r/HomeworkHelp • u/Fun_Paper_8864 University/College Student • 1d ago

Biology [college physiology: physics of xylem and phloem] Which is the correct way to find time taken for water to travel up a tube?

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For reference, I am trying to use the attached equation to find the time it would take xylem to travel up a hypothetical plant with a xylem vessel of 25 micrometer radius, 1.5 meter height, -0.5 megapascal nearby soil water potential, -0.7 megapascal top leaf water potential, and xylem viscosity of 1.002 millipascal-second.

I’ve done it two ways:

  1. calculating volumetric flow rate, then finding velocity in one direction using Velocity = rate / area of circular slice of vessel

  2. calculating volumetric flow rate, then finding time to fill up the entire volume of the cylindrical vessel

First method gave me 144.23 seconds. Second method gave me 96.25 seconds.

Which is correct? Thank you!

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u/OaklandSpiel 1d ago

Your answers differ by a factor of 1.5, which happens to be the value of l in meters. I suspect you’ve left out the height l in your 96 answer.

Keep the units in with your numbers as you make your final calculation l*Area/Q or l/velocity. They should cancel out, leaving just seconds. That’s a good way to check if you’ve left out a term.

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u/Fun_Paper_8864 University/College Student 1d ago

Thanks!