1. North : 2015
Kings Valley Charter School
Zarfas AP Biology 2014-2015
Friday February 13, 2015
Investigation of Photosynthetic Rate and Momentum of Rhododendron submerged in
Water versus Sodium Bicarbonate solutions
Abstract
Rhododendron plants grow in very high sun conditions giving them a dependence on high
photosynthetic rates and low photosynthetic momentum when in low light conditions. This was
demonstrated by evacuating leaf discs of air and sinking them in a 1% sodium bicarbonate
solution and placed under a light source. As photosynthesis occurs oxygen is produced and the
leaves float. The average time at which half of the leaf discs float(ET50) is a good indicator of the
average photosynthetic rate of the plant. The rhododendron disc in sodium bicarbonate solution
floated at an ET50 of 10 minutes while no other discs floated in any of the experimental groups.
This demonstrates that sodium bicarbonate does speed photosynthetic reactions. The experiment
was designed to focus on the photosynthetic momentum of Rhododendron leaves after being
removed from a light source compared to tap water however the discs did not all rise as quickly
as expected. This could be due to a flaw in experiment design of the length of time before
relocating two of the cups to a dark environment and starting the initial experiment.

2. Investigation of Photosynthetic Rate and Momentum of Rhododendron submerged in Water versus Sodium Bicarbonate solutions
North : 2015
Background
Photosynthesis can be summarized in the equation in Figure 1. If air in a leaf is removed and
replaced with water the leaf will sink. Next if CO2 is present while the leaf is underwater
photosynthesis will resume and O2 will accumulate in the leaf, eventually causing it to float. The
average time it takes 50% of leaf samples to float, ET50, is a good measure of photosynthetic rate
of a plant. Rhododendrons, native to tropical and alpine climate regions, are full-sun broadleaf
deciduous shrubs, this indicates they require large amounts of light to sustain photosynthetic
reactions for any amount of time.
Sodium Bicarbonate(NaHCO3) is a compound that when dissolved into water forms a carboxylic
acid that can readily supply CO2 to hasten photosynthetic reactions and lower ET50 consequently.
Figure 1: The chemical equation for photosynthesis using both the chemical formula and the
common names of reactants and products. Color tracks the movement of molecules before and
after the reaction according to the composition of reactants and products. Note: Light is not a
reactant, rather a catalyst for the reaction to occur.
https://students.ga.desire2learn.com/d2l/lor/viewer/viewFile.d2lfile/1798/12441/photosynthesis_
print.html

3. Investigation of Photosynthetic Rate and Momentum of Rhododendron submerged in Water versus Sodium Bicarbonate solutions
North : 2015
Experimental Methods
Hypothesis
It was hypothesized that the leaf chads submerged in 1% sodium bicarbonate solution will
photosynthesize at a faster rate, therefore a lesser ET50 than the leaf discs submerged in tap
water. The sodium bicarbonate solution will provide CO2 to fuel photosynthetic reactions faster
than tap water with only H2O. Cups with Rhododendron leaf chads placed in light will
photosynthesize faster with access to light to perform reactions, due to the full-sun nature of the
plant’s photosynthesis.
Materials and Procedures
Rhododendron leaves were used to demonstrate the effect of removing light on the
photosynthesis rate when submerged in different solutions. Leaf discs were punched from the
vein section of a rhododendron leaf with a Standard single hole punch. All leaf discs were then
placed in a large syringe with about twice the volume of water and sealed. The plunger was
pulled and released so as to remove CO2 from the chloroplasts of the discs, which was then
replaced with water, sinking the discs which were then relocated to four Dixie cups. Two cups
containing 10 discs each and approximately 100 mL of a 1% wt/wt solution of sodium
bicarbonate created by dissolving 8 g of food grade baking soda in 800 g of tap water and two
cups containing 10 discs each and approximately 100 mL of tap water were both placed directly
under a 60 watt incandescent lamp fitted with an 8” aluminum reflector dish. Each cup contained
about 1 mL of Dawn dish soap solution, created by mixing approximately 5 mL of dawn dish
soap concentrate in 250 mL of tap water was added to each cup to create more favorable
conditions for CO2 uptake at the surface of the leaf discs. A large glass petri dish filled with
water was placed on top of each cup to serve as a heat sink to maintain experiment validity by
keeping temperature constant. The number of leaf discs floating in each cup was recorded at five
minutes, where one cup with sodium bicarbonate solution and one with tap water were moved to

4. Investigation of Photosynthetic Rate and Momentum of Rhododendron submerged in Water versus Sodium Bicarbonate solutions
North : 2015
a dark environment and the experiment proceeded to record every two minutes for every cup
which is compiled in Table 1.
Results and Analysis
Time (min) 0 5 (initial) 7 9 11 13 15-25
Light
Normal 0 0 0 0 0 0 0
Baking 0 1 2 3 7 9 10
Dark
Normal 0 0 0 0 0 0 0
Baking 0 0 0 0 0 0 0
Table 1: The number of floating leaf discs in each solution before and after half of the group was
moved to the dark environment. The cups were relocated at five minutes, highlighted in orange
on the table
The ET50 of the Rhododendron in baking soda left in the light was approximately 10 minutes,
and no other cup had an ET50, because there was no activity in any other cup. Although two cups
were left in the light the cup with only water in it did not show any photosynthetic activity. The
reason for this could be that the sodium bicarbonate hastened photosynthetic reaction as can be
seen in Graph 1 below.
Graph 1: The photosynthetic rate of all four cups in the experiment over a period of 15 minutes.
The cups were moved at 5 minutes and only the discs in the cup of sodium bicarbonate solution
that were left in the light rose making for an inaccurately conclusive set of experiment data

5. Investigation of Photosynthetic Rate and Momentum of Rhododendron submerged in Water versus Sodium Bicarbonate solutions
North : 2015
pertaining to differences in photosynthetic rates and photosynthetic momentum after leaf discs
are removed. The greatest rate of photosynthetic reaction for baking soda in light occurred
between 9 minutes and 11 minutes, directly between both of which is the ET50 of the data set,
shown as an S-curve patterned set of data points.
Conclusion
The data seems to show a flaw of experiment design because only one of the four experimental
groups expressed any numerical change that would influence the outcome. If the experiment
were to be redone, the interval before relocation of the dark group of leaves would be 15-25
minutes in length after 0(zero) minutes. This would most likely increase the amount of numerical
data an experimenter could draw conclusions from however the experiment did not reveal any
conclusive patterns besides the S-shaped increase in photosynthetic rate for a single experimental
group.