Complementarity in biology : a reassessment in relation to molecular-reductionist and systemic approaches

At the beginning of the twentieth century, a range of seemingly unexplainable observations challenged the Newtonian view of the physical world and plunged physics into a major crisis. This crisis and the resulting intellectual advances led to the birth of quantum mechanics in the 1920s, a theoretical field that provided a new framework for the explanation of phenomena that were defying classical laws of mechanics. One of the main insights that emerged from quantum physics was the principle of complementarity, which Niels Bohr (1885-1962) formulated to explain the fact that, in quantum physics, two theories regarded as mutually exclusive are required to explain a single phenomenon (Bohr, 1958). In particular, Bohr was dealing with wave-particle duality: light exhibits different properties depending on the means of observation. Some phenomena, such as diffraction patterns, are best explained if light is considered as electromagnetic waves, whereas others, such as the photoelectric effect, require that light behaves like particles. Bohr's principle of complementarity established that, for investigations at the subatomic level, light could be considered as either, depending on the methods of measurement; that these two descriptions should not be regarded as contradictory, but as complementary; and that it is not possible to formulate a synthesis of the two. Interestingly, Bohr also believed that complementarity could be applied to contexts other than quantum mechanics and the realm of subatomic particles. In a lecture titled Light and Life that he gave to a congress on light therapy in Copenhagen in 1932 (Bohr, 1933), he proposed that a notion of complementarity might be needed for understanding biological phenomena. Bohr suggested that experiments to analyse the molecular properties of organisms and biological functions were basically incompatible--the complex organization of living systems cannot, in fact, be preserved under the experimental conditions of a complete quantum mechanical analysis (McKaughan, 2005). Instead, both types of experiment are needed.