  Photosynthesis offers a convenient means of sustaining biospheres. We
quantify the constraints for photosynthesis to be functional on the permanent
nightside of tidally locked rocky exoplanets via reflected light from their
exomoons. We show that the exomoons must be at least half the size of Earth's
moon in order for conventional oxygenic photosynthesis to operate. This
scenario of photosynthesis is unlikely for exoplanets around late-type M-dwarfs
due to the low likelihood of large exomoons and their orbital instability over
long timescales. Subsequently, we investigate the prospects for photosynthesis
on habitable exomoons via reflected light from the giant planets that they
orbit. Our analysis indicates that such photosynthetic biospheres are
potentially sustainable on these moons except those around late-type M-dwarfs.
We conclude our analysis by delineating certain physiological and biochemical
features of photosynthesis and other carbon fixation pathways, and the
likelihood of their evolution on habitable planets and moons.
