Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 46

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.