  Charge separation in light-harvesting complexes occurs in a pair of tightly
coupled chlorophylls at the heart of photosynthetic reaction centers of both
plants and bacteria. Recently it has been shown that quantum coherence can, in
principle, enhance the efficiency of a solar cell, working like a quantum heat
engine (QHE). Here, we propose a biological quantum heat engine (BQHE)
motivated by Photosystem {\rm II} reaction center (PS{\rm II} RC) to describe
the charge separation. Our model mainly considers two charge-separation
pathways more than that in the published literature. The two pathways can
interfere via cross-couplings and work together to enhance the
charge-separation yields. We explore how these cross-couplings increase the
current and voltage of the charge separation and discuss the advantages of
multiple pathways in terms of current and power. The robustness of the BQHE
against the charge recombination in natural PS{\rm II} RC and dephasing induced
by environments is also explored, and extension from two pathways to multiple
pathways is made. These results suggest that nature-mimicking architectures
with engineered multiple pathways for charge separations might be better for
artificial solar energy devices.
