Plant photosynthesis

Demonstrate photosynthesis and cellular respiration in plants by measuring CO₂ and O₂ exchange in light and darkness with FizziQ Connect sensors.

Photosynthesis is a fundamental chemical process of life: green plants use light energy to combine water (H₂O) and carbon dioxide (CO₂), producing carbohydrates and releasing oxygen (O₂). But plants also respire continuously, consuming O₂ and producing CO₂. By measuring gas concentrations in a sealed chamber with a CO₂ sensor, you can observe the striking reversal that occurs when the light is turned off: CO₂ consumption stops instantly and CO₂ production begins.

Activity overview:

The student records CO₂ and O₂ concentrations in a sealed chamber with plants, first in light then in darkness, observing the dramatic slope change.

Level:

High school

Marie-Anne Dejoan

Author:

Duration (minutes) :

30

What students will do :

- Demonstrate the gas exchange associated with photosynthesis and cellular respiration
- Measure the evolution of CO₂ concentration in a sealed chamber containing plants
- Identify the role of light as an energy source for photosynthesis
- Compare the rates of CO₂ consumption (light) and production (dark)
- Understand the net gas exchange balance of a plant

Scientific concepts:

- Photosynthesis
- Cellular respiration
- CO₂/O₂ gas exchange
- Chloroplasts and mitochondria
- Plant metabolism
- Photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Sensors:

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

Material needed:

- Smartphone or tablet with FizziQ Connect
- SCD40 sensor (CO₂, temperature, humidity)
- O₂ sensor (optional)
- M5 Stack module and multiport hub
- Transparent sealed chamber
- Fresh green leaves (lettuce, spinach)
- Opaque cloth to block light

Experimental procedure:

Connect the O₂ and SCD40 sensors to the I2C port of the M5 Stack module via the multiport hub.
Open FizziQ Connect on your smartphone or tablet and connect to the M5 Stack via Bluetooth.
Verify that both sensors are detected: you should see the CO₂ concentration (in ppm) and the O₂ concentration (in %).
Place freshly cut plant material (lettuce leaves, spinach) in the transparent sealed chamber, along with the sensors.
Seal the chamber and place it in daylight (near a sunny window).
Start simultaneous recording of CO₂ and O₂ concentrations in FizziQ Connect.
Leave the chamber in the light for 10 minutes. Observe the evolution: photosynthesis dominates, CO₂ decreases.
Without opening the chamber, cover it completely with the opaque cloth to block all light.
Continue recording for 10 more minutes in darkness. Observe the slope change: CO₂ now increases (only respiration occurs).
Stop the recording and analyze the graphs. Identify the exact moment of the slope change and compare the CO₂ consumption rate (light) with the production rate (dark).

Expected results:

A very clear slope change in CO₂ concentration occurs nearly instantly when the chamber is covered. In light, CO₂ decreases as photosynthesis consumes it faster than respiration produces it. In darkness, CO₂ increases because only respiration is active. The CO₂ consumption rate in light is typically 2-5 times the production rate in darkness.

Scientific questions:

- Why is the slope change in CO₂ concentration nearly instantaneous when switching to darkness?
- If respiration occurs both day and night, why is the net CO₂ exchange negative in light?
- What would happen if the experiment continued for several hours in the dark?
- How does light intensity affect the rate of CO₂ consumption?
- What is the compensation point where photosynthesis exactly balances respiration?
- Why do forests act as carbon sinks during the growing season?

Scientific explanations:

Photosynthesis is the process by which green plants convert light energy into chemical energy. The overall equation is: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. It occurs in the chloroplasts.

Cellular respiration is the reverse process: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). It occurs in the mitochondria and operates continuously, day and night.

In light, both processes coexist in the plant cell. However, photosynthesis is more active than respiration, so there is a net consumption of CO₂ and net production of O₂.

In darkness, photosynthesis stops completely (it requires light energy), but respiration continues. The plant becomes a net CO₂ producer and O₂ consumer.

The SCD40 sensor measures CO₂ concentration using a photoacoustic principle, with a range of 0 to 40,000 ppm and precision of about ±50 ppm.

The O₂ sensor works on an electrochemical principle. Its precision (about ±1%) is often insufficient to detect small O₂ changes over 10 minutes, so CO₂ measurement is more reliable for this experiment.

The intensity of photosynthesis depends on several factors: light intensity, light wavelength, temperature, CO₂ concentration, and the type of plant.

Extension activities:

Frequently asked questions:

Q: The O₂ sensor does not show a clear variation. Is this normal?
R: Yes. The O₂ sensor has a precision of about ±1%, which is often insufficient to detect the small O₂ changes over 10 minutes. Focus on the CO₂ data, which is much more sensitive.

Q: The CO₂ does not decrease in light.
R: Ensure the leaves are fresh and green. Wilted or yellowed leaves have reduced photosynthetic activity. Also check that the light is bright enough.

Q: Can I use artificial light instead of sunlight?
R: Yes, but use a bright white or blue-red LED. Green light is not absorbed by chlorophyll and will not drive photosynthesis.

Q: How long should I record?
R: 10 minutes in each condition is usually sufficient. For more dramatic changes, extend to 15-20 minutes.