Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
The captured light energy generates an "uphill" flow of electrons.

How does the light energy captured by METABOLISM/14.html">Chloroplasts induce an "uphill" flow of electrons?

When light excites a chlorophyll molecule embedded in the thylakoid membrane, one of its electrons is promoted to a higher energy level determined by the energy of the absorbed photon, thereby placing the molecule in an excited state. The excitation energy (exciton) then rapidly migrates through a network of light-harvesting pigment molecules to the reaction center of the photosystem, imparting a large amount of energy to one of the photosystem's electrons. This "hot" electron leaves the reaction center and is transferred to the primary electron acceptor in the Electron Transport Chain. Consequently, the first carrier in the chain becomes reduced as it accepts the electron, while the reaction center becomes oxidized as it gives up its electron. A reaction center in this oxidized state is said to possess an electron hole. This energy-rich electron, possessing a very high reducing "pressure," now travels down The electron transport chain to NADP⁺, reducing it to NADPH (Fig. 23-12). Clearly, the standard reduction potential of this photochemical reaction center must have a large negative value in order for electrons to flow "downhill" from the reaction center to NADP⁺, given that the standard potential of the conjugate redox pair NADP⁺/NADPH is itself quite negative, at −0.32 V (Section 17.5).

Two questions now arise. How is the electron hole created in the reaction center refilled? And how can we account for the evolution of O₂ from Water? To answer these questions, we must examine the overall scheme of photosynthetic electron flow.