1. Data Skills

2. How does CO2 concentration affect rate of photosynthesis?
Carbon dioxide was added to the water in varying concentrations
After setting up the apparatus the plant was allowed to acclimatize for 5 minutes
The bubbles coming from the plant in one minute were counted

3. Graph Drawing: Skill A Here are some data. Draw a graph Repeat 1
[CO2 ] (%) Volume of oxygen collected per minute (bubbles)
Repeat 1
Repeat 2
Repeat 3
Average
0.04 10 11 9 1 37 35 33 2 46 3 52 50 4 54

4. Did it look like this? Points drawn correctly
Independent variable (what you change) on bottom
Fills at least half page
Plot averages
Points drawn correctly
Axes labelled with units
Line of best fit drawn
No dodgy extrapolations

5. Describe the graph: Skill B
Say what you see

6. Did you say?
As the CO2 concentration increases, so does the volume of oxygen collected
This happens quickly at first but then begins to slow and levels off at around 3% CO2
Say as much as you can
Use er…er sentences
Identify where changes happen
You have got 4 points

7. To get 6
Average
Change in oxygen collected for 1% increase in CO2 (bubbles)
Rate of change in oxygen collected (bubbles per % CO2)
0.04 10 1 35 25 26 2 46 11 3 50 4 52
You need to do some further “mathematical” processing
This depends on the graph you have. For this one…you could work out how the rate of oxygen release is changed for every 1% increase in CO2
This tells you that changing the carbon dioxide concentration has a bigger effect when the carbon dioxide concentration is low.
This will be very useful in skill D

8. Evaluate the experiment
How good was the experiment?
How good were the results?
Could it be improved?

9. Did you comment on…?
The apparatus allows the concentration of CO2 to be changes and the volume of oxygen per minute to be collected
However, there may be some inaccuracies because some of the oxygen produced in photosynthesis might have been used in respiration and so we cannot measure it. This would make the measurement below the actual value
The data are reliable because all of the repeats are close together
The exception to this is at 3% CO2 where one repeat is much less than the other two. The point on the graph at 3% CO2 is therefore less reliable than the others
Further limitations include
Some of the gas may not be oxygen
Some of the gas may dissolve in the water
The size of the bubbles may vary
An improvement could be to collect all the gas given off and measure the volume more accurately in a gas syringe
Method
(Strengths)
Weaknesses
Accuracy (closeness of your measurement to the real thing)
Data
Reliability (closeness of repeats)
Anomalous points
You have got 4 points
Six point stuff
Detailed discussion of limitations
Suggested improvements

10. Explain your results Why did the graph look like it did Use science
“A” grade science should get 6 points

11. For 4 marks did you mention…?
An increase in CO2 concentration causes an increase in oxygen collected.
This is because oxygen is produced by photosynthesis and carbon dioxide is a reactant in photosynthesis. As seen in the equation…
Carbon dioxide + water → glucose + oxygen
Therefore the more carbon dioxide the faster the rate of photosynthesis

12. For 6 marks… The symbol equation for photosynthesis…
6CO2 + 6H20 → C6H12O6 + 6O2
Shows that you would expect the increase in O2 given off the be proportional to the increase in CO2 however the graph shows that this is not the case. The rate of increase slows down at high CO2 concentrations.
This is because at low concentrations CO2 is the limiting factor for photosynthesis, but at high concentrations it is not, something else, e.g. temperature or light intensity must be.
0.04% is close to the normal CO2 concentration in the atmosphere this means that for much of the time CO2 concentration is the limiting factor for the photosynthesis of most plants on Earth

13. Further planning
Plan a similar experiment that investigates the effect of changing light intensity on the rate of photosynthesis
What method will you use?
What factors will you control?
How will you change the independent variable?
What range will you use?
4 marks
Discuss the science behind your results and the implications of your results
6 marks

14. Did you? Use the same method
Make it clear how to carry out the experiment (use amounts not vague statements about “darker” or “further away”
Have a range of 5 independent variable values
Discuss the science behind potential results
Set up the apparatus as in the original experiment but keep CO2 concentration at 4% and all other factors constant as in original experiment
Do the experiment in a darkened room with a single light source (e.g. a lamp).
Count oxygen bubbles over one minute.
Begin with lamp 10 cm from plant
Do the experiment again with lamp 20, 40, 100, and 200 cm from plant
Repeat whole experiment twice and plot results on a graph as in the original experiment
Lamp distance is related to light intensity and the results should show over what range of distances light intensity is the limiting factor.
If LI has a big effect on the rate of photosynthesis it is the limiting factor. If it has little effect then something else is the limiting factor.
I would expect light intensity to be a limiting factor at low light intensities. This would mean that light is the main factor controlling the rate of photosynthesis at night, dusk and dawn