Q&A - PSLE Science

[kittycat01]

I have a question.


The picture in the question shows a plant in a sealed beaker of soil (only the bottom part of the plant that means the root part is sealed in the beaker). But there’s some water in the sealed beaker. The whole plant is sealed in a clear container and put out in the sun. Quesion says the rate of photosynthesis is decreasing…and is due to what? is it carbon dioxide or water. To answer the question must ask ourselves which one is used up first - carbon dioxide or water?

[kittybank]

[Verysuperkiasu]

wormy:

If circle is not big enough, the corners of the square will show. Hence it will look like a square

but height of circle (diameter) is same as the length of square. so everyone thinks it's a square. think is a trick question. u have to look at the distance between the light source and the objects. It is a square only when the light source is very far away. The only way to find out in the question is to draw straight lines from light source to the objects. Whichever one blocks the light will be that shadow on the screen. seems like circle Think so...

[atutor2001]

kittycat01:

I have a question.


The picture in the question shows a plant in a sealed beaker of soil (only the bottom part of the plant that means the root part is sealed in the beaker). But there's some water in the sealed beaker. The whole plant is sealed in a clear container and put out in the sun. Quesion says the rate of photosynthesis is decreasing.....and is due to what? is it carbon dioxide or water. To answer the question must ask ourselves which one is used up first - carbon dioxide or water?

This is a terrible question. Lets try to list down the effects of :
1. sealing the soil
2. sealing the plant
3. placing it under the sun.

1. Sealing the soil means water condensing in the outer container cannot return to the soil. Water from the soil cannot evaporate away under the sun. It also means the roots will use up the oxygen for respiration (an additional confusing factor) and will rot (an additional confusing factor).

2. Supply of carbon dioxide to the plant is limited and will be used up eventually. Sealing the plant means water vapour given out by the plant cannot escape (but also cannot return to the soil)

3. Placing the plant under the sun means light will be converted to heat inside the set-up and is trapped (greenhouse effect). Temperature inside the set-up will rise and the plant will give out more water vapour to cool down the plant. i.e. rate of transpiration will increase. However, with bright light from the sun, the rate of photosynthesis will also increase.

From the able analysis, we can see that both the rate of usage of water and carbon dioxide have increased. Therefore, to determine which is the main cause for the rate of photosynthesis to decrease, (i.e. which is used up first), the amount of water (i.e. size of the pot) and carbon dioxide available (i.e. size of the outer enclosure) will be the determining factor.

Based on my reading of the mindset of the person who set the question, I think the answer he/she wants is \"water\" because water is used for both transpiration and photosynthesis while carbon dioxide is used only for photosynthesis.

Anyway, it is a lousily set question as I can argue that the amount of water in the pot was \"so much so\" that carbon dioxide is used up first. The saddest truth is in real life, the photosynthesis will decrease not due to lack of water nor carbon dioxide. The plant will be dying because it cannot remove heat fast enough because of the intense heat inside the set-up (like the interior of a car parked under the hot sun)

PS once I bought a pot of orchid, left it in the car (under the sun) and continued shopping for less than an hour. The orchid looks fine when I reached home. I was shocked the next morning to find all the leaves had turned brown - it was just amazing.

[kittycat01]

sorry, atutor, the description is not that accurate. it’s actually in the psle science paper this year. According to my daughter, the plant is in a sealed beaker of water (no soil) but only the root part is immersed in the sealed beaker. and it’s placed under the lamp, not the sun. The rest of the details are correct. Sorry for the confusion. I should have clarified with her first before putting the question down on this forum.


Anyway, just to comment - lots of dd’s classmates put CO2 being the answer but others put water.

[P5G]

kittycat01:

sorry, atutor, the description is not that accurate. it's actually in the psle science paper this year. According to my daughter, the plant is in a sealed beaker of water (no soil) but only the root part is immersed in the sealed beaker. and it's placed under the lamp, not the sun. The rest of the details are correct. Sorry for the confusion. I should have clarified with her first before putting the question down on this forum.


Anyway, just to comment - lots of dd's classmates put CO2 being the answer but others put water.

Hi,

My answer is carbon dioxide as the plant is in a container with limited air. Air only contains 0.03% carbon dioxide. Most of my classmates also choose carbon dioxide as their answer.

[atutor2001]

deleted because information was wrong

[kittycat01]

atutor2001:

kittycat01:

sorry, atutor, the description is not that accurate. it's actually in the psle science paper this year. According to my daughter, the plant is in a sealed beaker of water (no soil) but only the root part is immersed in the sealed beaker. and it's placed under the lamp, not the sun. The rest of the details are correct. Sorry for the confusion. I should have clarified with her first before putting the question down on this forum.


Anyway, just to comment - lots of dd's classmates put CO2 being the answer but others put water.

I may still put \"water\" because there is insufficient water moving through the plant to cool it down (although it is not due to lack of water from the source, but rather it is due to the plant not being able to take in water fast enough to cool it down).

My apology for not being of much help.

I think the answer lies in how the actual question is phrase - then we can have a better interpretation of the requirement.

Thank you, atutor. Just wanted to see what you all think.

[atutor2001]

kittycat01:

atutor2001:

[quote=\"kittycat01\"]sorry, atutor, the description is not that accurate. it's actually in the psle science paper this year. According to my daughter, the plant is in a sealed beaker of water (no soil) but only the root part is immersed in the sealed beaker. and it's placed under the lamp, not the sun. The rest of the details are correct. Sorry for the confusion. I should have clarified with her first before putting the question down on this forum.


Anyway, just to comment - lots of dd's classmates put CO2 being the answer but others put water.

I may still put \"water\" because there is insufficient water moving through the plant to cool it down (although it is not due to lack of water from the source, but rather it is due to the plant not being able to take in water fast enough to cool it down).

My apology for not being of much help.

I think the answer lies in how the actual question is phrase - then we can have a better interpretation of the requirement.

[/quote]Hi kittycat01

My apology again.

The answer is \"carbon dioxide\". I didn't take note of your clarification that it is placed under a lamp and not sun. So the issue of heat is no longer valid.

[Tang]

P5G:

kittycat01:

sorry, atutor, the description is not that accurate. it's actually in the psle science paper this year. According to my daughter, the plant is in a sealed beaker of water (no soil) but only the root part is immersed in the sealed beaker. and it's placed under the lamp, not the sun. The rest of the details are correct. Sorry for the confusion. I should have clarified with her first before putting the question down on this forum.


Anyway, just to comment - lots of dd's classmates put CO2 being the answer but others put water.

Hi,

My answer is carbon dioxide as the plant is in a container with limited air. Air only contains 0.03% carbon dioxide. Most of my classmates also choose carbon dioxide as their answer.

Hi,

I think the correct answer is \"water\" because when carbon dioxide is used up, the plant can replenish it through respiration and photosynthesis can continue until the water in the beaker is depleted.