The study of biological phenomena in terms of quantum mechanics. Hypotheses about the role of quantum behaviour of subatomic particles in living processes date from the 1930s, but only in recent years has evidence emerged to substantiate them. For example, the dual wave-particle nature of excited electrons, or excitons, is now held to explain the high efficiency with which light energy is ‘trapped’ during photosynthesis in green plants, algae, and photosynthetic bacteria. The excitons are generated by photons of light being absorbed by chlorophyll molecules in the light-harvesting complex, or antenna pigments, of the photosynthetic apparatus of the cell (see photosystems I and II). They rapidly transfer between multiple pigment molecules to reach the reaction centre, where the energy is absorbed to drive the chemical reactions of photosynthesis. The wavelike nature of an exciton enables it to simultaneously ‘explore’ various alternative routes to the reaction centre via the pigment molecules, and hence find the quickest pathway. Another phenomenon, called quantum tunnelling, accounts for the dramatic way in which enzymes can boost the rate of chemical reactions. Lightweight atoms, such as hydrogen, can transfer across seemingly insurmountable energy barriers by virtue of their wavelike nature, and effectively re-emerge instantaneously in another location. In this way the enzyme molecule is able to manipulate carbon–hydrogen bonds in ways that are energetically highly unfavourable. Quantum tunnelling is also proposed as a mechanism underlying the ability of olfactory receptors to detect very subtle differences in odorant molecules. Research suggests that quantum mechanical effects may occur in diverse other areas of biology, including bird navigation.