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Lecture 3

BIOLOGY 1A03 Lecture Notes - Lecture 3: Sodium Bicarbonate, Sodium Carbonate, Carbon Fixation

Course Code
Dr.Da Silva

of 3
Floating Leaf Disk Photosynthesis Lab
Post-Lab Questions (41 points)
Please answer each of the following questions using complete sentences. Please type your answers using a
legible font. Include them at the end of your formal lab report.
1. What was the function of sodium bicarbonate in this experiment? (2 points)
To provide the leaves with a source of CO2 after all gases (O2 and CO2) had been removed from the
2. What is the purpose of the “vacuum” you created using the plunger? (2 points)
To remove trapped air and infiltrate the interior of the leaf with sodium bicarbonate solution, which will
be the source of CO2.
3. What was the purpose of the total darkness trial? (2 points)
To determine the effect of no light on the rate of photosynthesis
4. Explain the process of carbon fixation. (2 points)
CO2 is attached to another sugar by an enzyme, rubisco.
5. Explain the process that causes the leaf disks to rise. (3 points)
As photosynthesis proceeds, CO2 is converted to oxygen gas, which accumulates and fills the interior
(mesophyll layer) of the leaf, changing the buoyancy of the leaf and causing it to rise.
6. What was the purpose of using the water/soap solution for one of the trials? (2 points)
To serve as a control, and reveal the effect of no CO2 (which is a reactant for photosynthesis) on the rate
of photosynthesis
7. What is the effect of darkness on photosynthesis? Explain. (3 points)
Darkness slows the rate of photosynthesis. Even in darkness (unless total darkness), there is typically still
some light. However, without a lot of light, photosynthesis cannot occur at a high rate, as
photosynthesis requires light as a reactant.
8. What variables or features of plant leaves might affect the rate of photosynthesis? How and why? (2 points)
Veins, species of plant leaves, leaf color, leaf age, etc.
Explains How and Why
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9. Using class data, write the equation you used to determine the ET50. This should take the form of y=mx + b, but
instead of using these variables, you should state what the “y” variable is, what the “x” variable is, and write in
the numbers for the slope (m) and the y-intercept (b). (2 points)
30cm from light w/ NaHCO3: Percent of leaf disks floating = 5.324(time) + 10.7
50cm from light w/ NaHCO3: Percent of leaf disks floating = 4.212(time) + 1.275
Dark: Percent of leaf disks floating = 5.424(time) + -17.455
Control: Percent of leaf disks floating = 0
10. Using the equation for number 9, calculate ET50. Show all calculations/work. (4 points)
30cm from light w/ NaHCO3: 7.38 min
50cm from light w/ NaHCO3: 11.57 min
Dark: 12.44 min
Control: cannot solve
11. The net rate of photosynthesis is the inverse of ET50. Therefore, 1/ET50 = net rate of photosynthesis. What is the
net rate of photosynthesis? Use class data to determine this. (2 points)
30cm from light w/ NaHCO3: 0.135 min-1
50cm from light w/ NaHCO3: 0.086 min-1
Dark: 0.0804 min-1
Control: photosynthesis does not occur, so 0 min-1
12. The same experiment was conducted where 10 leaf disks were placed in a sodium bicarbonate solution and
placed in the light. Every minute, the number of floating disks were counted and recorded. After 14 minutes, the
leaf disks were moved into the dark and the number of floating disks was recorded every minute. Below is a
graphical representation of the data.
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a. Why did the leaf disks begin to sink after being placed in the dark? (2 points)
There were no gases left in the leaf disks. As respiration ensued, O2 produced by photosynthesis
was consumed during cellular respiration. All of the CO2 from the sodium bicarbonate was also
used up during photosynthesis. With all of the O2 and CO2 used up, there were no gases to make
the leaf disks continue to float, so they sank.
b. What could the rate of leaf disks sinking be correlated to? Explain why. (3 points)
The rate of cellular respiration, because as the CO2 provided by the sodium carbonate is used up
during photosynthesis, O2 is consumed as a product of photosynthesis. As all of the O2 is then
consumed during cellular respiration, no gases will be left to ensure that the leaf disks continue
13. Design an experiment using the same setup to investigate a different variable in the rate of photosynthesis.
Make sure you explain how you would test this variable, what you expect your results to be and why. Also
explain why you chose this variable. Make sure you do this part well! (10 points)
Possible variables: different carbon dioxide sources, different concentrations of carbon dioxide, different
light conditions, different light sources, different plants for leaf disks, different temperatures
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