Energy from Food Topic

Explain How Light Affects Photosynthesis?

This science revision note aims to help your child

– identify the factor affecting the rate of photosynthesis.

– explain your answer using the proper scientific keywords.

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Greg wanted to conduct an experiment with his plant. He placed the plant in a dark room for 24 hours to remove all the starch in it.

After 24 hours. he took out the plant and placed it beside the window. He chose a leaf and covered Region A of the leaf with black strip of paper, while leaving Region B alone.

After 3 days, he removed the leaf and added a few drops of iodine to each region. The iodine tests for the presence of starch. The iodine solution is brown but changes to blue-black when it detects starch.

What was the colour of the iodine in Region A? Explain your answer.

STEP 1 – Analyse the question and link them to the concept(s)

To figure out what this question is asking for, identify the purpose of iodine in the experiment.

Iodine tests for the presence of starch –> Plants make food through photosynthesis –> The excess food is then stored as starch in the leaf –> hence, this question is about photosynthesis

STEP 2

What does the black strip of paper do? Consider how it affects photosynthesis.

The black strip of paper blocks out light from Region A of the leaf. Without light, photosynthesis cannot occur at Region A.

STEP 3

How does photosynthesis link to the colour of iodine?

Without photosynthesis, no food is made. No starch is produced at Region A. Iodine will not detect starch and will remain brown.

You can watch this video for a clearer understanding of the photosynthesis concept.

FINALLY, put your answer together

Question: What was the colour of the iodine in Region A? Explain your answer.

Answer:

The iodine in Region A remained brown. The black strip of paper blocks out light from region A of the leaf. Hence, region A of the leaf cannot photosynthesise and cannot make food. No starch is produced at Region A. Iodine will not detect starch and will remain brown.

Common mistakes

Incomplete explanation:

The iodine in Region A remained brown. There is a black strip of paper that prevents photosynthesis from occurring at Region A. Without photosynthesis, starch is not produced at Region A. Iodine will not detect starch, and will remain brown.

Teachers’ comments:

Although this is not wrong, it is unclear how the black strip of paper prevents photosynthesis from occurring at Region A. You need to clearly state the effect of the black strip of paper by specifying the factor that affects the ability to photosynthesise – the presence/absence of light.

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