I recently saw a lovely video that has nothing to do with biology. It is a time-lapse video of an artist jolting a plywood board with 15,000 volts of electricity. The accompanying text on an EarthSky blog page concisely connects the video’s evocative imagery with branching structures in biology.
15,000 Volts from Melanie Hoff on Vimeo.
That got me thinking about whether introductory biology students would be able to make that connection. For example, you could show students the plywood video, and as they watched, you could ask them to jot down ideas about what the branching patterns on the burned plywood resemble. I read the EarthSky post before I watched the video, so blood vessels and leaf veins came immediately to mind. But the more I thought about it and discussed it with others, the more examples of branched structures I could list.
- Plants: Leaf veins, tree trunk/branches, root branches
- Animals: Blood vessels, respiratory passages, neurons, kidneys, moth antennae, scorpion pectines
- Fungi: Hyphae
- Protists: Plasmodial slime molds
- Quasi-biological examples: Phylogenetic trees, food webs
- Nonbiological examples: Satellite images of watersheds, roadways
You could then ask your students to consider just the biological examples. Clearly the structures have similarities, but what do their functions have in common? I hope that students would recognize that some of these examples represent distribution networks, whereas others gather information or resources. Either way, the network must have a high surface area to do its job efficiently.
Countless tiny root hairs, for example, absorb nutrients and water from soil and channel these resources into a large-scale distribution pathway (the xylem). Many xylem vessels from the roots converge at the plant’s stem. Then, as xylem enters the plant’s branches and leaves, it divides into smaller and smaller veins that eventually deliver water and minerals to small groups of cells.
Now, compare that plant example with a vertebrate example such as blood vessels. Tiny capillaries at the lungs absorb oxygen. These capillaries converge into ever-larger blood vessels that eventually empty into the heart. That organ, in turn, pumps the oxygenated blood through the aorta, then through additional arteries and arterioles of decreasing diameter, until it reaches the tiniest capillaries. There, the blood delivers oxygen to small groups of cells.
If students work out multiple examples of this sort, they should start to see that high-surface-area structures are common and that many of them serve similar functions. This insight can lead them to a lasting appreciation of the unity of life.
One final note: Melanie Hoff, the artist who zapped the plywood, also has posted a beautiful, wordless demonstration of the chemical process used to extract bismuth from Pepto-Bismol. Bonus points if you can find a way to apply that video to your biology classroom!
