contribution to the history of photosynthesis: theodor
wilhelm engelmann and hugo von mohl

Théodor Wilhelm Engelmann (1843 -1909) was a German botanist and physiologist. He demonstrated that the site of photosynthesis is the chloroplast, the cytoplasmic organelle in plant cells which is responsible for the green coloration of leaves and other organs.

He also shed important enlightenment on the functions of light in photosynthesis, particularly light quality. He showed that not all colors or wavelengths of light are effective in photosynthesis. He therefore made significant addition to Ingenhousz’ finding that the degree of shade and brightness of the day (light intensity) affects the ability of plants to purify the air (photosynthesis). It is now a well accepted fact that the process of photosynthesis is affected by the properties of light which include light quality, light intensity, and light duration.

But long before Engelmann’s work on Spirogyra which was published in1881, the presence of chloroplasts in plant cells was already discovered. In 1837, Hugo von Mohl (1805–1872), also a German botanist and the discoverer of the protoplasm in plants, first described the ‘chlorophyllkörnern’ or chlorophyll granules in detail. It was proposed in 1883 to substitute the term ‘plastid’ for chlorophyll granule, but eventually ‘chloroplast’ was adapted (Staehelin 2003).

Von Mohl also recognized the presence of starch in the chloroplasts and developed the concept that this starch is a reserve food (Spoehr 1926).

In 1881, Theodor Wilhelm Engelmann designed an experiment for the simultaneous study of the light requirements and biochemistry of photosynthesis. He used the green alga Spirogyra, which has long chloroplast in each cell, as test organism. He placed the alga first in a drop of water which contained aerophilic (oxygen-requiring) bacterium to serve as indicator of oxygen evolution. He then illuminated the alga with different colors of light using a prism.

Using a microscope, he discovered that the bacteria gathered around the parts of the chloroplast that were illuminated by red and blue light. He concluded that the red and blue colors of light were most effective in producing oxygen (Moore et al. 2003; Govindjee and Krogmann 2004). This means that the wavelengths of light corresponding to the red and the blue colors are most effective in photosynthesis.

It has long been established that some forms of algae constitute the nano-plankton. These are organisms which form the basis of the food chain from fish to man. It was estimated that the contribution of planktonic organisms to the overall photosynthetic processes on earth exceeds 90 percent (Boedijn 1968). As to generation of oxygen, Moore et al. (2003) suggested that unicellular marine algae probably contributes to 50-70% of the oxygen in the earth’s atmosphere.

Note: The lists of contributors and Literature Cited are in the History of Photosynthesis Mainpage.

(Ben G. Bareja. July 2012)

Julius Robert von Mayer <<<   Engelmann and von Mohl   >>> Julius von Sachs

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