The Effects of a Sodium Chloride Solution on the Rate of
Photosynthesis of an Egeria Densa Plant
Abstract:
Ken Meyer 3/7/09 6:10 PM
Comment: Italics, lower case d
Ken Meyer 3/7/09 6:10 PM
Comment: Title - 5
The purpose of this lab was to alter the rate of photosynthesis in an Egeria
densa plant in some way by changing any one variable. For our purposes the
rate of photosynthesis was defined as the number of oxygen bubbles
released per minute from the submerged Egeria Densa plant. Photosynthesis
is the process that provides energy to all living things either directly or
indirectly. In plants, photosynthesis consists of two main steps: the lightdependent reaction and the carbon fixation (light-independent) reaction.
NADPH and ATP are produced in the light-dependent reaction and are then
used in the carbon fixation reaction to incorporate CO2 into organic
molecules, such as glucose. Water plays a vital role in the production of ATP
and NADPH and oxygen and thus photosynthesis cannot occur without
water. In our control experiment we submerged the Egeria densa plant in
water; in our variable experiment we submerged it in a 30% NaCl solution.
Our aim was that the hypertonic NaCl solution surrounding the plant would
cause the water inside the plant cells to diffuse outside in response to the
concentration gradient. Since photosynthesis cannot occur without water we
hypothesized that this would cause the rate of photosynthesis to reduce
significantly. We measured the number of oxygen bubbles produced per
minute for 30 minutes for both the plant submerged in water and the plant
submerged in NaCl solution. We found that the control produced an average
of 1.833 bubbles per minute and the variable produced an average of 0.033
bubbles per minute. The data supports our hypothesis, but we also observed
that other factors such as the accumulation of a precipitate on the plant and
the use of a glass rod to keep the plant submerged may have affected the
rate of photosynthesis.
Ken Meyer 3/7/09 6:12 PM
Comment: Very good
Abstract - 20
Introduction:
Photosynthesis is a process that provides energy to all living things on
earth either directly or indirectly. It takes place in the chloroplasts of plants
in two steps. The first part is called a light-dependent reaction. When light
reaches a plant it is absorbed by photosystems- bunches of chlorophyll and
accessory pigment molecules. One pigment molecule in the antenna complex
captures the photon of light, which causes the electrons in the pigment to
become excited. This excitation energy is the passed from molecule to
molecule making its way to the reaction center chlorophyll. Here, the
energized electrons in the chloroplast are transferred to an electron carrier.
In photosystem II a water molecule donates electrons to the chlorophyll,
restoring it to its original state. The oxidized water splits up, releasing
oxygen and two H+ protons. The excited electrons leave photosystem II to
go to photosystem I. On the way they pass through the b6-f complex which
uses their energy to pump protons into the thylakoid creating a proton
gradient that is used to produce ATP by chemiosmosis. In photosystem I the
process begins the same: energy from the absorption of photons makes its
way to the reaction center chlorophyll where excited electrons are
transferred to an electron acceptor. The difference is that the excited
electrons are passed outside the thykaloid membrane to NADP+, reducing it
to NADPH. Electrons lost by the chlorophyll are replaced by those coming
from photosystem II (Raven et al., 2008, pp. 150-157).
In the light independent reaction, plants convert energy from the sun into
ATP. The NADP+ in the plants is also converted into NADPH. These products
are then utilized in the second part of photosynthesis which is a carbon
fixation (light-independent) reaction, so called because it can take place in
the dark as well as the light. Carbon fixation describes the process of
incorporating CO2 into organic molecules, such as glucose. ATP provides
Ken Meyer 3/7/09 6:14 PM
Comment: ????
energy for this reaction. The NADPH provides protons and the energetic
electrons needed to bind them to carbon atoms. The resulting C-H bonds
hold much of the energy obtained in photosynthesis. (Meyer 2008).
Water plays a vital role in photosynthesis. In photosystem II water acts
as an electron donor to the oxidized reaction center chlorophyll, returning
the chlorophyll to its original state. Without water the chlorophyll can never
regain the electrons lost during previous photosynthesis and when the
energy from a photon reaches it there are no electrons to excite and pass
on. Without excited electrons passing through the b6-f complex it does not
have energy to establish a proton gradient and ATP cannot be made.
Without the excited electrons from photosystem II replacing those lost to
NADPH production in photosystem I, there are no electrons to continue
NADPH production. With both NADPH and ATP production stopped, carbon
fixation cannot occur. Oxygen is produced during photosynthesis from the
splitting of an oxidized water molecule so the absence of water also stops
oxygen production (Raven et al.,2008). A plant with no water cannot
photosynthesize and produce oxygen and energy for itself. With this
knowledge our group decided to limit the amount of water in the plant cells
as a way to stop our plant from photosynthesizing. By emerging the plant in
a 30% NaCl solution we created a hypertonic environment around the plant.
