A stochastic Liouville theory is presented for the superexchange electron-transfer reactions involving three paraboloidal potential surfaces in a two-dimensional reaction coordinate. Its close relationship with the spin-boson model for photosynthesis is discussed. A triangle representation is used to explain the relationship between the reorganization energy and the degree of correlation for the solvent fluctuations experienced by the donor, the intermediate, : and the acceptor. Explanations for a small reorganization energy for an efficient superexchange mechanism in natural photosynthesis are offered.