The leaf discs were placed in a beaker of sodium hydrogencarbonate (NaHCO₃) solution. Explain why sodium hydrogencarbonate solution was used instead of water.
How to approach this question
1. What are the reactants of photosynthesis?
2. Which of these reactants might be in short supply in plain water?
3. How does sodium hydrogencarbonate help to provide this reactant?
Full Answer
Sodium hydrogencarbonate solution provides a source of carbon dioxide (CO₂). Carbon dioxide is a reactant needed for photosynthesis. This ensures that a lack of CO₂ does not become a limiting factor in the experiment.
Photosynthesis requires two main reactants: water and carbon dioxide. The leaf discs are already in an aqueous solution, so water is readily available. However, the amount of dissolved CO₂ in tap water can be low and variable, and it would quickly be used up by the photosynthesising leaf discs. Sodium hydrogencarbonate (NaHCO₃) solution releases carbon dioxide when dissolved in water. By using this solution, the student ensures that there is a constant and plentiful supply of carbon dioxide for the leaf discs. This is important for making it a fair test, as it prevents CO₂ concentration from becoming the limiting factor, allowing the effect of light colour to be investigated properly.
Common mistakes
✗ Just saying "it helps photosynthesis" - you must specify that it provides carbon dioxide.
✗ Confusing carbon dioxide with oxygen or carbon.
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