Data Skills

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

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

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

Describe the graph: Skill B Say what you see

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

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

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

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

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

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

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

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

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