Lesson 5 of 12 / Nutrition and transport in flowering plants
Measuring photosynthesis
What would make a pondweed comparison fair?
I can plan and interpret investigations of light, carbon dioxide or temperature using oxygen output as a rate indicator.
I can investigate and describe light, CO2 and temperature effects on photosynthesis.Syllabus
Syllabus K224 / K225, Topic 8(h). Investigate and describe light, CO2 and temperature effects on photosynthesis.
About 6 min
Make a guess. You are not marked.
You move a lamp closer to some pondweed and it gives off more bubbles. Can you be sure light caused the change?
Show the answer
The lamp may also have warmed the water.
A close lamp raises temperature as well as light. Control or measure temperature before blaming light.
The key idea
Change one factor, control the others and measure oxygen output over a fixed time. Repeated measurements make the comparison more reliable.
- Change
- Change measured light intensity or lamp distance.
- Keep comparable
- Keep temperature, CO2 availability, plant amount and timing comparable.
| Condition | Volume (cm3) | Rate (cm3/min) |
|---|---|---|
| Low light | 2 | 1 |
| Medium light | 4 | 2 |
| Higher light | 6 | 3 |
A closer lamp may warm the water. Use temperature control and allow adjustment before measuring. Divide volume by 2 minutes to obtain the rates shown.
Green: aquatic plant. White gas in the inverted tube: collected oxygen. This schematic omits support stands. Bubble sizes vary; oxygen volume per time is generally a better measure than bubble count, and dissolved/respired oxygen is not collected.
Explanation
A water plant such as pondweed gives off oxygen when it photosynthesises. Collecting the gas and measuring its volume per minute shows how fast it is photosynthesising. Counting bubbles is easier but less accurate, because bubbles can be different sizes.
To test the effect of light, move a lamp nearer or further away. Keep the temperature and the carbon dioxide supply the same, use similar pieces of plant and time each test for the same length of time. A lamp also gives out heat, so use a heat shield or a water bath to keep the temperature steady.
To test carbon dioxide, use sodium hydrogencarbonate solutions of different strengths, and keep light and temperature the same. To test temperature, use water baths at different temperatures, and keep light and carbon dioxide the same. Let the plant settle for a few minutes before each reading.
Repeat each test and work out a mean rate. More light or more carbon dioxide can increase the rate, until the rate levels off. A limiting factor is the factor in shortest supply, which holds back the rate. Increasing it increases the rate; increasing any other factor does not. When the rate levels off, the plant is still photosynthesising, at a steady rate.
Warming the plant from a low temperature can speed up photosynthesis, because its enzymes work faster. If it gets too hot, the enzymes are damaged and the rate falls. Describe what your own data show.
Step by step
- 1
Design the comparison
Choose light, CO2 or temperature and state how the other two are controlled.
- 2
Measure a rate
Collect oxygen volume over a known time; repeat under comparable conditions.
- 3
Interpret with limits
Use the data trend, noting bubble-size differences or oxygen dissolution where relevant.
Worked example
Convert output to rate
In an illustrative experiment, 6.0 cm3 of oxygen is collected in 3 minutes. Calculate the rate and state one reason it estimates rather than directly measures total photosynthesis.
One way to explain it
The measured oxygen-output rate is 6.0 / 3 = 2.0 cm3 per minute. Some oxygen may dissolve in the water or be used in respiration rather than collected.
Why this answer works
- Divide volume by time.
- Include rate units.
- Link the limitation to the measured output rather than dismissing the experiment.
Watch out for this
A student says: "Moving a lamp closer changes only light intensity." What is wrong with this?
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It can also increase temperature. A fair test must control or monitor this additional change.
Check your understanding
Why is oxygen volume usually a better measure than bubble count?
Show the answer
Bubbles can differ in size, so equal counts need not represent equal gas volumes.
Measuring volume accounts for size differences, although other practical limitations remain.
Exam-style questions
Check your understanding
Hydrogencarbonate indicator is red at normal carbon dioxide levels, yellow when carbon dioxide is higher and purple when it is lower. A leaf in bright light turns the indicator purple. Why?
Show the answer
Photosynthesis takes in carbon dioxide faster than respiration releases it
In bright light the net uptake of carbon dioxide lowers its level, so the indicator turns purple.
Quick recall
What would make a pondweed comparison fair?