Key Ideas

• A plant’s bulk does not come from soil. Van Helmont’s willow gained 164 pounds while the soil lost a quarter of a pound.
• A plant is built largely from carbon taken out of carbon dioxide in the air, through pores in its leaves called stomata.
• Photosynthesis and respiration are the same reaction run in opposite directions. Plants use light to build carbon links; animals break them for energy.
• Carbon atoms cycle endlessly between air, plants, and animals. That is why the air keeps its oxygen instead of running out.
• The entire cycle is driven by sunlight. Without light, nothing rebuilds the links and the whole system winds down.

• Photosynthesis: Plants using light energy to build carbon dioxide from the air into the molecules of their own bodies, releasing oxygen as a leftover.
• Respiration: Animals using oxygen to break the links between carbon atoms in food, releasing stored energy and giving off carbon dioxide.
• Stomata: The tiny pores covering a leaf’s surface. They let carbon dioxide in and let oxygen and water vapor out.
• Carbon cycle: The endless loop of carbon atoms: out of the air into plants, into animals, and back to the air.

1. A wooden log weighs fifty pounds. Where were those fifty pounds a hundred years ago? Trace it back as far as you can.
2. You are designing a sealed spacecraft for a two-year trip. What would you have to bring, and why might plants be part of the answer?
3. Van Helmont ran the right experiment and got the wrong answer, because he did not know carbon dioxide existed. Was his experiment still worth doing? Why?

• A timer, some floor space (for a minute of jumping jacks)