Photosynthesis is the biological process by which the energy of the Sun is collected, converted and stored in chemical bonds needed to power life. Therefore, photosynthesis serves as the vital link between the light energy of the Sun and almost all living organisms on Earth. In this chapter we will focus on the first steps of photosynthesis: energy collection and conversion, i.e. light-harvesting and charge separation, highlighting the role of quantum effects on the ultrafast dynamics and quantum efficiency of these two remarkable processes. Both experimental and theoretical approaches will be described and combined.

Photosynthesis

In photosynthesis solar energy is absorbed by the light-harvesting antenna and transferred to the photosynthetic reaction centre (RC) within several tens of picoseconds. In the RC, the absorbed excitation energy is converted into electrochemical energy by means of an ultra fast charge separation. Photosynthetic purple bacteria employ a single reaction centre, in contrast, in photosynthesis of plants, algae and cyanobacteria, two reaction centres, Photosystem II (PSII) and Photosystem I (PSI), operate in series. PSII uses light to extract electrons from water (to produce oxygen), while PSI uses light to reduce NADP+ to NADPH. The subsequent electron transfer from PSII to PSI is coupled to the build-up of a proton motive force (pmf) that is used to form ATP. NADPH and ATP are required in the Calvin–Benson cycle to produce a reduced sugar. In the following we will discuss photosynthetic charge separation and photosynthetic light-harvesting with an emphasis on the role of quantum effects.