Photosynthesis Project
Plants make sugar, storing the energy of the sun in the form of chemical energy, by the process
of photosynthesis. This process uses light energy to convert carbon dioxide and water into sugar,
oxygen, and other organic compounds. It is often summarized by the following reaction:
H2O + CO2(g) + light energy CH2O + O2(g)
When plants require energy, they can tap the stored energy in sugar by a process called
respiration. This process converts the chemical energy of organic molecules into water, carbon
dioxide and energy in a chemical form (phosphate bond in ATP). It is often summarized by the
following reaction:
CH2O + O2(g) H2O + CO2(g) + energy
Using a Vernier infrared CO2 gas sensor attached to the Vernier Lab Quest, you will monitor the
carbon dioxide consumed or produced by plants. The Vernier CO2 Infrared Gas Sensor measures
gaseous carbon dioxide levels in the range of 0 to 10,000 ppm (low range setting) or 0 to 100,000
ppm (high range setting) by monitoring the amount of infrared radiation (4260 nm) absorbed by
carbon dioxide molecules. Current atmospheric CO2 levels are about 380 ppm. Exhaled human
breath has a carbon dioxide concentration of about 50,000 ppm. Your y-intercept values (b) will
likely be above 380 ppm thanks to the presence of several Homo sapiens exhaling in the
laboratory at the time the photosynthesis chamber is closed.
You will also be monitoring the light emitted from the Xtreme LED lamp, which can put out
75,000 lux! You might want to bring sunglasses to class today! The Vernier Light Sensor uses
a Hamamatsu S1133 silicon photodiode. It produces voltage proportional to light intensity. The
spectral response covers the range of photosynthetically active radiation (PAR).
• The 0–600 lux range is useful for very dim light.
• The 0–6000 lux range is useful for common indoor light.
• The 0–150,000 lux range is useful for PAR above the compensation point.
Setting Up The Photosynthetic Chamber and Lamp
Record the temperature of the room in which the tests will be conducted.
Harvest a plant cutting from the greenhouse as directed.
1. Place the plant cutting in a 100 mL beaker with 50 mL of water.
2. Place the beaker with the cutting in the 2L chamber.
3. Insert the Vernier CO2 sensor in a port in the chamber lid. Wiggle the shaft gently,
supporting the rubber grommet on the back side. Do NOT twist the top or the shaft!
4. Plug the other port with a #6 rubber stopper.
5. Rotate the chamber so the clearest side will face the lamp; the red graduation line should
likely be on the side away from the light. Make sure the plant cutting is positioned so that
light from the lamp will hit most of the leaf area.
6. Secure the lid of the chamber so that the CO2 sensor does not block the light path to the
plant leaves. The sensor should be on the side of the chamber with the red graduation line
(i.e. in the shadow from the cuttings).
7. Position the light sensor on top of the chamber with the sensor tip facing the light source
and so that the tip is in the same plane as the chamber surface closest to the sensor.
8. Tape the light sensor in place with electrical tape. Is the tip facing the light directly?
9. Position the Xtreme LED lamp at the end of the table with the intake grills (on the sides)
facing to the side (NOT facing the table and ceiling!). Turn the lamp on by plugging it in.
Be sure NOT to look directly into the LED lights! It is OK to look around and have them
vaguely in your peripheral vision, but do not stare directly into the LED sources. Treat it
as if it were the sun in the sky! Note: the red and blue LEDs are releasing only
photosynthetically active wavelengths, but the resulting magenta light will deplete the
inactive rhodopsin in your corresponding cones. Your green cones and all of your rods
are relaxed in this light. If you leave the room, do not be surprised if your red and blue
cones do not respond to white light, but your rods and green cones DO. So your vision
will be greenish until your cones recover (one minute with your eyes closed should give
you a strong recovery of your red and blue cone sensitivity).
Setting Up the Vernier Lab Quest Instrument
1. Connect the power adapter to the power port on the left side of the LabQuest and plug the
power adapter into the nearest power outlet.
2. Connect the wires from the CO2 and light sensors into channel ports 1 and 2 on the top of
the LabQuest, respectively.
