6. Cell Energy - Photosynthesis in a Jar

Summary
These experiments use a bell jar (or any other very large, clear, glass jar) to determine the identity of the gas produced by plants. It mirrors the famous experiments of Joseph Priestley and Jan Ingenhousz from the 1700’s that first demonstrated the existence of oxygen and its importance to plants and animals.

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.

Materials

  • Small plant in a pot, well watered
  • Candle or row of matches (I found the matches easier to light with the converging lens although it produces significantly more smoke.)
  • Flood lamp with a very high wattage bulb or bright sunlight
  • Magnifying glass
  • Bell jar or other large glass jar (Don’t use a plastic container like I did initially. Not only will it collapse with the vacuum produced following the burning of the candle but the smoke from the candle flame often deposits itself on the inside of the plastic, obscuring everything inside from view.)
  • Vacuum plate or large tray full of water (If you use a tray of water, beware that as the candle heats the air, the expanding gas will escape out from under the rim of the jar. When the air cools again, the level of the water inside the jar will rise so be sure to prop your plant up on a pedestal of some sort to prevent the whole thing from getting swamped. Also use a lot of water in the tray initially or air from outside will be pulled into the jar as the air cools.)
  • Heavy black cloth or dark closet

Procedure
A candle uses up the oxygen in the jar:

  1. Place a burning candle inside the bell jar and seal it. The candle will eventually go out.
  2. Focus a beam of light on the candle wick with the converging lens to show that the candle cannot be relit.

A plant restores the oxygen in the jar:

  1. Place a burning candle and a plant inside the bell jar and seal it. The candle will eventually go out.
  2. Try relighting the candle to show that the candle cannot be relit.
  3. Place the setup in a sunny window for 2 days.
  4. Try relighting the candle. The candle should relight.

A plant in darkness does not restore oxygen:

  1. Place a burning candle and a plant inside the bell jar and seal it. The candle will eventually go out.
  2. Try relighting the candle to show that the candle cannot be relit.
  3. Place the setup in the dark for 2 days.
  4. Try relighting the candle. The candle should not relight.

A plant in darkness uses oxygen:

  1. Place a burning candle and a plant inside the bell jar and seal it. The candle will eventually go out.
  2. Try relighting the candle to show that the candle cannot be relit.
  3. Place the setup in a sunny window for 2 days.
  4. Remove the setup from the sunny window and place it in darkness for 2 more days.
  5. Try relighting the candle. The candle should not relight.

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