Practical: The Effect of Light on Gas Exchange in Plants
Concentration Gradients: In bright light, the rate of photosynthesis is significantly higher than the rate of respiration. This causes the internal concentration of carbon dioxide to drop below atmospheric levels, leading to a net diffusion of CO₂ into the leaf cells.
pH and Carbonic Acid: Carbon dioxide reacts with water to produce carbonic acid (), which dissociates to release hydrogen ions. An increase in CO₂ lowers the pH (making the solution more acidic), while a decrease in CO₂ raises the pH (making it more alkaline).
The Compensation Point: This is a specific light intensity where the volume of carbon dioxide produced in respiration exactly equals the volume consumed in photosynthesis. At this point, there is no net exchange of gas with the environment, and the indicator remains its original orange color.
| Condition | Net CO₂ Movement | Indicator Color | Metabolic State |
|---|---|---|---|
| Bright Light | Net Uptake | Purple | Photosynthesis > Respiration |
| Total Darkness | Net Release | Yellow | Respiration only |
| Dim Light | Little to no change | Orange | Photosynthesis ≈ Respiration |
| Control | No change | Orange | No biological activity |
Respiration vs. Photosynthesis: Students often assume respiration stops in the light; however, it continues at a steady rate while photosynthesis fluctuates with light intensity. The 'purple' result is a measure of the difference between the two rates, not the absence of respiration.
Foil vs. Muslin: Aluminum foil creates a binary 'dark' condition, while muslin acts as a semi-permeable light filter. This distinction allows students to investigate the compensation point where metabolic rates reach equilibrium.
Identify the Indicator's Baseline: Always mention that hydrogen-carbonate indicator starts as orange. Examiners look for the specific starting color to verify that the student understands the direction of the pH shift.
Explain 'Why' for Every Color: Do not just state that the tube turns purple; explain that it turns purple because the rate of photosynthesis exceeds respiration, leading to a decrease in carbon dioxide concentration and an increase in pH.
CORMS Evaluation: When asked to design this experiment, ensure you mention keeping the leaf size, species, and temperature constant (Same), repeating the test for reliability (Repeat), and measuring the color change after a set time (Measurement).
The 'No Respiration' Error: A common misconception is that plants only respire at night. In reality, respiration provides the ATP needed for cellular maintenance 24/7; the indicator only shows the 'net' result of gas exchange.
Leaf Positioning: If the leaf falls into the indicator, the results may be invalid because the solution might absorb cellular contents or the leaf surface may be blocked from gas exchange. Always ensure the leaf is suspended securely in the air space.
Inadequate Time: If the experiment is checked too early, the color change may be too subtle to distinguish. It is standard practice to leave the tubes for several hours or overnight to allow a measurable concentration gradient to develop.