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The Discovery of Photosynthesis
Hands On Activity: Repeat Baptista van Helmont, Joseph Priestley and Jan Ingenhousz Experiments

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  • Who Discovered Photosynthesis?

    Photosynthesis is a very important and complex process in nature and some of its phases are still not completely understood.

    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.

    Famous Historic Experiments

    Is Water the Source of Energy in Plants?

    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:

    The Interaction of Plants With Air

    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:

    Plants and Light

    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


    Photosynthesis Basics
    An Introduction to Photosynthesis and Its Applications - Wim Vermaas
    Illuminating Photosynthesis - NOVA
    Photosynthetic organisms - Julian Andres Mina Caicedo & Francisco J. Romero-Campero
    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
    What Is Photosynthesis? - Aparna Vidyasagar
    Physiology: Plant Growth and Development - University of Arizona
    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 -
    History of Plants -
    Photosynthesis Links - Biochemistry Department, The University of Sydney
    Photosynthesis - Peter V. Sengbusch
    Chlorophyll - Paul May
    Discoveries in Oxygenic Photosynthesis - Kluwer Academic Publishers

    Photosynthesis Books


    Provides a clear, concise and vivid account of the process of photosynthesis. Discusses the details of photosynthetic processes at the macro and molecular level

    Molecular Mechanisms of Photosynthesis

    Aimed at undergraduate and beginning graduate students, the text begins with a review of the principles of energy storage. Other topics include evolution, electron transfer pathways, kinetics, and genetic manipulations. The text is accompanied throughout by detailed diagrams.

    Light and Photosynthesis in Aquatic Ecosystems

    The purpose of the first part of this book is to describe and explain the behavior of light in natural waters

    Photosynthetic Adaptation: Chloroplast To Landscape : With 94 Illustrations (Ecological Studies)

    The authors question whether photosynthetic adaptations take place primarily at the metabolic and biochemical level or through changes in structure and form, or both. In the interest of genetic engineering and agricultural applications, the authors analyze the relative importance of genes that control both metabolic and light reactions as well as the structure, arrangement, and orientation of photosynthesis.

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