Photosynthesis factors

Study the influence of different factors (light, temperature, plant type) on photosynthesis intensity by measuring the rate of CO₂ consumption with FizziQ Connect.

Photosynthesis is not a fixed process: its intensity varies depending on many environmental parameters. Light intensity, light wavelength, temperature, and even the plant species all influence how quickly a plant consumes CO₂. By designing a controlled experiment that varies one factor at a time while keeping all others constant, you can quantify these effects and understand the biological mechanisms behind them.

Learning objectives:

The student designs and carries out a controlled experiment to study the influence of one factor on photosynthesis rate, measuring CO₂ consumption with the SCD40 sensor.

Level:

High school

Marie-Anne Dejoan

Author:

Duration (minutes) :

50

What students will do :

- Design an experimental protocol with identified variables (independent, dependent, controlled)
- Formulate a testable hypothesis about the influence of a factor on photosynthesis
- Perform multiple measurements under controlled conditions
- Calculate and compare CO₂ consumption rates under different conditions
- Draw conclusions based on data and relate them to biological mechanisms

Scientific concepts:

- Limiting factors of photosynthesis
- Enzymatic activity and temperature
- Absorption spectrum of photosynthetic pigments
- Light compensation point
- Experimental design and controlled variables

Sensors:

- SCD40 sensor (CO₂ in ppm, temperature, humidity)
- O₂ sensor (optional)

What is required:

- Smartphone or tablet with FizziQ Connect
- SCD40 sensor
- O₂ sensor (optional)
- M5 Stack module
- Sealed transparent chamber
- Fresh leaves
- Light sources of varying intensity and color
- Thermometer and water bath

Experimental procedure:

Choose the factor you wish to study: light intensity, light color, temperature, or plant type.
Identify the variables: the independent variable (the factor you vary), the dependent variable (CO₂ consumption rate), and the controlled variables (everything else held constant).
Prepare the basic setup: connect the SCD40 sensor to the M5 Stack module, connect FizziQ Connect, and verify the readings.
Perform a first control measurement: place the plants in the sealed chamber in daylight for 10 minutes. Calculate the CO₂ consumption rate (slope of the CO₂ curve).
Modify the chosen factor for the second condition. For example: reduce light intensity (move the lamp farther or use a filter).
Perform the second measurement under the same conditions (same duration, same plant volume, same chamber), changing only the tested factor.
Repeat for at least a third condition (for example: bright, medium, and dim light; or 5°C, 20°C, 35°C).
For each condition, calculate the slope of the CO₂(t) curve during the linear phase. This slope represents the CO₂ consumption rate.
Plot a summary graph: CO₂ consumption rate versus the studied factor.
Compare your results with your initial hypothesis. Conclude on the influence of the factor and propose explanations based on the underlying biology.

Expected results:

Results depend on the factor studied. For light intensity: CO₂ consumption rate increases with intensity up to a plateau (pigment saturation). For light color: red and blue light give the highest rates, green the lowest. For temperature: rate increases up to 25-35°C then drops. For plant type: C4 plants (corn) are faster than C3 plants (spinach) at high light and temperature.

Scientific questions:

- Why is photosynthesis more efficient under red and blue light than under green light?
- How does temperature influence enzyme activity, and why is there an optimum?
- What is the light compensation point and why does it matter for plant ecology?
- How would increasing CO₂ concentration affect the photosynthesis rate?
- Why is it important to vary only one factor at a time?
- How do your results relate to the global carbon cycle?

Scientific explanations:

Photosynthesis depends on several environmental factors that act on the two phases of the process: the light-dependent reactions (in the thylakoids) and the Calvin cycle (in the stroma).

Light intensity directly affects the light-dependent reactions. More photons means more photosystems activated, more ATP and NADPH produced, and faster CO₂ fixation. Above a saturation intensity, the rate plateaus.

Light wavelength is crucial because photosynthetic pigments do not absorb all colors equally. Chlorophyll a and b absorb strongly in red (660-680 nm) and blue (430-450 nm) but reflect green, which is why leaves appear green.

Temperature affects enzyme activity, particularly RuBisCO (the enzyme that fixes CO₂). Activity increases with temperature up to an optimum (typically 25-35°C), then decreases rapidly as enzymes denature.

The light compensation point is the light intensity at which photosynthesis exactly balances respiration. Below this point, the plant is a net CO₂ producer even in daylight.

In an experimental project, it is essential to vary only one factor at a time while keeping all others constant. This is the principle of controlled experimentation that ensures any observed effect is due to the tested variable.

Extension activities:

Frequently asked questions:

Q: How do I maintain a constant temperature during the experiment?
R: Use a water bath (container filled with water at the desired temperature) in which you immerse the sealed chamber.

Q: The CO₂ consumption rate is very low or zero.
R: Ensure the leaves are fresh, green, and not wilted. Use strong light. If the rate is still low, try a larger mass of plant material.

Q: My results are inconsistent between trials.
R: Environmental conditions may have changed between trials. Repeat under the same conditions and calculate averages.

Q: Can I test multiple factors simultaneously?
R: This is not recommended for an initial study because it makes it impossible to determine which factor caused the observed change. Test one factor at a time.