Can you explain how this conclusion could be drawn?
The conclusion that CO₂ is required for photosynthesis can be drawn through a simple logical analysis of the experiment. In this setup, one part of the leaf is enclosed in a test tube with KOH soaked cotton, which absorbs CO₂, while the other half remains exposed to air containing CO₂.
When both parts are placed in light and later tested for starch, the exposed part shows positive for starch, whereas the portion inside the tube tests negative. Since both parts receive equal light, the only variable is the presence of CO₂. The absence of starch in the KOH-enclosed portion indicates that photosynthesis did not occur there due to lack of CO₂. This clearly demonstrates that CO₂ is essential for photosynthesis to produce starch.
Explanation
This question tests the student's understanding of experimental design and the role of CO₂ in photosynthesis. The key concept is that KOH absorbs CO₂, creating a CO₂-free environment. When the leaf portion lacks CO₂, it cannot perform photosynthesis and thus produces no starch. The exposed portion with access to atmospheric CO₂ produces starch normally. By comparing these two outcomes under identical light conditions, students can conclude that CO₂ is necessary for photosynthesis. This is a classic example of a controlled experiment where all variables except one (CO₂ availability) are kept constant.
Solution Steps
Step 1: Identify the variable being tested - presence/absence of CO₂
Step 2: Note that KOH absorbs CO₂, creating a CO₂-free zone
Step 3: Compare starch test results - positive in exposed part, negative in enclosed part
Step 4: Conclude that the difference in results is due to CO₂ availability alone