AQA GCSE Measuring & calculating rates of photosynthesis (Biology)
Measuring & calculating rates of photosynthesis
The rate of photosynthesis cannot be measured directly, so we measure something linked to it such as:
1.Counting oxygen bubbles released by pondweed per minute
2.Measuring the volume of oxygen produced using a gas syringe
Oxygen is produced during photosynthesis, so more oxygen = a faster rate.
Example
Counting bubbles:
If 24 bubbles are counted over 2 minutes
Rate = number of bubbles/time
Rate = 24/2 =12 bubbles per minute
Measuring volume of gas:
If 1.5cm3 of oxygen gas is collected over 10 minutes
Rate = volume of oxygen gas/time
Rate = 1.5/10 = 0.15 cm3/min
Rearranging this formula
Plotting Graphs Correctly
When drawing a graph, you must:
When drawing a graph, you must:
Axes
Choose a sensible scale that uses most of the graph
Use equal intervals
Label both axes with units
Plotting
Plot points accurately
Draw a smooth curve of best fit or straight line of best fit
Do NOT join dot to dot
Sample Data: Rate of Photosynthesis (Bubble Count Method)
Independent variable: Light intensity (arbitrary units / lamp distance)
Dependent variable: Rate of photosynthesis (bubbles per minute)
| Light intensity (units) | Bubbles per minute |
|---|---|
| 0 | 0 |
| 10 | 3 |
| 20 | 7 |
| 30 | 11 |
| 40 | 14 |
| 50 | 15 |
| 60 | 15 |
Example questions on these graphs
1.Describe the relationship between light intensity and rate of photosynthesis.
As light intensity increases rate of photosynthesis increases up to a maximum value.
2.Identify the limiting factor between 0–30 units of light intensity.
Light intensity
3.Explain why the rate does not increase beyond 50 units.
Light intensity is no longer a limiting factor
4.What is the rate of photosynthesis at 40 units of light intensity?
14 bubbles per minute
Practice Questions
1.State two ways that we can measure the rate of photosynthesis
2. A graph is plotted using volume of oxygen gas produced against time. Suggest how the rate of photosynthesis can be measured and calculated at a specific point in time
3. In the diagram of the experiment above a tank of water is placed between the lamp and the pond weed. Suggest the role of the water tank.
Factors affecting the rate of photosynthesis
Measuring & calculating rates of photosynthesis
Inverse square law and photosynthesis
Economics of enhancing the conditions in greenhouses
Investigating the effect of light intensity on the rate of photosynthesis