2.2.8Transport Over Larger Distances
Required Practical 10: Light and Photosynthesis
5 min read 8 parts Lesson 18 of 20 1 quiz
Section 1 of 8Method with pondweed
1
Set up a beaker of water containing a short piece of Cabomba or Elodea, with a funnel over the weed to collect gas.
2
Add a little sodium hydrogencarbonate to the water so carbon dioxide is not limiting.
3
Place a lamp at a measured distance, and put a second water bath between lamp and beaker to absorb heat.
4
Allow the plant to adjust for a few minutes, then count the bubbles released in one minute.
5
Move the lamp to new distances, such as 10, 20, 30, 40 and 50 cm, repeating the count each time.
6
Repeat each distance and take a mean.
Section 2 of 8Variables
| Type | Variable |
|---|---|
| Independent | Distance of lamp from the plant, which changes light intensity |
| Dependent | Bubbles of oxygen per minute, or volume of gas collected |
| Control | Temperature, CO2 concentration, same piece of weed, lamp power, time |
Section 3 of 8Processing results
1Mean rate
- Mean rate Average the repeated bubble counts for each distance.
2Light intensity
- Light intensity Calculate 1 ÷ distance2 because intensity is inversely proportional to distance squared.
3Graph
- Graph Plot rate against light intensity.
4Conclude
- Conclude Rate rises with intensity until another factor becomes limiting.
Section 4 of 8Evaluation and safety
Sources of error
- Sources of error Counting bubbles is subjective and bubble size varies;
- collecting gas in a syringe or capillary tube gives a volume instead.
- Lamp heat can warm the water, so use the water-bath shield or a cool LED lamp.
Safety
- Safety Keep electrical equipment away from spilt water and wash hands after handling pond plants.
Section 5 of 8Worked example: processing results
| Distance (cm) | Bubbles in 1 minute (3 repeats) | Mean rate | 1 ÷ d2 |
|---|---|---|---|
| 10 | 48, 51, 51 | 50 per minute | 0.0100 |
| 20 | 14, 12, 13 | 13 per minute | 0.0025 |
| 40 | 4, 3, 5 | 4 per minute | 0.000625 |
Section 6 of 8Interpreting the pattern
Trend
- Trend As the lamp moves further away the light intensity falls and fewer bubbles of oxygen are released each minute.
Explanation
- Explanation Less light energy is absorbed by chlorophyll, so the light-dependent stage slows and less glucose and oxygen are produced.
Limits of the method
- Limits of the method At very short distances the graph may level off because carbon dioxide or temperature becomes limiting,
- so the straight-line relationship no longer holds.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Pondweed | A water plant, such as Cabomba, used to show oxygen production. |
| Light intensity | Amount of light energy reaching a surface. |
| Inverse square law | Intensity is proportional to 1 divided by distance squared. |
| Sodium hydrogencarbonate | A salt added to water as a source of carbon dioxide. |
| Mean | The sum of repeated readings divided by the number of readings. |
Section 8 of 8Exam technique
1
Plotting rate against distance and treating the graph as direct proportionality.
2
Not allowing the plant time to adjust after moving the lamp.
3
Forgetting to control temperature, which also changes the rate.
4
Counting bubbles from several different plants.
5
Counting bubbles before the weed has adjusted and before the stream of bubbles becomes steady.
Marked practiceQuizzes for this lesson
Biology 0610 & 9700 Exam Model
Limiting Factors Master
Photosynthesis: Limiting Factors & Graph Plateau
6CO2 + 6H2O → C6H12O6 + 6O2 (Light & Chlorophyll required)
- Limiting Factor Definition: Something present in the environment in such short supply that it restricts life processes.
- Graph Plateau: When rate levels off as light intensity increases, light is no longer the limiting factor; CO2 concentration or temperature has become limiting.
- High Temperature Drop: Beyond optimum temp (~40°C), enzymes (e.g. RuBisCO) denature, permanently decreasing rate.