What Do We Know? Chlorophyll, Light and CO2
Before we explore chloroplasts, electron transport and the Calvin cycle, we must be secure in the three requirements that simple, repeatable experiments established long ago: chlorophyll, light and carbon dioxide.
These classical demonstrations are not childish exercises — they are the evidential bedrock of the entire chapter. Their logic is beautifully economical:
- A variegated leaf or a leaf partly masked with black paper tests the roles of chlorophyll and light in a single stroke, because starch appears only where green tissue meets illumination.
- A leaf partly enclosed with KOH-soaked cotton tests the role of carbon dioxide, because the enclosed portion is starved of ##\ce{CO2}## and consequently fails the starch test.
This quiz will check whether you can read the reasoning behind each setup — from the starch test itself to the conclusion it licenses. Mastery here makes the historical experiments of Priestley and Ingenhousz intelligible rather than mysterious.
Q1. According to the simple experiments described in the lesson, which three factors are required for photosynthesis to occur?
Simple experiments from earlier classes showed that chlorophyll (the green pigment of the leaf), light and ##\ce{CO2}## are required for photosynthesis to occur.
Q2. In the experiment using a variegated leaf, what does the leaf carry alongside green tissue?
A variegated leaf carries patches of non-green tissue alongside green tissue, allowing the role of chlorophyll to be isolated.
Q3. A leaf was partially covered with black paper before being exposed to light. Which factor does this setup isolate?
The masked leaf isolates light, since chlorophyll is present everywhere but the covered portion receives none.
Q4. On testing the variegated leaf and the partially black-paper-covered leaf for starch, what was observed?
On testing these leaves for the presence of starch it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light.
Q5. In the variegated leaf experiment, why does starch fail to appear in the non-green patches even though light reaches them?
The variegated leaf isolates chlorophyll: light reaches green and non-green regions alike, but only the green regions can absorb it usefully.
Q6. In the ##\ce{CO2}## experiment, what is the role of the KOH-soaked cotton placed in the test tube?
A part of a leaf is enclosed in a test tube containing KOH-soaked cotton, which absorbs ##\ce{CO2}##, while the other half is exposed to air.
Q7. In the ##\ce{CO2}## experiment, how is the leaf arranged before the setup is placed in light?
A part of the leaf is enclosed in a test tube containing KOH-soaked cotton while the other half is exposed to air, and the setup is then placed in light for some time.
Q8. After the ##\ce{CO2}## setup was placed in light for some time, what did the starch test reveal about the two parts of the leaf?
On testing for starch, the exposed part of the leaf tested positive while the portion that was in the tube tested negative.
Q9. What conclusion was drawn from the result of the KOH-soaked cotton experiment?
The exposed part tested positive for starch while the portion in the tube tested negative, showing that ##\ce{CO2}## was required for photosynthesis.
Q10. How can the conclusion that ##\ce{CO2}## is required be drawn from the starch test in the KOH experiment?
The enclosed portion is starved of ##\ce{CO2}## and consequently fails the starch test, while the exposed portion passes it — isolating ##\ce{CO2}## as the required factor.
Q11. Which experimental material is used to demonstrate that chlorophyll is necessary for starch formation?
A variegated leaf, carrying patches of non-green tissue alongside green tissue, is used to look for starch formation and isolate the role of chlorophyll.
Q12. In the masked-leaf experiment, chlorophyll is present everywhere in the leaf. What does the covered portion lack?
The masked leaf isolates light: chlorophyll is present everywhere, but the portion covered with black paper receives no light.
Q13. Which statement best summarises the combined result of the variegated leaf and masked leaf experiments?
The result is a double confirmation: where tissue is green and illuminated, starch accumulates; where either condition fails, it does not.
Q14. In the ##\ce{CO2}## experiment, the setup is placed in light for some time. Why is this step essential to the logic of the experiment?
The setup is placed in light so that photosynthesis can proceed; only then does the presence or absence of starch in the two parts become meaningful.
Q15. Which of the following correctly lists the three requirements of photosynthesis as established by the simple experiments in this lesson?
Simple experiments carried out in earlier classes have shown that chlorophyll, the green pigment of the leaf, light and carbon dioxide are required for photosynthesis to occur.