3. Set the switch on the CO2 sensor to low (0-10,000 ppm) and the switch in the box along
the cord of the light sensor to 0-150,000 lux.
4. Turn on the LabQuest with the button in the upper left corner just above the screen.
5. Use the stylus to tap the Mode button and set the data collection rate to 0.2 samples per
second and the length to 300 seconds.
6. Use the metric ruler to place your photosynthesis chamber 15 cm away from the lamp.
7. Tap the green triangular play button to start recording data.
8. After a minute or so, hit the red square stop button to stop recording data.
9. Tap the Analyze Tab.
10. Tap the Statistics choice, at the left click the Illumination choice.
11. Tap the Curve Fit choice, at the left click the CO2 choice. In the popup dialog, select the
Linear choice.
12. Tap the graph area to return to your data.
Collecting Photosynthesis Data
1. Tap the green triangular play button to start recording data.
2. In the popup dialog, select the Discard choice.
3. After 5 minutes (=300 seconds) of data collection, the recording should automatically
stop. If needed, hit the red square stop button.
Determining Parameters To Record
1.
2.
3.
4.
On the right of the screen, the LabQuest has already analyzed your new data.
Next to the Light Graph, Tap down arrow to scroll through its statistical data.
Record the Mean (average) lux value on your datasheet (whole number, no decimals).
Next to the CO2 Graph, Tap the down arrow to scroll through the regression analysis
data.
5. Record the slope (m), the y-intercept (b), and the absolute value of r2 (correl.) on the
datasheet (round m to 3 places, b to a whole number, and r2 to 3 places). Pay close
attention to the sign of the numbers as you record your data.
Varying Light Intensity (Photon Flux Density)
1. Tap the green triangular play button to start recording data.
2. In the popup dialog, select the Discard choice.
3. Observe the illumination measurement as you move the chamber away from the lamp
until the light sensor reports a value about ½ of the previously recorded value. Remember
the LabQuest is only recording a data point every 5 seconds, so you have to wait a few
seconds after each small movement of the chamber.
4. Tap the red square stop button to stop recording data.
5. Repeat the three steps for Collecting Photosynthesis Data
6. Repeat the five steps for Determining Parameters to Record
7. Repeat these steps for Varying Light Intensity until you have reached a point where the
regression analysis provides a positive (rather than negative slope).
One Final Data Set
Finally cover your photosynthesis chamber with a corrugated box to keep out as much light as
possible. There is some light leakage, so the mean value will not be 0 lux.
1. Repeat the three steps for Collecting Photosynthesis Data
2. Repeat the five steps for Determining Parameters to Record
Cleaning Up
1.
2.
3.
4.
5.
6.
7.
8.
Turn off the LabQuest and unplug it. Pull the plug-body, never pull the wire!
Disconnect the sensors.
Place the power supply and the LabQuest into its box.
Remove the electrical tape from the chamber and discard it.
Put the sensors and cables into their corresponding box.
Remove the cutting from the chamber and lay it on some paper towel.
Dump the beaker of water down the drain.
Rinse the beaker and put it away as directed.
Determining the Leaf Area of your Cutting
1.
2.
3.
4.
5.
6.
7.
8.
Cut off the blades of the leaves from the cutting.
Blot the blades dry with paper towel. Be thorough; no need to be gentle!
Lay the DRY blades on a piece of copier paper.
Carefully trace around the blades on the paper lightly in pencil.
Discard the leaves.
Cut out the traced shapes carefully with fine scissors.
Weigh all of the traced shapes collectively as a group to the full precision of the balance.
From another sheet of the same copier paper, measure out and cut a 15 x 15 cm square
from one corner.
9. Weigh this square of paper to the full precision of the balance.
10. Clean up your workspace and return all supplies as directed.
Photosynthesis Rate Calculations
Your Lab Quest system provided uncorrected lux values and rates of CO2 concentration change
by linear regression of the CO2 ppm data streamed to it by the infrared gas analyzer sensor.
