Ingenhousz (1730–1799) was a Dutch physician and man of the world; he was at one‐time court physician to the Austrian empress Maria Theresa and had also visited England on several occasions (Magiels, 2010). On one of these trips in 1773, he heard a lecture by John Pringle, the President of the Royal Society, in which he presented a medal to Priestley. In his lecture, Pringle lauded Priestley's accomplishments. Ingenhousz thus became interested in the subject, but took no action until several years later when, in 1779, he rented a house in the English countryside and in the short space of three months conducted more than 500 experiments on the properties of plants and their effects on the air. During this frenzied summer, Ingenhousz discovered the essential role of light in the process of photosynthesis. By October of the same year, he had already written up his results in the form of a book entitled Experiments upon Vegetables, Discovering Their Great Power of Purifying the Common Air in Sunshine and Injuring It in the Shade and at Night. In addition to discovering the effect of sunlight, Ingenhousz discovered plant respiration, although he greatly overemphasized the effects of it, as can be seen from the title of his book. Ingenhousz was a brilliant, but not very careful, scientist and often jumped to conclusions with little evidence. Nevertheless, his contributions to the understanding of photosynthesis were extremely important, including a book published in 1796, Food of Plants and Renovation of the Soil. In this book, he summarized and presented for the first time the process of photosynthesis in terms of the new chemistry proposed by Lavoisier.
3.4 Senebier and the role of carbon dioxide
Jean Senebier (1742–1809) was a Swiss minister, contemporary, and bitter rival of Ingenhousz. His most important contribution to the developing understanding of photosynthesis was his finding in 1783 of the essential role that carbon dioxide, or “fixed air,” has in the process. Senebier and Ingenhousz engaged in a long‐standing and acrimonious priority dispute. This was not helped by Senebier's poor writing style, such that he would take entire volumes to describe his results in tedious detail, with no summaries provided. For his part, Ingenhousz tried to claim in his 1796 book that he had actually discovered the role of carbon dioxide in his 1779 experiments, although he had clearly denied that it was important in writings in 1784.
3.5 De Saussure and the participation of water
The final contribution to the overall equation of photosynthesis was made by Nicolas de Saussure (1767–1845), a Swiss scientist. He confirmed the observations of Ingenhousz and Senebier and added careful measurements of the relative amounts of carbon dioxide taken up and oxygen given off by plants. He proved by careful weighing experiments, published in 1804, that the increase in dry weight of the plant is considerably greater than the weight of carbon in the carbon dioxide taken up. He correctly surmised that the balance of the weight came from water, although he mistakenly thought that the assimilation of water was a process separate from the incorporation of carbon dioxide.
3.6 The equation of photosynthesis
The period between 1771 and 1804 was exciting in the history of photosynthesis. In this short time, the basic chemical equation of photosynthesis was established, which in 1804 could be written as:
(3.1)
This was not yet a balanced equation, as the nature of the organic matter had not been established, and even the proper chemical formula for water had not yet been established! The concept of atoms combining to form molecules was just being formulated by John Dalton (1766–1844) near the end of this period, but unambiguous chemical formulas were not established for another 50 years.
It should be clear that the elucidation of the equation of photosynthesis was an amazing feat, which required contributions from many brilliant and hardworking scientists of differing backgrounds and points of view. It should also be clear from this example that science is very much an enterprise carried out by human beings, who are often blinded by preconceived ideas, ego, and jealousy.
3.6.1 The balanced equation for photosynthesis
It was another 60 years before the chemical equation of photosynthesis could be written in modern chemical symbols and properly balanced. A key aspect of this development was the determination of the redox state of the organic matter produced during photosynthesis. This involved careful measurements of the photosynthetic quotient, the ratio between the carbon dioxide assimilated and the oxygen produced. The first accurate measurements of the photosynthetic quotient were made in 1864 by a Frenchman, Jean Baptiste Boussingault (1802–1887). This determination consisted of measuring the volumes of CO2 taken up and O2 produced during photosynthesis. Boussingault found that the photosynthetic quotient was close to 1 for a number of plants. This established that the fixed carbon is at the redox level of carbohydrate (where the ratio of H to O is 2:1). Bolstering this view, Julius von Sachs, a German plant physiologist, found in the same year that the carbohydrate, starch, accumulates in leaves only when they are illuminated, and only in those parts of the leaf that are directly illuminated. This effect can be illustrated dramatically by actually printing photographs on leaves! The process is accomplished by taping a photographic negative over a leaf, illuminating it to form starch, extracting the pigments, and then developing the image by treating it with iodine, which forms a dark‐colored complex with starch. Remarkably high‐quality images can be obtained in this manner (Walker, 1992).
The information that the photosynthetic quotient is 1, and that the organic matter is a carbohydrate such as starch or sugar, allows us to write a minimally balanced equation for photosynthesis:
(3.2)
where (CH2O) is representative of a carbohydrate. One example of a carbohydrate is glucose, C6H12O6, which makes the overall balanced photosynthetic equation:
(3.3)
As we will see in Chapter 9, glucose is not the carbohydrate directly formed in photosynthesis, but it has almost the same energy content, so this is adequate for our present needs.
Julius Robert Mayer (1814–1878), a German physician and physicist who first enunciated the law of conservation of energy, proposed in 1845 that in photosynthesis light energy is converted to chemical energy, thus completing the formulation of the equation of photosynthesis.
3.7 Early mechanistic ideas of photosynthesis
When the overall reaction of photosynthesis had been established, attention turned to elucidating the details of the mechanism of the process. The early ideas in this regard were erroneous and much too simplistic. Richard Willstätter (1872–1942) and Arthur Stoll (1887–1971) proposed in 1918 that the product was actually formed directly as a molecular species, formaldehyde (CH2O), in a direct, concerted process involving chlorophyll, CO2, and H2O. This view was revived later by Otto Warburg (1883–1970) in support of his unorthodox formulation of photosynthesis, which we will discuss a little later. We now know that these mechanistic ideas, which were certainly reasonable at the time, are not valid, because there are literally dozens of intermediate states that have been identified, and the reduction in CO2 can be separated from the production of oxygen, and vice versa. The key to understanding in more detail how the mechanism of photosynthesis really works came from the analysis of simple photosynthetic organisms by van Niel and by Hill's experiments showing that CO2 reduction and O2 evolution can be decoupled.
3.7.1 Van Niel and the redox nature of photosynthesis
The cornerstone of our current understanding of photosynthesis is that it is a light‐induced