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Question 5 on Midterm 1

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Post  katleu Thu Jan 31, 2008 9:03 pm

Can someone please explain why E) is the correct answer to this problem?

The questions is as follows:

5. To collect maple sugar, one inserts a tube into the trunk until the end reaches the phloem. If instead a tube were inserted with the end reaching the xylem, the result would be:

A) Water and minerals flowing steadily out only if it had rained recently.
B) Water and minerals flowing steadily out only if the sun were shining.
C) Water and minerals flowing steadily out unless the air was extremely humid.
D) Water and minerals flowing out only until xylem pressure was relieved.
E) No water flow or minerals.

I wrongly presumed that the answer was C: due to transpiration.

katleu

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Join date : 2008-01-24

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Post  laurenbettencourt Fri Feb 01, 2008 3:55 am

I think that water doesn't flow out because movement of water/nutrients in the xylem doesn't depend on pressure, but rather on the cohesive abilities of water (one molecule pulls the next up as it transpires through the stomata). So if there is a hole in the xylem, I don't think the water molecules will be able to move at all. If you think of it like a drinking straw, you see that no water is able to move when there is a puncture in the capillary. But...yeah, I'm not a TA so please correct me if I'm wrong.

laurenbettencourt

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Post  TA Brian Fri Feb 01, 2008 4:34 am

Yeah, I think Lauren is right. It seems very counterintuitive that water wouldn't flow out of the tube, but water wouldn't flow b/c the xylem flow isn't dependent on pressure flow but instead more of hydrogen-bonding. And yes, that straw analogy is really good; cohesion with the tube is probably also a contributor to the fact that water doesn't flow.

TA Brian

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Post  katleu Sat Feb 02, 2008 10:07 pm

oh, that makes a lot of sense. thank you!

katleu

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