A model for the protein control of energy transfer in photosynthetic light-harvesting antenna is proposed. Thermal fluctuations of the energy gap between the excited donor and acceptor molecules for the pairwise energy transfer step are assumed when the latter is a bottleneck for the overall transfer from the antenna to the reaction center. The model is motivated by fluorescence decay experiments on photosystem I particles. It emphasizes the dynamical aspect of spectral heterogeneity rather than the conventionally employed static picture. A comparison and discussion between experimental data and theoretical predictions are given.