O₂ evolution measured with a gas syringe (mL O₂ min⁻¹)
Procedure (outline)
Collect healthy, fully expanded leaves from the same plant species.
Cut uniform leaf discs (e.g., 1 cm diameter) and place each disc in a sealed chamber containing a known volume of water.
Introduce the leaf disc to the chamber and allow it to equilibrate for 5 min.
Adjust the experimental variable to the first value in the series.
Record the change in gas volume every minute for 10 min using a gas syringe.
Repeat steps 3–5 for each value of the variable, ensuring that all other conditions remain constant.
Carry out three replicates for each treatment and calculate the mean rate of O₂ evolution.
Data Presentation
Example table for light intensity (mean rates):
Light intensity (µmol m⁻² s⁻¹)
Mean O₂ evolution (mL min⁻¹)
0
0.00
200
0.45
400
0.88
600
1.20
800
1.35
1000
1.38
Graphical Representation
Plot rate of photosynthesis (y‑axis) against the variable being tested (x‑axis).
Identify the region where the curve rises sharply (limiting region) and where it plateaus (saturation).
Suggested diagram: Typical photosynthesis rate vs. light intensity curve showing a hyperbolic rise and plateau.
Suggested diagram: Rate vs. CO₂ concentration curve – initially linear then leveling off.
Suggested diagram: Rate vs. temperature curve – optimum temperature with decline at higher temperatures.
Theoretical Explanation
1. Light intensity – Light provides the energy for the light‑dependent reactions. At low intensities, the rate is directly proportional to photon flux:
\$r_{\text{light}} \propto I\$
Beyond a certain intensity, the photosynthetic apparatus becomes saturated and the rate reaches a maximum (light‑saturated rate, \$r_{\max}\$).
2. CO₂ concentration – CO₂ is the substrate for the Calvin cycle. The relationship can be described by Michaelis–Menten kinetics:
where \$Km\$ is the half‑saturation constant. At high CO₂ the rate approaches \$r{\max}\$.
3. Temperature – Temperature influences enzyme activity. The rate generally follows an Arrhenius‑type increase up to an optimum (\$T_{\text{opt}}\$), after which enzyme denaturation causes a decline:
(\$\sigma\$ reflects the temperature tolerance of the enzymes).
Interpretation of Results
Light intensity: A rapid rise up to \overline{800} µmol m⁻² s⁻¹ indicates light limitation; the plateau beyond this point shows that other factors (e.g., CO₂) become limiting.
CO₂ concentration: Linear increase at low concentrations demonstrates substrate limitation; the leveling off indicates saturation of Rubisco activity.
Temperature: The curve peaks around 25–30 °C for most C₃ plants, reflecting the optimum for Rubisco and electron transport. Decline above 35 °C signals thermal inhibition.
Conclusions
Photosynthetic rate is directly proportional to light intensity, CO₂ concentration and temperature only within specific ranges.
Each factor exhibits a characteristic saturation point beyond which it no longer limits the rate.
Understanding these limiting factors helps explain plant distribution, crop yield optimisation and responses to climate change.
Possible Extensions
Investigate the effect of water availability (humidity) on stomatal conductance and photosynthetic rate.
Compare C₃ and C₄ species to illustrate differences in CO₂ saturation curves.
Use chlorophyll fluorescence as a non‑destructive proxy for photosynthetic efficiency.
Safety and Ethical Considerations
Handle glass gas syringes carefully to avoid breakage.
Dispose of plant material according to school policy.
Ensure that any chemicals used to manipulate CO₂ levels (e.g., NaHCO₃) are handled with appropriate PPE.