The biological physics of photosynthesis can be operationally described by target theory. The photochemical reaction center receives excitation energy primarily through a Forster dipole resonance/trapping model. The excitation coupling between the light harvesting complexes and the photochemical redox reaction defines an effective absorption cross section, which, to a first order, follows a cumulative one-hit Poisson distriubution. This cross section interacts with the biochemical machinery of the cell to optimize light harvesting at low photon fluences, but at high photon fluence rates, electron transport is limited by "dark" reactions. Numerous processes have evolved to allow dynamic changes in the cross section on time scales ranging from minutes to days. These processes include competative trapping through non-radiative energy dissipating systems (e.g. to non-coupled transition states of carotenoids), reversible coupling of light harvesting pigments to the antenna system(s), and active destruction/synthesis of new antenna complexes. All these processes are mediated by a redox sensor within the electrical circuit of the cells; one of the redox components (a pool of quinone molecules) acts as a "light meter". The feedback processes provide extraodinary plasticity of the photosynthetic appparatus to accomodate to wide variations in solar radiation.