Following the Path of Discovery
Repeat Famous Experiments and Inventions
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Photosynthesis in plants and a few bacteria is responsible for feeding nearly all life on Earth. It does this by taking energy from the sun and converting it into a storable form, usually glucose, which plants use for their own life processes. Animals that consume plants also make use of this energy, as do those that consume those that consume plants, and so on to the top of the food chain.
As important a job as making all of the world's food is, there's another vital function that photosynthesis performs: It generates the oxygen that oxygen-breathing animals need to survive. But here we animals repay the favor. We exhale the carbon dioxide that plants need for photosynthesis.
From PBS's "NOVA" program
Many scientists contributed to the discovery and understanding of photosynthesis throughout the ages; in this page are outlined some of those crucial milestone experiments that contributed to this effort.
Jan Baptista van Helmont, Flemish physician, chemist, and physicist, in the 1600s carried out a famous experiment by growing a willow tree in a pot for five years. At the end of this period the tree had increased in mass by 74kg but the mass of the soil had changed little. Van Helmont believed that water was the source of the extra mass and the plant's source of life.
Repeat Helmont's experiment:
John Woodward, a professor and physician at Cambridge University in the late 1600s, tried to design an experiment to test Van Helmont hypothesis that water was the source of the extra mass. In a series of experiments over as many as 77 days, Woodward measured the water consumed by plants.
For example, one plant showed a mass gain of about 1 gram, while Woodward had added a total of almost 76,000 grams of water during the 77 days of plant growth - this was a typical result. Woodward correctly suggested that most of this water was “drawn off and conveyed through the pores of the leaves and exhaled into the atmosphere. So the hypothesis that water is the nutrient used by plants was rejected.
Repeat John Woodward's experiment:
In August of 1771, Joseph Priestley, an English Chemist, put a sprig of mint into a transparent closed space with a candle that burned out the air (oxygen was not discovered yet) until it soon went out. After 27 days, he relit the extinguished candle again and it burned perfectly well in the air that previously would not support it. And how did Priestley light the candle if it was placed in a closed space? He focused sun light beams with a mirror onto the candle wick (Priestley had no bright source of light, and had to rely on the sun). Today, of course, we can use more sophisticated methods to light the candle like focusing light from a flood light through converging lens, or by an electrical spark.
So priestly proved that plants somehow change the composition of the air.
In another celebrated Experiment from 1772, Priestley kept a mouse in a jar of air until it collapsed. He found that a mouse kept with a plant would survive. However, we do not recommend to repeat this experiment and hurt innocent animals.
These kinds of observations led Priestley to offer an interesting hypothesis that plants restore to the air whatever breathing animals and burning candles remove.
Repeat Joseph Priestley's Experiments:
Jan Ingenhousz took Priestley’s work further and demonstrated that it was light that plants needed to make oxygen (oxygen was discovered a few years earlier, in 1772 by Carl Wilhelm Scheele).
However, Jan Ingenhousz was the first person to show that light is essential to the plant process that somehow purifies air fouled by candles or animals.
In 1779 Ingenhousz put a plant and a candle into a transparent closed space. He allowed the system to stand in sunlight for two or three days. This assured that the air inside was pure enough to support a candle flame. But he did not lit the candle. Then, he covered the closed space with a black cloth and let it remain covered for several days. When he tried to light the candle it would not light.
Ingenhousz concluded that somehow the plant must have acted in darkness like an animal. It must have breathed, fouling the air. And in order to purify the air plants need light.
Repeat this Experiment:
In another experiment, Ingenhousz, placed a small green aquatic plant in a transparent container of water and exposed the container to bright sunlight. He observed gas bubbles forming around the leaves and the green parts of the stems. When the system was placed in darkness, the bubbles stopped. These bubbles might be what the plant produced that purifies air fouled by animals or candles. In this experiment Ingenhousz demonstrated that plants are dependent on light and their green parts for nutrients and energy.
For more photosynthesis experiments look here:
Photosynthesis Science Fair Projects and Experiments
Illuminating Photosynthesis - NOVA
Photosynthesis: Capturing Energy - Elizabeth Anne Viau
Photosynthesis: Key Questions and Answers - The Learning Center
Photosynthesis, energy, and life - FT Exploring
Photosynthesis Interactive Tutorials - Florida State University
Photosynthesis: Light Reactions - Merritt College, Oakland
An Introduction to Photosynthesis and Its Applications - Wim Vermaas
Physiology: Plant Growth and Development - University of Arizona
What is Photosynthesis? - M.J. Farabee
ASU Photosynthesis Center
Green Plants - Channel 4 Learning
Photosynthesis - Encarta
Photosynthesis - Overview Part I - ODU Biology
Photosynthesis - Wikipedia
Photosynthesis History and Discovery Links
Highlights in Photosynthesis Research - Nobelprize.org
History of Plants - saskschools.ca
Photosynthesis Links - Biochemistry Department, The University of Sydney
Photosynthesis - Peter V. Sengbusch
Chlorophyll - Paul May
Discoveries in Oxygenic Photosynthesis - Kluwer Academic Publishers