Science · Biology
Chapter 1: The Nature of Life
Every definition of life fails somewhere.
In 1935 Wendell Stanley purified tobacco mosaic virus into needle-shaped crystals that could sit in a jar like salt. Rubbed onto a tobacco leaf, the crystals still caused a disease that spread and multiplied.
Is the virus alive, and does the question have a clean answer?
Make your guess to turn the page.
Still to come: 9 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students expect a list of characteristics to sort everything cleanly into living and non-living. The book scores three cases against seven criteria and finds that fire and a virus each pass some and fail others, and the lab shows that none of the criteria can be seen by looking, because each is a process. Ask: which criterion would you give up first, and what would that let in? The idea to reach is a cluster weighted toward metabolism and cells, with disputed edges.
Every definition of life fails somewhere — Lists of characteristics exclude mules and dormant spores and admit fire in appearance. Stanley crystallized tobacco mosaic virus in 1935 and shared the 1946 Nobel Prize in Chemistry; the first electron micrographs of the virus followed in 1939. Brownian motion, first described by Robert Brown in 1827, is why movement under a microscope is not evidence of life.
Life is a property of organization — No single molecule in a cell is alive. The property emerges from the arrangement and interaction of parts, which is why taking a cell apart and reassembling it does not work.