8–2 Photosynthesis: An Overview

Investigating Photosynthesis
Research into photosynthesis began centuries ago with a simple question: When a tiny seedling grows into a tall tree with a mass of several tons, where does the tree's increase in mass come from? From the soil? From the water? From the air?

Van Helmont's Experiment In the 1600s, the Belgian physician Jan van Helmont devised an experiment to find out if plants grew by taking material out of the soil. Van Helmont determined the mass of a pot of dry soil and a small seedling. Then, he planted the seedling in the pot of soil. He watered it regularly. At the end of five years, the seedling, which by then was a small tree, had gained about 75 kg. The mass of the soil, however, was almost unchanged. Van Helmont concluded that most of the mass the plant gained had come from water, because that was the only thing that he had added to the pot.

Van Helmont's experiment accounts for the "hydrate," or water, portion of the carbohydrate produced by photosynthesis. But where does the carbon of the "carbo-" portion come from? Although van Helmont did not realize it, carbon dioxide in the air made a major contribution to the mass of his tree. The carbon in carbon dioxide is used to make sugars and other carbohydrates in photosynthesis. Van Helmont had only part of the story, but he had made a major contribution to science.

Priestley's Experiment More than 100 years after van Helmont's experiment, the English minister Joseph Priestley performed an experiment that would give another insight into the process of photosynthesis. Priestley took a candle, placed a glass jar over it, and watched as the flame gradually died out. Something in the air, Priestley reasoned, was necessary to keep a candle flame burning. When that substance was used up, the candle went out. That substance was oxygen.

Priestley then found that if he placed a live sprig of mint under the jar and allowed a few days to pass, the candle could be relighted and would remain lighted for a while. The mint plant had produced the substance required for burning. In other words, it released oxygen.

Word Origins
Jan Ingenhousz Later, the Dutch scientist Jan Ingenhousz showed that the effect observed by Priestley occurred only when the plant was exposed to light. The results of Priestley's and Ingenhousz's experiments showed that light is necessary for plants to produce oxygen. key The experiments performed by van Helmont, Priestley, Ingenhousz, and other scientists reveal that in the presence of light, plants transform carbon dioxide and water into carbohydrates and release oxygen. Biology and History: Show Detail