#68: Solved: The Mystery of Gravity-Defying Sap

One synthetic tree accomplishes what loads of scientists never could.

By Karen Wright|Wednesday, December 10, 2008
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tree
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The world’s first synthetic tree consists of nothing more than a super-skinny tube (the “trunk”) connecting two networks of microcapillaries (one forms the “roots” and the other the “leaves”) embedded in a porous polymer. But this fake tree has managed a feat that researchers have struggled for more than a century to duplicate in the lab: transpiration, the process of pumping water from the soil to leaves and then into the air. “This was an unsolved problem, and we took it on as a biophysics challenge,” says chemical engineer Abraham Stroock of Cornell University, who published the research in September in Nature [subscription required].

Water is pulled upward in plants when evaporation from the leaves creates a tension in the water column that reaches down to the roots. The tension is maintained by bonds between water molecules that are drawn up through the narrow channels of a plant’s plumbing. Stroock created the same tension in his transpiring system by using a high-tech fabric of cross-linked polymers called a hydrogel. With pores little larger than the water molecules themselves, the hydrogel stabilizes intermolecular bonds, allowing water to flow through the synthetic trunk without breaking the tensile chain. Stroock showed that the hydrogel roots of his tree can pump water from even a partially dry medium, and the hydrogel leaves release water vapor the same way real ones do.

The synthetic tree proves that transpiration can occur without any energy input from the plants themselves. “Plants may use a hydrogel-like approach,” Stroock says.

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