The Effect of the Concentration of Carbon Dioxide on the Rate of Photosynthesis
In this course work, I will be discussing how the concentration of
carbon dioxide affects the rate of photosynthesis. To find this out I
will do an experiment to find out how much oxygen a piece of Elodea
(Canadian pondweed) produces over a set period under controlled
Back Ground Knowledge
All plants photosynthesise, They do this in order to create their own
food by converting sunlight, carbon dioxide
and water into glucose and
oxygen, which is a by-product (6Co + 6H O à C H O + 6O ) Plants have
a part in their cell called chloroplast which contain chlorophyll the
What are the limiting factors of photosynthesis?
Although the diagram shows you what you need for photosynthesis it
does not tell you what the limiting factors are. The limiting factors
in photosynthesis are carbon dioxide, water, light and chlorophyll.
This is because up to a point the more of each factor that is
available to the plant will result in an increase in the
Why did I use pondweed?
I used pondweed so I could measure the amount of oxygen it produced
accurately. I can do this by catching the oxygen bubbles under water,
because of this choosing an aquatic plant was the best choice. I will
use 5cm of elodea because it is a big enough piece to get a measurable
result. I will know how quickly the elodea has photosynthesised by
taking readings of how much solution was left in the burette after 24
hours and again after 48 hours.
Why use the sodium bicarbonate?
I used sodium bicarbonate solution in the experiment because it reacts
with water to make carbon dioxide, which meant I could control the
amount of carbon dioxide available to the pondweed. However, because
of the time limitations I got my results by sharing the class results,
this meant that I had to use the following concentrations of sodium
bicarbonate: 0% 1.25% 0.5% 1% 5%. I used these percentages so that I
had a good range of percentages but this didn’t give very good results
because I made a big jump from 1% to 5%. If I did the experiment,
again I would use these percentages: 0% 0.5% 1% 2% 3% 4%and 5% so that
it had a steady increase.
How will I control the variables other than the Co concentration?
To control the other variables (water, light and chlorophyll). I will
be using a set amount of water, I will be using the same lamps at the
same voltage and I will be using a set amount of pondweed so that the
amount of chlorophyll level should be the same. They will all have the
same window letting in natural light and the temperature will be the
same because they are in the same room.
I plan to set up an experiment using pondweed to see how much oxygen
is produced over 24 hours and 48 hours. I will then put my results in
tables and graphs to help me answer my main question.
In my preliminary experiment, I tested the concentrations I had chosen
to check that the Elodea photosynthesised. I used a plastic funnel and
a ray box, after using these in the preliminary I decided to replace
them with a glass funnel that was transparent so that more light could
get through. I also decided that a ray box was not giving out enough
light because the elodea was not photosynthesising well and so I
decided to change it to a desk lamp to give it a more directed and
stronger light source.
Here is a diagram of my preliminary experiment:-
Here is a diagram of the experiment I used to get my results:-
1. set up the equipment in the apparatus list as the diagram above.
2. Record the volume of solution at the start of the experiment
3. turn the light source on
4. record the volume of solution after 24 hours of the first
5. record the volume of solution after 24 hours of the second
To make my results accurate I will be measuring the water level from
the bottom of the meniscus. I will do this by taking the results with
my eyes level to the bottom of the meniscus. I will repeat the
experiment twice so that including the average I have three sets of
results, which I will then plot on a graph. If the results do not turn
out how I am expecting I will look back at my work to check any
problems I may have had.
I will be leaving my elodea for 2 days and taking readings after 24
hours and 48 hours. This is because it will give it a reasonable
amount of time to photosynthesise. I will find out how much oxygen the
plant has produced over that time by taking my original reading for O
in the burette and deducting my readings at 24h and again at 48h.
What do I expect to happen?
I expect that as the sodium bicarbonate concentration increases that
the rate of photosynthesis
will increase until it hits its maximum
rate of photosynthesis. I can tell this because as the sodium
bicarbonate reacts with the water it will produce carbon dioxide,
which will increase the rate of photosynthesis.
What I expect my graph to look like:
Why I expect this to happen:
I expect this to happen because this is the normal graph you would
expect for a plant photosynthesising.
Conc. Of sodium bicarbonate
Amount of NaHCO before
Expt 1 Expt 2
Amount of NaHCO after 24h
Expt 1 Expt 2
Amount of NaHCO after 48h
Expt 1 Expt 2
Average oxygen produced over 24h
Expt 1 Expt 2
Average oxygen produced over second 24h
Expt 1 Expt 2
As you can see, from the results I have some missing results this is
because while doing the investigation some burettes were knocked which
meant that the liquid fell to the bottom of it. My group did 0.5%
The result in red is highlighted because I did not use this result to
get the average. This is because the pondweed must have died because
it did not produce any oxygen.
Concentration of NaHCO (%)
Sum used to calculate average amount of O
Average amount of oxygen produced in a 24hour period (ml)
0.10 + 0.20 + 1.00
0.70 + 0.55
1.10 + 0.10 + 5.50 + 4.50
5.05 + 3.00 + 0.40
3.65 + 0.10 + 2.45 + 0.50
My graph shows that when I increased the concentration of sodium
bicarbonate from 0% to 1% the rate of photosynthesis increased. At
first, it increased quite quickly but then it slowed down until
eventually when we did 5% it decreased rapidly the only problem with
my graph is there are not enough results between 1% and 5% so I cannot
be sure how the rate of photosynthesis drops.
My graph starts similarly to my prediction however then it drops when
my prediction says it should hit its optimum and stay there but
instead it drops. I think this happened because when I increased the
percentage of sodium bicarbonate the alkalinity of the solution goes
up and disrupts the plants enzymes. A way to find out if this was
happening would be to test the alkalinity of the solution. A way you
could do this would be to use an electronic device that measures the
PH in the water. If the alkalinity was different in each concentration
then it would be an unfair test of how the concentration of carbon
dioxide effects the rate of photosynthesis because the PH would also
be a variable and I wouldn’t know which variable, the carbon dioxide
or the PH was effecting the plant and its rate of photosynthesis.
From my investigation, I can conclude that to a point an increase in
sodium bicarbonate concentration the rate of photosynthesis will
increase. However because I did not use any concentrations between 1%
and 5% I don’t know how the rate of photosynthesis decreases but by
the time it had gotten to 5% it had dropped to 1.7ml of oxygen
produced by photosynthesising. There are two possible reasons for this
happening, it could be an anomaly or it could just be a drop at that
point. To find this out I would have to redo the investigation to
include experiments with 2, 3 and 4 percent concentration of NaHCO .
The results from the concentrations are not very close but the 0.50
and 1.00 were very similar with only 0.02ml difference but the same
jump from the 0.25 to 0.50 had the difference of 2.17ml. This suggests
that because the spread of results were so different that the evidence
may not by trustworthy.
To improve my experiments I could have used a different light source
that did not heat up the solution so much. I also would use more
pondweed so that I would get a clearer set of results. I would also
have to find a different way to supply the carbon dioxide so that the
PH was not a variable.