The water in the plant cells, hypotonic relative to the NaCl solution, should
diffuse through the cell membrane into the salt water solution in response to
the concentration gradient (Meyer, 2009).
We referred to the diffusion lab that we performed previously as we
designed this experiment. In the diffusion lab we placed solutions with
concentrations of 1%,1%, 10%, and 25% of sugar into bags and let one of
the 1% bags sit in a 25% sugar solution and let the 1%, 10%, and 25%
bags sit in a 1% sugar solution. We observed how the varying differences in
concentrations affected the amount and rate of diffusion that occurred in the
Ken Meyer 3/7/09 6:16 PM
Comment: spelling
bags. The bag with a higher concentration of solute inside than the solution
outside of it diffused water into the bag. The bag that had the same
concentration both inside of it and out did not diffuse water in or out. The
bags that had lower concentrations in them than outside of them diffused
water out of them.
In our experiment we plan to implement what we observed in the
diffusion lab. We hypothesize that the solution outside of the plant, which
will have a higher concentration of solute (salt) than the water in the plant,
will cause the water in the cell to be diffused outward. The plant will then be
unable to perform photosynthesis because it will be deprived of water.
Methods:
Ken Meyer 3/7/09 6:18 PM
Comment: A little too wordy and
detailed, but I understand what you
are doing and why you think it will
work.
Introduction - 20
First, we created the 30% NaCl solution. In order to prepare this solution
we measured out thirty grams of salt. We poured this amount of salt into
100mL of distilled water with 1% carbon dioxide added to it. We used a
heating plate and a magnetic stirring rod to ensure the solution was
homogenous. Following the solution preparation, we cut the end of
the Egeria densa plant (using as little pressure as possible) to reveal the end
of the vein in its stem. This allowed the oxygen bubbles to be released so we
could observe and record them. We set up our experiment using a test tube
stand and clamp to hold up one large test tube. The test tube was filled to
its rim with the NaCl solution and an 8 cm Egeria densa stem with as many
leaves as possible. We also had to use a glass rod to keep the plant fully
submerged due to the change in water density. We placed the test tube and
stand 10 cm away from a lamp with a 60 Watt bulb to give the plant the
photons necessary for the photosynthesis reaction. Next, we let the plant sit
in the light for approximately ten minutes to allow for it to get warmed up.
Then, we started counting and carefully recording the oxygen bubbles
Ken Meyer 3/7/09 6:19 PM
Comment: Amount in ml
released from the plant for each minute for a total of thirty minutes.
Ken Meyer 3/7/09 6:20 PM
Comment: Only thing missing is a
biggy, the control set-up.
Methods - 17
Results:
Table 1:
Minute
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
# of
Bubbles
in Water
2
1
1
2
1
2
2
1
2
2
1
2
2
2
2
# of
Bubbles
in 30%
NaCl
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
Minute
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
# of
Bubbles
in Water
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
# of
Bubbles
in 30%
NaCl
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Table 1: The effect of 30% concentration sodium chloride (salt) on the rate of
photosynthesis (number of oxygen bubbles produced) of a egeria densa plant per
minute.
Graph 1:
Ken Meyer 3/7/09 6:21 PM
Comment: ? it isn’t mentioned in
your caption
Ken Meyer 3/7/09 6:22 PM
Comment: # of bubbles or average
#?
Graph 1: This graph is a comparison between photosynthesis of an egeria densa plant
in water (with 1% CO2) and photosynthesis in a 30% NaCl solution.
Table 2:
Mean number of bubbles
per minute
Standard Deviation
Water Solution
Salt and water solution
1.833
0.033
.37904
.18257
P-Value
(2 Sample T-test)
5.921 X 10E-26
Table 2: The means and standard deviations were found based on the thirty trials (one
per minute) for each type of solution. The p-value was found using a two-sample ttest in order to test the null hypothesis that the salt in our salt-water solution did not
slow down the rate of photosynthesis (number of bubbles produced per minute) when
compared to the plant in the control water solution.
There is significant evidence from our two-sample t-test to say that the salt in our
salt-water solution slowed down the rate of photosynthesis in an egeria densa plant.
Therefore, we reject the null hypothesis and accept the alternative hypothesis which
states that the salt did in fact slow down the photosynthesis process.
In the first experiment, we used deionized water with 1% additional carbon dioxide
in it we had an average of 1.83 bubbles per minute. In our second experiment, we
used a 30% sodium chloride solution in place of the water that we had used in our
first lab. As we had expected, the plant's average number of bubbles per minute
decreased. However, we did not really expect it to decrease quite as much as it did.