1. The Xtreme LED lamp you used and the Vernier Light Sensor lux measurements have
been previously calibrated to photosynthetically active radiation (PAR) measurements
from a LiCor Quantum Radiometer. Multiply each mean lux value you obtained by
0.0341 to convert it to photon flux density (µmol photons m-2 sec-1).
2. Our uncorrected rate was measured as ppm CO2 sec-1. This number can be directly
converted to µL CO2 L-1 sec-1...because there would be a million µL in one L of chamber
volume...right? You multiply this value from step 1 by the volume of the chamber you
had (2L):
______ µL CO2 L-1 sec-1 •2 L of chamber volume = ______ µL CO2 sec-1
3. A liter of gas occupies 22.4 liters at STP. But our room temperature was _____°C
So we would add 273 to that temperature = ______ to get °K.
You would divide those °K by 273 °K and multiply that quotient by 22.4 = ______ L/mol
4. Now divide the value of µL CO2 sec-1 from step 2 by the L/mol from step 3.
That converts our CO2 change rate to = ______ µmol CO2 sec-1.
5. This absolute rate depends on how much leaf material you had in the chamber. To
convert this to a specific rate, we need to know how much leaf area we had in the
chamber. You traced out the shape of the leaves on a sheet of copy paper and cut out the
shapes. You weighed these pieces of paper as a group.
The combined weight of the leaf shapes was ______ g paper.
The 15 x 15 cm square of this same paper has an area of _______ cm2
This square of copier paper weighed ______ g.
Divide the area of the square by the weight of that square = ______ cm2 g-1
Multiply this conversion factor by the weight of the leaf shapes = ______ cm2 of leaf
6. Finally divide the CO2 rate from step 4 in µmol CO2 sec-1 by the leaf area from step 5.
Then multiply the answer by 10,000 to convert the answer to a meter-square rather than a
centimeter-square basis.
This gives us a CO2 change rate = ___________ µmol CO2 m-2 sec-1
7. These calculations need to be applied to each rate recorded at each photon flux
density. One sensible approach would be to reduce the six steps to an Excel spreadsheet
to apply to the six rate values you have collected.
Photosynthesis Amplification Material Name _______________________
Partner(s) ______________________________________
Table 1. The rate of photosynthesis in various photon flux densities at ______°C by leaves of
Abelmoschus esculentus (or __________________) having an area of ___________ cm2
Mean Lux
m
(ppm CO2 sec-1)
b
(ppm CO2)
[whole number]
[3 places]
[whole number]
ABS(r2)
PFD (µmol
photons
m-2 sec-1)
CO2 rate
(µmol CO2
m-2 sec-1)
[3 places]
[whole number]
[3 places]
The last two columns in this table are calculated from data collected on the rest of this page using
the calculation instructions on the previous page. In doing these calculations, be careful to make
proper units conversions and use the correct multipliers and signs (+/-)! To express this on a leafarea basis, you need to divide by the area of the leaves in the container, which is calculated from
the facts below:
Weight of leaf cutouts _______g Weight of 15x15 cm paper______g/225 cm2
Reporting
Plot the photosynthetic rate as a function of the photon flux density. Be careful to use a scatter
(xy) plot rather than Excel’s line graph type. Avoid scientific notation! Format your graph to be
biology-professional, including a proper legend with figure number, title, and clarifications
needed to make the figure free-standing. You have a handout on this in your notebook! Write an
abstract summarizing your project. Staple the abstract on top of the figure and this sheet filled
out above. Hand in your submission before the deadline.
Questions for thinking prior to graphing & writing (not to be answered laboriously!)
1. Were any of the rate values a positive number? If so, what is the biological significance of this?
2. Were any of the rate values a negative number? If so, what is the biological significance of this?
3. If you are plotting Photosynthetic Rate, are you plotting consumption or production? Are your data
providing values correctly or is a further manipulation (or two) needed? How do the terms net and
gross photosynthetic rate come into this, which would be the “telling” graph?
4. At some PFD the smooth-fit curve among these points gives a rate of 0. What would that PFD value
be called and what is the biological significance of this value?