box to 20 °C, which is around room temperature. Record the temperature to reference later.

1. Switch the plant light intensity of the plant light box to 5, the maximum. This setting is located next to the gray Start button in the upper right corner of the plant light box. Record the plant light intensity to reference later.
2. Set the timer to 120 minutes.

Part 2: Collecting Oxygen Produced by the Plant

The rate of photosynthesis can be measured by collecting the oxygen produced by the plant.

1. Take a gas syringe from the Instruments shelf and place it onto the plant light box. Record the initial volume in mL of gas in the syringe at 0 min of the experiment (countdown timer reads 120 min) to reference later. To see the volume, double-click on the gas syringe.
2. Press the gray Start button in the upper right corner of the plant light box.
3. After 30 simulated minutes in the plant light box (countdown timer will read 90 min), press the lab pause button in the lower left corner of the lab (Figure 1) to note the gas volume in the syringe.
   
   Figure 1. Lab Pause Button
4. After recording the simulated time and gas volume to reference later, press the lab play button (Figure 2) to resume the experiment.
   
   Figure 2. Lab Play Button
5. Repeat this pausing and playing sequence to note and record the gas volume in the syringe after:
   • 60 simulated minutes (countdown timer reads 60 min)
   • 90 simulated minutes (countdown timer reads 30 min)
   • 120 simulated minutes (countdown timer reads 0 min)
6. When the door of the plant light box opens to indicate this run is done, make sure to leave everything in place for the next experiment.

Experiment 2: Respiration in the Dark

1. Change the light intensity of the plant light box all the way down to 0 for dark conditions. Record the plant light intensity and temperature settings to reference later.
2. Set the timer to 120 minutes. Record the initial volume at 0 min of the experiment (countdown timer reads 120 min) of gas in the syringe to reference later.
3. Press the gray Start button.
4. Using the lab pause and play buttons as needed, record the syringe’s gas volume after:
   • 30 simulated minutes (countdown timer reads 90 min)
   • 60 simulated minutes (countdown timer reads 60 min)
   • 90 simulated minutes (countdown timer reads 30 min)
   • 120 simulated minutes (countdown timer reads 0 min)
5. When the door of the plant light box opens, move the flask to the waste to empty it.
6. Place the empty flask in the sink.
7. Double-click the gas syringe and reset the plunger. Make sure the volume goes back to 0.00 mL.

Experiment 3: Effect of Light Intensity

1. Repeat the set-up outlined in Experiment 1, Part 1, steps 2 – 9. However, this time set the plant light intensity to 4.
2. Record the initial volume of gas in the syringe at 0 min of the experiment (countdown timer reads 120 min) to reference later.
3. Press the gray Start button, then use the lab pause and play buttons to note and record the syringe’s gas volume after:
   • 30 simulated minutes (countdown timer reads 90 min)
   • 60 simulated minutes (countdown timer reads 60 min)
   • 90 simulated minutes (countdown timer reads 30 min)
   • 120 simulated minutes (countdown timer reads 0 min)
4. Record the plant light intensity and temperature settings to reference later.
5. When the door of the plant light box opens, move the flask to the waste to empty it.
6. Place the empty flask in the sink.
7. Double-click the gas syringe and reset the plunger. Make sure the volume goes back to 0.00 mL.
8. Repeat the procedure outlined in steps 1 – 7 for the following plant light intensity settings:
   • 3
   • 2
   • 1

Experiment 4: Effect of Environmental Temperature

1. Set the plant light intensity of the plant light box to 5 and the timer to 60 minutes. Record the light intensity setting to reference later.
2. Take a 250 mL Erlenmeyer flask from the Containers shelf and place it onto the workbench.
3. Take a branch of Elodea from the Materials shelf and add it to the flask.
4. Add 100 mL of 0.1 M sodium bicarbonate from the Materials shelf to the flask.
5. Place the flask into the plant light box.
6. Set the temperature of the plant light box to 10 °C. Record the temperature setting to reference later.
7. Record the initial volume of gas in the syringe at 0 min of the experiment (countdown timer reads 60 min) to reference later.
8. Press the gray Start button.
9. Record the volume in the gas syringe after 60 simulated min (countdown timer reads 0 min) to reference later.

1. Move the flask to the waste to empty it.
2. Place the empty flask in the sink.
3. Double-click the gas syringe and reset the plunger. Make sure the volume goes back to 0.00 mL.
4. Repeat steps 2 – 12 for two additional temperatures:
   • 30 °C
   • 40 °C
5. Clear the bench of all materials, containers, and instruments, then return to your course page to complete any assignment for this lab.