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6. Cell Energy - Photosynthesis in a Jar
In 1771 and 1772, Priestly conducted a series of experiments using a bell jar. It was known that a candle placed in a sealed bell jar would eventually burn out and could not be relighted while still in the jar. Priestly discovered that a plant can survive indefinitely within a jar. Thus, he tried placing a plant into the jar with the burning candle. The candle went out as before and could not be relit right away. Priestly waited several days and tried again. The candle could be relit! The plant had restored the air inside the jar! (Do not try the next series of experiments since it harms animals!) Next priestly investigated what would happen to animals. He found that a mouse placed inside a sealed jar will eventually collapse. However, a mouse can survive in a sealed jar with a plant since the plant restores the air. Priestly was the first to demonstrate that oxygen is necessary for fire and animals but that given time, plants can create oxygen, allowing fires to burn and animals to breathe.
A few years later, Jan Ingenhousz investigated the effect of light on a plant’s ability to restore air. He found that a plant left in darkness cannot restore the air for a candle. To demonstrate this, he burned up all the oxygen in a jar with a plant then left the plant in sunlight for a few days to restore the air. Then without relighting the candle, he put the plant into darkness for several more days. At the end of the dark period, he was unable to relight the candle. He concluded that a plant in darkness acts like animals, using up the oxygen that it had created. Ingenhousz had discovered that plants can photosynthesize and create oxygen but only in the light. If left in the dark, plants do not photosynthesize and thus no oxygen is produced. Moreover plants are always performing respiration, just like animals. In sunlight, the rate of photosynthesis outstrips respiration so there is an excess of oxygen being produced. But in darkness, no photosynthesis takes place but respiration continues to occur. Thus, by keeping a plant in darkness you can demonstrate that plants need oxygen and use it up, just like animals.
For more information, see the wonderful website of Julian Rubin which describes many of these experiments and offers links and resources on how to recreate them in the classroom. Also, NSTA has produced a great set of photosynthesis related inquiry activities including the bell jar experiments.
A plant restores the oxygen in the jar:
A plant in darkness does not restore oxygen:
A plant in darkness uses oxygen: