Photosynthesis: Why does it occur?

Respiration and photosynthesis are exactly opposite only at the reversible boundary between the two. This metastable equilibrium state is maintained as long as the incident light promoting photosynthesis exerts a force equal and opposite to that exerted by respiration. When the force induced by light is less, respiration can occur and does so irreversibly. (The free-energy decrease is not all used as work; some is wasted as heat.) When the force is greater, photosynthesis occurs.