Method:
I shall cut a shoot of Canadian pondweed (with an approximate length of 5 cm) and place it in a boiling tube (containing 40 cm3 of distilled water), placing this in a water bath, heated to 25°C. To keep the shoot down below the surface of the water, we attached a paper clip. To keep the experiment fair, I shall try to cut shoots of similar ‘greenness’, length, and width, when the experiment carries over to another lesson. Then, having let the water equilibrate for half a minute, I shall count the number of bubbles given out at the cut end of the shoot (indicating the rate of photosynthesis) during a minute. Then, using a small syringe, we added 0.25 cm3 of sodium hydrogen carbonate to the boiling tube, let the contents of the boiling tube equilibrate, and count the number of bubbles in a minute again. The light intensity was kept constant by keeping a 60W-lamp 10-cm from the beaker, and the same amount of distilled water (40 cm3) was used for each experiment.
Taking and use of Results:
I will change the variable, the amount of CO2 available for photosynthesis, by adding sodium hydrogen carbonate to distilled water in 0.25 cm3. Results will be taken between 0 and 2 cm3. We shall use a 2% sodium hydrogen carbonate solution, that is to say, 2g of sodium hydrogen carbonate made up with distilled water to 100 cm3. The formula needed for photosynthesis is this:
CO2 + H2O C6H12O6 + O2
(The elements in italics are the variables, those underlined are the constants, and the other is not of direct interest to the experiment) As sodium hydrogen carbonate decomposes in water, it releases H2 (g) and CO2 (aq). This is the full equation for how sodium hydrogen carbonate decomposes:
NaHCO3 + H2O → HCO3- + H+ → H2O + CO2 + NaOH
This can be condensed to:
HCO3- → H2O + CO2
Thus I can control the amount of CO2 available for photosynthesis, because theoretically distilled water contains no dissolved gases, including CO2. I can add accurately measured amounts of NaHCO3 to 50 ml. of distilled water; allow the Canadian pondweed to equilibrate, and measure the number of bubbles produced in a minute, using the results as a crude method of the rate of photosynthesis.
Diagram:
OBTAINING EVIDENCE
Results:
The experiment was carried out under the following conditions:-
Constant Temperature at 25°C
Distance of light from Beaker; 10 cm.
Then with altering concentrations as indicated the following results were obtained:
* The figures in the column are an average of three results taken.
ANALYSIS
Conclusion:
From the graph my hypothesis can be seen to be true, as the rate of photosynthesis increases as the amount of carbon dioxide available increases (decomposition of sodium hydrogen carbonate releases carbon dioxide in the solution), eventually tailing off to a plateau – also as predicted. The increase can be seen between 0 and 1.75 cm3 of sodium hydrogen carbonate added, and the plateau between 1.75 and 2 cm3 added.
There is an anomalous result, the number of bubbles should be zero when no sodium hydrogen carbonate has been added, because the experiment was carried out in distilled water – which should have no dissolved gases, including carbon dioxide. This should therefore not allow photosynthesis to occur; however this is not the case and two bubbles form – showing that some carbon dioxide was able to dissolve into the water from the surrounding atmosphere.
The results show that as the concentration of carbon dioxide increases the rate of photosynthesis increases, but not quite quantitatively; as the concentration of carbon dioxide doubles, there at first appears to be a slight hint of direct proportion, but that quickly disappears, and the results show that the maximum rate of photosynthesis occurs when 1.75 cm3 of sodium hydrogen carbonate is added to 40 cm3 of distilled water.
Improvements:
To make sure that 1.75 ml of 2% NaHCO3 solution is the turning point for the maximum rate of photosynthesis the experiment should have been carried on further – to approximately 3.00 ml, to see if the plateau developed properly as shown briefly in the graph. Some other improvements for the experiment are as follows:
- Re-boil the water, then cool whilst covered up to make sure that there are no dissolved gases.
- Change the water each time the concentration is increased, so that the plant is using the amount of solution you put in, because, when just adding the solution, the plant has already used some of this.
- Carry out the experiment in a dark room, so the lamp is the only source of light, as the weather conditions of the day of the experiment may have affected the results, as may have done the lighting in the laboratory, which may or may not have been on the different days that the experiments were carried out, even as far as it may have affected the pilot studies, and my predictions made from them.
- Use more accurate methods of measuring the gas given off – as the bubbles could have varied in size so much that my results could be completely wrong in the way that carbon dioxide affects photosynthesis. A set up as shown below may provide a more accurate measurement of the oxygen given off during photosynthesis, but to gain a significant amount, the length of time may have to be increased.
Essentially, the procedure was valid for this experiment, but grossly inaccurate. There were too many variables that could not be controlled that were mentioned above; background light, chlorophyll in the shoots, and surface area available for photosynthesis. Also, the distilled water was not pure, gases were still dissolved in it, therefore decreasing the credibility of the experiment.