How Does the Amount of C02 Affect the Rate of Photosynthesis?

How does the Amount of CO2 Affect the Rate of Photosynthesis?

There are a number of variables that can affect the photosynthesis rate of Canadian water weed (Elodea). They are as follows:
 The amount of light: an increased light level increases the rate of photosynthesis to a certain point where at another factor limits the photosynthesis rate.
 The temperature: Increased temperature increases the rate of photosynthesis to a certain temperature, after which, essential enzymes are denatured, thus, in the long run, killing the plant.
 The CO2 levels: Increased CO2 levels also increase the rate of photosynthesis.

The variable I am going to investigate is the level of CO2, however, the addition of NaHCO3 (sodium bicarbonate) to the water may affect the osmotic ability of the water. Just 2g of sodium bicarbonate in 1000ml of water gives the equivalent salinity of seawater, and as Elodea is a fresh water plant, it cannot survive in salt water. I will need to take this into account when measuring the NaHCO3.

To work out the rate of photosynthesis I am going to count how many bubbles of O2 are given off in a minute.

Prediction
I think that the more NaHCO3 there is in the solution, up to a certain point, the greater the number of O2 bubbles given off, showing a faster rate of photosynthesis.

Fair testing
To keep this experiment as fair as possible, the only variable I will change is the amount of NaHCO3 in the water. All other variables, i.e. the light level, will be kept the same throughout the experiment.

Equipment
 Large (1000ml) beaker
 Piece of Elodea
 Water
 Funnel
 Test tube
 Lamp
 Stopwatch
 NaHCO3 3x 0.2g, 3x 0.4g, 3x 0.6g, 3x 0.8g & 3x 1.0g

Method
1. Draw up a results table. It should, ideally, resemble the table below.
Amount of NaHCO3 (g) 1st Attempt 2nd Attempt 3rd Attempt Average number of Bubbles
0.2
0.4
0.6
0.8
1.0

2. Set out all equipment as shown in the diagram below using the first of the smallest masses of NaHCO3and start the stopwatch.
3. Count the number of bubbles given off in the allotted time of a minute and log the result in the table.
4. Repeat for the remaining masses of NaHCO3.

Results
Amount of NaHCO3 (g) Number of bubbles: 1st Attempt Number of bubbles: 2nd Attempt Number of bubbles: 3rd Attempt Average number of Bubbles
0.2 6 10 9 8.3
0.4 17 17 18 17.3
0.6 21 22 21 21.3
0.8 28 28 29 28.3
1.0 31 33 32 32

Analysis and Evaluation
My prediction was proved correct in that, the more NaHCO3 there is in the water, the faster photosynthesis is. On my graph, however, instead of the results showing a straight line of best fit, it is curved, giving evidence that another limiting factor, be it temperature or light intensity, was coming into effect towards the end of the experiment. Another thing I did not expect was that the line of best fit does not go through the origin (see Point A on graph). Upon thinking more carefully about this, I came to the conclusion that below0.1g of NaHCO3 dissolved in the water, no detectable bubbles are given off. This doesn't mean photosynthesis is not happening, it could mean that a) bubbles are being given off, but cannot be detected, or b) O2 production equals the O2 usage of the plant, so photosynthesis and respiration rate are balanced.

The experiment wasn't overly accurate for a number of reasons. Firstly, all the bubbles counted would have been different sizes etc. Secondly, not all the NaHCO3 may have been either added to the beaker, or dissolved in the water, thus changing the true mass of NaHCO3 in the water. Also, because we were using a warm, and bright, lamp, the temperature of the water could have fluctuated somewhat during the experiment.