The tone of science fictionfilms is often a guide to the tone of the times the film was made: other worlds and species are viewed in turn as wonderful, terrifying, threatening, incomprehensible, and childishly benign.
... by the pigment molecules.
The typical overall chemical reaction of photosynthesis is:
6H2O + 6CO2 + light → C6H12O6 (glucose) + 6O2
In simple English ...