Energy From Food Topic

Question on Photosynthesis and Respiration

This science revision note aims to help your child

– identify the relevant concepts after analysing the setup that has plants and fishes

– explain your answer using the proper scientific keywords.

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Megan had two identical tanks filled with equal amounts of water. She placed an identical plant of equal size in each of the tanks. She then added a few fish into Tank X only. The tanks were placed under the sun in her balcony. The fish do not feed on the plant in tank X during the entire experiment.

After a few days, she noticed that one of the plants grew slightly bigger than the other. Which plant grew bigger? Explain your answer.

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

The only difference between the tanks is that Tank X has fish while Tank Y does not. Hence, identify the effect of having fish in the tank.

The fish in Tank X respire, giving out dissolved carbon dioxide.

STEP 2

How does carbon dioxide affect the growth of plants?

Plants require carbon dioxide to photosynthesise.

More carbon dioxide –> faster rate of photosynthesis –> more food for the plant –> grows faster.

STEP 3

So which plant grew bigger after a week?

Tank X has more dissolved carbon dioxide as it has fish respiring –> the plant in Tank X takes in more carbon dioxide to photosynthesise faster –> providing more food for itself –> grows faster than the plant in Tank Y

Hence, the plant in Tank X grew bigger than the plant in Tank Y after a week.

FINALLY, put your answer together

Question: Which plant grew bigger? Explain your answer.

Answer:

The plant in Tank X grew bigger. The fish in Tank X respire, giving out dissolved carbon dioxide. Water in Tank X has more dissolved carbon dioxide so the plant in Tank X takes in more carbon dioxide to photosynthesise faster, providing more food than the plant in tank Y. Hence, the plant in X grows faster than the plant in Tank Y.

Common mistakes

Incomplete explanation:

There is more carbon dioxide in Tank X, so the plant in Tank X takes in more carbon dioxide to photosynthesise faster, providing more food for itself, allowing it to grow faster than the plant in Tank Y.

Teachers’ comment

The answer is not complete. The student did not explain why there is more carbon dioxide in Tank X. You need to explain where the carbon dioxide comes from (the fish) and how the fish provide it – respire is a keyword here.

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