The photosynthesis seemed to stop and only one bubble was produced in all thirty
minutes combined, resulting in an average of only 0.033 bubbles per minute. Before
performing the experiment we were not sure that we would be able to diffuse all the
water out of the plant cells with our salt solution. We thought that perhaps the plant
would start photosynthesizing slower than the plant in the pilot study, but not stop
completely. In the diffusion lab the bag with the 1% concentration inside and 25%
concentration outside did not have all of the water in it diffused out of it. It gradually
diffused out. We expected that this would happen in our lab as well: the rate of
photosynthesis would start out slow and gradually get even slower. However, in our
experiment the plant appears to have stopped photosynthesizing from the very start.
Discussion:
Our results indicated that the NaCl solution had a significant effect on the rate of
Ken Meyer 3/7/09 6:24 PM
Comment: Not needed, you said
everything that needed to be said in
the previous paragraph
Results - 18
photosynthesis of the Egeria densa.The rate did decrease; in fact, the plant only
produced one bubble during the whole thirty minute period (a .033 bubbles per
minute average) when subjected to the solution instead of control water. Our pilot
study plant produced an average of 1.83 bubbles per minute. We thought that the salt
water would cause the rate of photosynthesis to decrease because the plant would not
have as much water for the process to take place, but our observations during the
experiment make us think that there might have been other contributing factors.
During our experiment we noticed that a white precipitate collected on the top of the
stem and on the leaves of our plant. It is likely that this was solid NaCl precipitating
from the solution. It seems possible that this could have affected our results by
blocking the light from getting to the leaves or blocking oxygen bubbles from escaping
from the top of the stem vein. Also,in our experiment our plant started floating
upward and out of the NaCl solution. The solution's density had increased from the
control and was now higher then that of the plant. In order to counteract the plant
Ken Meyer 3/7/09 6:25 PM
Comment: Very good
floating out of the water we placed a glass rod in the water to push the plant down.
This glass rod may have partially intercepted or lessened the intensity of the light rays
reaching the plant.
These factors would have caused an additional decrease in oxygen produced beyond
the variable we were trying to measure (less water in plant cells).
This lab emphasized the need for water during photosynthesis. When the water
Ken Meyer 3/7/09 6:26 PM
Comment: How might you correct
this problem?
diffused out of the plant it stopped, or at least greatly reduced, the rate at which it
was photosynthesizing. This also shows how and why plants are unable to be watered
with salt water or other minerals in it. "When the water in the soil is salty, water tends
to be sucked out of the roots into the soil."(McGuinness, 1997). Water with a high
level of solute in it becomes a hypertonic solution, causing the water in the plant cells
to diffuse outward and disrupt the cycle of photosynthesis.
If we were able to do the experiment again we would try decreasing the
concentration of solute in the water to about ten percent. We predict that this would
cause the rate of photosynthesis in the plant to be slightly faster than the 30%
solution caused it to be, but still slower than the control. The plant may produce a few
Ken Meyer 3/7/09 6:28 PM
Comment: If you want to do this, feel
free. It would make for a good
science fair project next year.
more bubbles than it did with the 30% NaCl solution, but the plant probably would not
photosynthesize much longer than it did in our experiment because all the water will
still eventually be diffused out of the plant. However, there would most likely not be
as much precipitate settling onto our plant which would help to decrease that potential
source of error. With a decrease in the solution's density we may not need to use a
glass rod to keep the plant submerged, eliminating that possible source of error.
Ken Meyer 3/7/09 6:28 PM
Comment: Very good discussion
Discussion - 20
References:
Ken Meyer 3/7/09 6:29 PM
Comment: Alphabetical,
References 4
Vodopich, D.S., and Moore, R. (2008). Osmosis and the rate of diffusion along a
concentration gradient. In Biology laboratory manual eighth edition (pp. 93-95). New
York, NY: McGraw-Hill
Vodopich, D.S., and Moore, R. (2008). Plasmolysis of plant cells. In Biology laboratory
manual eighth edition (pp. 98-99). New York, NY: McGraw-Hill
Raven, Johnson, Losos, Mason, and Singer (2008). Photosynthesis. In Biology eighth
edition (pp.143-164). New York, NY:McGraw-Hill
Meyer, Ken. (2009, February). Photosynthesis. Lecture presented for College Biology
class, Coon Rapids High School, Coon Rapids, MN.
McGuinness, Keith.(1997) Re:How does saltwater affect the plants on land?. Retrieved
February 28, 2009, from http://www.madsci.org/posts/archives/199703/853362106.Bt.r.html
104/110