Theodor Wilhelm Engelmann

Theodor Wilhelm Engelmann (born 1843 in Leipzig – died 1909) was a German botanist, physiologist, microbiologist, university professor, and musician whose 1882 experiment measured the effects of different colors of light on photosynthetic activity and showed that the conversion of light energy to chemical energy took place in the chloroplast.

Academic history
Englemann studied natural science and medicine first at the University of Jena, from 1861 to 1862, and later at the University of Heidelberg, the University of Göttingen and the University of Leipzig. In 1867, he received a doctoral degree in medicine at Leipzig.

He later taught physiology at the University of Utrecht, becoming a professor in 1888. In 1897, he began teaching physiology at the University of Berlin, where he also became the editor of the Archiv für Anatomie und Physiologie. He retired in 1908, but continued to serve as editor until his death.

Scientific investigations
Engleman's major contribution to the field of physiology emerged from a study lasting from 1873 to 1897, in which he observed the contractions of striated muscles. Focusing on the visible bands of fibers in the muscles, he noted that the volume of the anistropic band increased during contraction, whereas the volume of the isotropic band decreased. He theorized that it was this interaction between the two bands which allowed for muscle contraction.

He also demonstrated, after experiments with dissected frogs in 1875, that contractions of the heart were caused by the heart muscle itself, not an external nerve stimulus, as was previously believed.

In 1881, he demonstrated that photosynthesis takes place inside the chloroplasts of plants and that the chloroplasts receive more energy from red and blue light than from other colours. He illuminated a filamenous alga Spirogyra with light passed through a prism, exposing different segments to different wavelengths. Aerobic bacteria, which grow in the presence of oxygen, were used to determine where the alga was releasing the most oxygen and thus photosynthesizing most. They accumulated under red and blue light, showing that only specific parts of the electromagnetic spectrum are used. A year later Engelmann discovered that purple bacteria utilise ultraviolet light in the same way.