Lesson focus: Photosynthesis investigations require a valid proxySomething you measure instead because it is linked to what you really want to know, such as tree rings for past rainfall., control variableSomething you keep the same in an investigation so the test is fair, such as using the same amount of water each time. and cautious interpretation of limiting factorThe one thing in shortest supply that stops a process going any faster, such as light for photosynthesis..
Plan and evaluate investigations of photosynthesis using starch formation or oxygen production as evidence.
Build on earlier learning
LeavesLeaf structure showed how light, gases and water reach photosynthetic cells. This lesson tests factors that affect the process.
Planning an investigation?
Design an investigation with CORMMSSA step-by-step Logic Lab guide to planning a fair test, with a worked example and practice.
Practical
Testing a leaf for starch
- Boil the leaf in water to soften it.
- Place it in hot ethanol in a water bath to remove the chlorophyll.
- Rinse and add iodine: blue-black shows starch.
- Compare a leaf kept in the dark with one kept in the light.
Record: The colour of each leaf and what it shows about photosynthesis.
Safety: Ethanol is flammable: heat it only in a water bath, with flames out.
You will be successful when you can…
- Identify limiting factors.
- Describe a safe leaf-starch test.
- Calculate oxygen-production rate.
- Evaluate proxy measurements and control variables.
Words that carry the science
- limiting factor
- A factor in shortest supply that restricts the rate of a process.
- proxy
- A measurable quantity used as indirect evidence for another quantity.
- destarch
- To reduce stored starch before an investigation, usually by keeping a plant in darkness.
- control variable
- A factor kept sufficiently constant to make a comparison valid.
- rate
- Change in a measured quantity divided by time.
- saturation
- A region where increasing one factor no longer increases rate because another factor limits.
Use evidence without overstating what it measures
01Limiting factors interact
Light intensity, carbon dioxide concentration and temperature can limit photosynthesis. Water shortage can also restrict photosynthesis through stomatal closure and damage. Raising one factor increases rateHow fast something happens, such as how much changes each second or each minute. only while that factor is limiting.
A plateau does not mean photosynthesis has reached a universal maximum. It means another factor limits under those conditions or the measurement cannot resolve further change.
02The leaf-starch test detects stored product
destarchTo remove the stored starch from a plant, usually by keeping it in the dark for a couple of days. a plant, expose selected leaf regions to defined conditions, then heat the leaf in water to stop reactions. Decolourise it in ethanol heated in a water bath, rinse it, and add iodine solution. Blue-black indicates starch.
Ethanol is flammable, so it must be kept away from naked flames. The result shows that starch accumulated; it does not directly measure the instantaneous rate of photosynthesis.
03Aquatic-plant oxygen is another proxy
Gas volume collected per unit time is stronger quantitative evidence than counting bubbles, because bubble size varies. Keep temperature, plant length, carbon dioxide supply and measurement time controlled.
A lamp can heat the water as it is moved closer. Use a heat shield or LED source, monitor temperature and measure light intensity when possible. Oxygen may dissolve or be used in respiration, so measured gas can underestimate photosynthetic production.
Photosynthesis and light intensity
Move the lamp and count the bubbles of oxygen from the pondweed. The rate rises with light until something else runs short: the limiting factor.
Controls
Halving the distance gives four times the light. The model keeps the water temperature fixed, as a heat shield or LED lamp would.
Calculate an oxygen-production rate
An aquatic shoot produces 7.2 cm³ of gas in 12 minutes. Calculate the mean rate.
- Use rate = measured change ÷ time.
- Substitute 7.2 cm³ ÷ 12 min.
- Calculate 0.60.
- State the unit: cm³ min⁻¹.
How to make the reasoning visible: The result is a mean gas-collection rate and should be described as a proxySomething you measure instead because it is linked to what you really want to know, such as tree rings for past rainfall. for photosynthesis under the stated conditions.
Interpret a limiting-factor investigation
The values are illustrative mean oxygen volumes collected in 10 minutes.
| Light intensity / arbitrary units | 20 °C / cm³ | 30 °C / cm³ | Pattern | Likely limitation |
|---|---|---|---|---|
| 10 | 1.1 | 1.2 | Similar low rates | Light |
| 20 | 2.0 | 2.3 | Rate rises | Mainly light |
| 40 | 3.1 | 4.0 | Temperature difference grows | Light and temperature |
| 80 | 3.3 | 5.8 | 20 °C plateaus | Another factor at 20 °C |
| 120 | 3.4 | 6.0 | Both near plateaus | CO₂ or biochemical capacity may limit |
Why is it unsafe to conclude carbon dioxide definitely caused the final plateau?
Why are means from repeats preferable to one result?
Check your understanding
Answer from memory, then check against the suggested answer.
Name three common limiting factors for photosynthesis.
What colour shows starch with iodine?
Why heat ethanol in a water bath?
A student counts bubbles at different lamp distances. Give one improvement to validity and one to measurement quality.
Explain why increasing temperature may increase photosynthesis at low values but decrease it above an optimum.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student measures the rate of photosynthesis by counting bubbles from pondweed while moving a hot filament lamp closer.
Evaluate the method. Identify two weaknesses.
(2)Suggest improvements and state what can be concluded.
(4)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., connect each limitation to its effect on the conclusion and then give a specific matching improvement.