Lab on Photosynthesis Essay Sample

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Aim: To test what effect CO2 concentration has on the rate of photosynthesis; by placing an Elodea plant in water solutions with varying amounts of CO2 due to varying amounts of Sodium Hydrogen Carbonate (NaHCO3) and observing the number of bubbles of oxygen that are released as a bi-product of photosynthesis

Hypothesis: As the concentration of CO2 increases the rate of photosynthesis will increase, until a certain point where the rate of photosynthesis will become constant because of other limiting factors on photosynthesis. The equation for photosynthesis is – 6H2O +6CO2 –> C6H12O6 + 6O2

Water + carbon dioxide –> glucose + water

As carbon dioxide increases in an environment with an adequate water supply photosynthesis should increase so more oxygen will be produced; eventually the rate of photosynthesis will be limited by aspects such as temperature and amount of light present.


Independent Variables:

* Amount of CO2 in water due to concentration of NaHCO3 placed in water

Dependent Variables:

* The number of bubbles which rise from Elodea plant in 10 minutes

Constant Variables:

* Time in which experiment is rate of photosynthesis is observed (10 minutes)

* Light Source (colour, intensity, distance from plants)

* Size of Elodea

* Method of Measurement

* Temperature

* Equipment in Experiment

* Amount of Carbon solution placed in solution


* Pencil

* Pen

* Paper

* Stopwatch

* 5 Beakers

* 10 test tubes (five for solutions, 5 for funnel)

* Elodea plant

* Water

* Varying percentage of carbon solution (0.1%, 0.2%, 0.3%, 0.4%)

* 2 lights

* 2 funnels

* Accurate scale

* Tiny spoon

* Test tube rack


1. The Elodea plant was cut into 5 equal pieces

2. Each piece of Elodea was placed in a beaker that had been filled with water (there were 5 500ml beakers each with water and Elodea)

3. The 10 test tubes were each filled with water

4. 1 gram of each NaHCO3 solution was weighed out (0.1%, 0.2%, 0.3%, 0.4%)

5. These varying solutions of NaHCO3 were placed in 4 test tubes of water (while one just contained water) and these 5 test tubes were labelled

6. The 2 lights were set up next to each other plugged in

7. A beaker was placed in front of each light

8. Each beaker in front of the light received the water and varying carbon solution from the test tubes mentioned in step 5

9. A funnel was placed inverted in each beaker so that the stem of the Elodea plant went up the funnel but the mouth of the funnel was above the water (refer to diagram)

10. An unlabeled (not containing a solution) test tube was placed inverted over the end of the funnel quickly so that all the water did not pour out

11. The light was turned on and the timer started for 10 minutes

12. Observations were made about how many bubbles rose and the amount recorded (2 people were needed because there were 2 beakers) in the 10 minutes

13. Steps 7 – 12 were repeated 3 times so that all 5 beakers with varying concentrations of CO2 being observed


Pictures of Experiment


Table 1/ Number of Oxygen Bubbles due to Percentage of NaHCO3

Percentage of Carbon solution

Number of Bubbles


44 (+ or – 2)


56 (+ or – 2)


78 (+ or – 2)


101 (+ or – 2)


128 (+ or – 2)

Table 2/ Rate of Photosynthesis due to Percentage of NaHCO3

Percentage of Carbon Solution

Rate of Photosynthesis (Amount of Bubbles per minute)


4.40 (+ or – .2)


5.60 (+ or – .2)


7.80 (+ or – .2)


10.1 (+ or – .2)


12.8 (+ or – .2)

Data Analysis:

Calculations for rate of photosynthesis

For 0.0%

Number of Bubbles / time = rate of photosynthesis

44/ 10= 4.4 bubbles per minute

For 0.4%

Number of Bubbles / time = rate of photosynthesis

128/ 10= 12.8 bubbles per minute

Calculations for absolute error

Error of Number of Bubbles / Time

(+ or – 2) / 10 = + or – .2


Graph1/ Rate of Photosynthesis of Elodea plant in water with varying concentrations of Sodium hydrogen Carbonate


Graph 1 shows the results. As the percentage of CO2 increases the rate of photosynthesis increases with the rate increasing from 4.4 to 12.8 (Table 2), which means the rate increased nearly by 300%. This trend is supported by external scientific evidence with the exception of the graph swallowing into a flat line as the rate stops increases because of other limiting factors. . I think this would of happened if we had done the experiment on more concentrations of carbon solution. I do think that the percentage of increase is a bit exaggerated but this could be due to flaws in the experiment


This experiment had many errors and limitations to observations, which hindered the accuracy of the results. The first limitation to the experiment was that the size of each Elodea was not exactly the same, although cut to equal lengths some pieces had more leaves than others. This meant that some Elodea pieces photosynthesized more effectively than others. This error could be reduced but not eliminated by measuring and weighing the Elodea pieces but one would still be unsure of the percentage of leaves (which photosynthesize) to stem (which doesn’t).

The weighing of the carbon solution also presented a problem because it was almost impossible to get each solution to exactly one gram; this would of resulted in a small change in the results. The counting of the amount of the bubbles was also problematic because some bubbles are larger than others and sometimes one could miscount or skip over a bubble, this error could be overcome by obtaining an instrument that measures the change of percentage of oxygen in a certain area which meant the observations would have been more scientific and less subjective. Also the light source posed a problem because if the lights were not at exactly the same angle of position in relationship with the beaker the rate of photosynthesis would be different. For example in one beaker the funnel appeared cloudier then the other which would have reduced the amount of light being received by the Elodea plant.

This problem could be easily resolved by having better equipment where the funnels were exactly the same shade and one larger and longer light such as a florescent light tube. The consistency of the water could also be an opportunity for error because if the carbon solution and the water in the beaker didn’t mix equally and the plant water under the funnel didn’t have the added carbon water; the results would be an inaccurate representation of the experiment. The time in which the experiment was also a limiting factor because the whole experiment was conducted in 80 minutes it meant one was rushed to set up and do the experiment, if one had a longer period of time in which to conduct the experiment many mistakes and careless errors could be overcome by being given the time to correct are mistakes. Also if we did the experiment for more solutions it would be more accurate. Overall the experiment worked and shows the general trend but if the experiment was improved it could be a much more accurate representation.

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