Early Research

Figure 9-1 Joseph Priestley (1733-1804). (Illustration by Donna Mariano)

As a general rule, knowledge is gained in a gradual, stepwise, cumulative fashion. Such itsh e case with what we know about photosynthesis. We started to learn about it several hundred years ago, and we are still learning.

The account could begin with J.B. van Helmont (1577-1644), who gave this report: "I took a vessel and placed in it 200 pounds of earth which had been carefully dried and then weighed. Then I placed in it a stock of willow weighing 5 pounds. I faithfully watered it always with rain water and after five years had gone by I took the tree from the soil and found that it weighed 159 pounds and 3 ounces. The earth in which the tree had grown I carefully protected from the accumulation of dust by placing over the vessel an iron plate perforated with holes. I again dried and weighed the earth in which the willow had grown and found that it weighed the same, 200 pounds minus about 2 ounces, and hence I concluded that 164 pounds of wood had been derived from the element of water alone."

This creditable experiment uncovered part of the truth about photosynthesis. Although van Helmont knew nothing of either the atmosphere or any role that the atmosphere plays in the process, he did learn something very significant: that water contributes to the growth in weight of a plant. This work indicates a careful investigator and a good thinker.

Henry Cavendish (1731-18101, an English chemist and physicist, was a pioneer in the study of gases and is credited with determining that water is a compound. He also determined that the air produced by fermentation in vats and by putrefaction has the same properties as the “fixed air” obtained from marble. His 1784 paper summarizing his findings was entitled Experiments on Air.

Joseph Black (1728-1799) discovered the existence of an atmospheric gas that differed from common air: carbon dioxide, or fixed air. He performed detailed studies on the fixed air produced in fermentation vats, in respiration, and by burning charcoal.

Joseph Priestley (1733-1804) was a dissenting English clergyman and a chemist. He discovered that plants make dephlogisticated air, or oxygen. Following is the essence of his report:

I took a mass of air made thoroughly noxious by mice breathing and dying and putrefylng in it and divided it into two parts. To one I added a sprig of mint and the other I let stand in the same position as that portion containing the sprig of mint. This was about the beginning of August, 1771, and after eight or nine days I discovered that a mouse lived perfectly well in that part of the air in which the sprig of mint had grown, but died the moment it was put in the other part of the same quantity of air which had no plant growing in it.

His report was titled On the Purification of Air by Plants.

Antoine Laurent Lavoisier (1743-1794) is credited with demolishing the phlogiston theory. He gave the name oxygen to dephlogisticated air. Although Joseph Priestley had prepared and discovered it, Lavoisier named it. He is also remembered for claiming that water is formed of hydrogen and oxygen.

Jan Ingenhousz (1730-1799), an English physician working in Holland, described the interchange of gases between plants and the atmosphere. He found that dephlogistication (the release of oxygen) takes place only in sunlight and that “plants behave like animals” (producing carbon dioxide) at night. In 1772 Joseph Priestley observed that plants could purify air; later, Ingenhousz learned that only the green parts of plants are able to restore air and that sunlight alone has no such power. Upon his return to London in 1779, Ingenhousz published a book entitled Experiments on Vegetables, Discovering Their Great Power in Purifying the Common Air in Sunshine, but Injuring it in the Shade or at Night. (Many years later, in 1932, Robert Hill was able to show that intact chloroplasts freed from all other cell constituents could trap light energy and liberate oxygen. Still later, Daniel Arnon established that isolated chloroplasts could use light energy to reduce carbon.) In 1782 a theologian named Jean Senebier discovered that fured air (carbon dioxide) was required for the process described by Ingenhousz. Then in 1804, Nicholas Theodore de Saussure (1767-1845) used Lavoisier’s methods of quantitative measurement to show that equal volumes of carbon dioxide and oxygen are exchanged during photosynthesis and that the plant retains carbon. He showed that plants gain more weight during photosynthesis than can be accounted for by the carbon alone, the rest of the matter, with the exception of minerals, coming from water (figure 9-2).

When it was determined that the green substance in plants is chlorophyll and that the first product of photosynthesis is glucose, a balanced chemical equation was produced (figure 9-3). This equation indicates that carbohydrate is formed from a combination of carbon and water molecules and that the oxygen liberated comes from splitting carbon dioxide. This seemed acceptable until van Niel reported on his studies of purple sulphur bacteria.

Figure 9-2 During photosynthesis, the volume of carbon dioxide consumed and the volume of oxygen liberated are equal.

Figure 9-3 A balanced chemical equation showing the end products of photosynthesis.

While at Stanford University, C.B. van Niel studied a group of bacterial organisms capable of photosynthesis. Such organisms have a chlorophyll-like pigment (unlike the pigment in flowering plants) that can catalyze photosynthesis without releasing oxygen. One such group of bacteria utilize hydrogen sulfide rather than water, and liberate free sulphur rather than oxygen (figure 9-4).

Figure 9-4 The photosynthetic reaction of purple sulphur bacteria. Hydrogen sulfide is utilized in the place of water, resulting in the liberation of elemental sulphur.

These bacteria seemed to manufacture water in the process. Van Niel, thus, proposed the generalized formula shown in figure 9-5. In this formula, A represents sulphur, some other substance used by photosynthetic bacteria, or water. Thus, van Neil proposed that it is the lysis of water rather than the breakdown of carbon dioxide that liberates oxygen during photosynthesis.

Figure 9-5 A generalized equation of photosynthesis that does not designate a specific compound in the reaction and indicates that water is manufactured in the process.