The question “What is life?” was proposed as “An activity to convey the complexities of this simple question” by Prud’homme-Généreux (2013). As supplementary material to the activity in that ABT article, I suggested, in a letter to ABT, some examples of physical/chemical systems that appear to have some of the features of living organisms (Stansfield, 2013).

Since then, a group of scientists (Bianchini et al., 2013) report that particles of tea leaves in water from one container can move upstream against a flow of pure water from a second container. Self-propulsion in biological entities, such as certain flagellated bacteria or ciliated animal cells, requires metabolic activity, indicated by the expenditure of energy. Tea leaves were once alive, but any dried leaf cells in tea brew have no mechanism for self propulsion, especially when moving upstream like spawning salmon. So by what mechanism can the movement of tea leaf particles be explained? It is hypothesized that the “particles overcome gravity and the current thanks to a property of water called surface tension. The linkup of hydrogen atoms among water molecules tends to create an elastic, trampoline-like surface. But small particles like tea leaves disturb that network, causing the hydrogen bonds to pull apart and thrust the particles toward purer water where the surface tension is higher” (Grant, 2013).

Cessation of metabolic activity and response to environmental stimuli are usually included in the definition of organismic or cellular death. Goodspeed et al. (2013) report that a head of cabbage cropped from its stalk continues to produce anti-caterpillar glucosinolates as an adaptive response to alternating 12-hour periods of light and dark (circadian cycles). “Caterpillars on a cycle offset by 12 hours to the cabbages’ (so the cabbages’ dawn was the caterpillars’ dusk) ate about 20 times more than did caterpillars on a schedule synchronized to their food” (Gelling, 2013). The clock-related pest resistance lasts about a week after harvesting.

References

References
Bianchini, S., Lage, A., Siu, T., Shinbrot, T. & Altshuler, E. (2013). Upstream contamination by floating particles. Proceedings of the Royal Society of London, Series A, 469, 20130067.
Gelling, C. (2013). Veggie bioclocks tick after picking: daily cycles help crop plants ward off hungry pests. Science News, 184(2), 14.
Goodspeed, D., Liu, J.D., Chehab, E.W., Sheng, Z., Francisco, M., Kliebenstein, D.J. & Braam, J. (2013). Postharvest circadian entrainment enhances crop pest resistance and phytochemical cycling. Current Biology, 23, 1235–1241.
Grant, A. (2013). Tension behind tea leaf mystery: property of water’s surface allows unexpected motion. Science News, 184(2), 11.
Prud’homme-Généreux, A. (2013). What is life? An activity to convey the complexities of this simple question. American Biology Teacher, 75, 53–57.
Stansfield, W.D. (2013). Letter to the Editor: “What is life?” American Biology Teacher, 75, 309.