Photosynthesis in plants- question

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Aerlinn
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Photosynthesis in plants- question

Post by Aerlinn » Tue Nov 15, 2005 10:14 am

This is a multiple choice question I have trouble with.
Plants growing in hot dry environments often show reduced photosynthetic activity in the middle of the day. This is because:
A. when the transpiration rate is high the stomata close, limiting the amount of carbon dioxide that enters the leaf.
B. the plant is unable to photosynthesise without plenty of water and there is not enough water available in a hot dry environment
C. the plant does not need to photosynthesise all the time in such a sunny environment and therefore restricts photosynthesis to early morning and late afternoon.
D. high temperatures in the middle of the day slow down the rate of photosynthesis.

I don't think it's C or D... but I could be wrong. Help please?
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Post by Navin » Tue Nov 15, 2005 11:25 am

I think its A.
Due to the closing of the stomata, no carbon dioxide is present for photosynthesis.

I don't think B as in fact very little water is used for photosynthesis by the plant.

D is a little off. I doubt temperatures can reach higher than 50 degrees celcius.

C has problem with logic.
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Post by Aerlinn » Tue Nov 15, 2005 11:44 am

I thought so too. Thanks!
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Post by MrMistery » Tue Nov 15, 2005 8:00 pm

Most plants do close their stomas in the middle of the day, especially if they are adapted to a dry climate(they need the extra water), so A is correct.
However, most plants show a decrease in photosynthesis at 30 degrees celsius, because of lack of CO2 but also because of celular lesions and protein denaturation happening at that temperature. 50 degrees celsius is where photosynthesis stops for most plants. so D is also correct.

I think your teacher wants A as an answer, but things are not that simple...
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Post by baikuza » Thu Nov 17, 2005 3:57 am

B !!

yeah...!
ho..ho..
quite enough of explanation

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Post by canalon » Thu Nov 17, 2005 4:14 am

baikuza wrote:B !!

yeah...!
ho..ho..
quite enough of explanation

I could do with some explanations to undersatnd how you cme to this surprising answer. Because if I remember correctly my botany (oh so long ago...) when the air is dry, the stomata close (I remember fuzzily a neat system with cells that would close it when they go dry and reopen it when the water level was up...) to save the water, ence no CO2 come into the cell and the photosynthesis stops...
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any proof. (Ashley Montague)

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Post by MrMistery » Thu Nov 17, 2005 7:11 pm

That is somewhat correct. While the basic mechanism of the function of stomata is activated by light, so we would expect them to be wide open in midday, most plants have developed internal biorithms of opening/closing their stomata to save water. However, if it is too hot, the plant can not keep it's stomas closed for ever, it needs to open them in order to get CO2
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Post by baikuza » Sat Nov 19, 2005 9:22 pm

sorry. i just want to answer it. just that

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Post by MrMistery » Sun Nov 20, 2005 3:37 pm

LOL... I wasn;t critising you. i was telling what i knew. This is a forum and debating is what should happen here...
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Post by baikuza » Mon Nov 21, 2005 6:53 am

oh.

photoshyntesise in the midle of day is slower than before is caused by the homeostasis activity.
because of high temperature, the plants may lost a large amount of water. in fact, because of the mechanism of water transport high because of it, and at a level, the plants can not hold this hydration. so that, the plants close its stomata to decrease the hydration(the hydration is higher when the stomata is opened-water can easily goes out to the air). when plants closes its stomata automaticly the CO2 which is available in the chloroplast is limited but the photoshyntesise still run. when there is no CO2 to do the job(photoshyntesise), it can run/ work.
not all of the stomata close.
practically, the photoshyntesise velocity is lower than before. (at this time, but after this level passes it should be normal again-depend it on the light after that level)

in the high dry environment.. it is the same one. but the stomata open at night ant at noon the photoshyntesise runs. the others activity not to much differ than in tropical environment.

hei, how about this? actually it is the same with the others, right?

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Post by victor » Mon Nov 21, 2005 12:24 pm

I thil what you mean is "de"hydration. :)
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Post by baikuza » Tue Nov 22, 2005 12:32 pm

kind of word which has meaning "we lost our water in our body".

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