September 9, 2005 | David F. Coppedge

# Reader Project: Calculate the Speed of Plant Package Delivery

Get out your pencil and hand calculator.  A team of Swedish and French scientists measured the velocity of a message traveling on the intraplant internet (see 08/12/2005, 11/09/2004, 10/04/2004 and 07/13/2001 entries).  Publishing in Science,1 they believe they have witnessed a signaling molecule, in the form of a messenger-RNA (mRNA; see yesterday’s entry) moving through the phloem, from leaf to shoot, telling the tip to begin flowering.  The leaf is sensitive to day length.  When the clock strikes that the days are right for flowering, messenger molecules travel to the tips to initiate the process.  This is one method by which a plant, distributed in space without a central nervous system, can keep synchronized with itself.
The mRNA moved 6 to 7 mm in an Arabidopsis leaf to the tip in 2 to 5 hours, which they calculated as 1.2 to 3.5 millimeters per hour.
Problem:  Calculate what speed range that would represent in miles per hour if the mRNA were the size of a delivery truck on a freeway.  You need to find out the approximate size of an mRNA molecule and work it out with proportions.  Send your answer here.

1Tao Huang et al., “The mRNA of the Arabidopsis Gene FT Moves from Leaf to Shoot Apex and Induces Flowering,” Science, Vol 309, Issue 5741, 1694-1696, 9 September 2005, [DOI: 10.1126/science.1117768].

Your answer can be reprinted here to help other readers visualize what a human might see if shrunk to the size of an mRNA molecule.  The answer will be simplified somewhat since it will not take into account how many twists and turns the molecule must make on its route, nor the amount of traffic and inspections it encounters, but it should be interesting.  One reader estimated 20-60 mph, one 23-66 mph or up to 68 to 200 mph, another 2000 to 6000 mph, and another, 8181 mph.  Give us your best calculation: write here and tell us how you arrived at it.  Hint: an RNA molecule is about 1 nanometer wide (10-9m).  You need to figure not only the length of an average mRNA, but whether or not it folds into a more compact structure before traveling.
The reader who seemed to work the hardest on the problem said, “if we use 5 nm as the width of the mRNA folded up on itself, the scaled speed increases dramatically, and would be in the range of 1100 to 3300 mph.”  Additional factors would need to be known, like how compact the mRNA folds, and whether it passively “rides along” in the sap for part of its journey.  Anyway, it was an interesting investigation – thanks to all who wrote in with their calculations.

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Categories: Botany, Genetics

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