Fireflies are a group of insects that possess a lantern organ composed of numerous light-producing cells (photocytes) in their abdomens. The light is produced by a chemical reaction involving the light-emitting organic compound luciferin. In each photocyte, luciferin is activated
in an ATP-dependent reaction that is catalyzed by the enzyme luciferase. In the presence of oxygen, the activated luciferin emits light as it is
converted to oxyluciferin (Figure 1).
The flashes of light are controlled by neurons that innervate the lantern organ. Activated neurons stimulate the release of nitric oxide (NO), an
inhibitor of the electron transport chain. The pattern of light flashes (Figure 2) emitted by males is a signal to attract females as a component
of mating behavior. Different species of firefly display different patterns of flashes (Figure 3). The evolutionary relatedness of these species,
as determined using multiple morphological characteristics, is shown in Figure 4.
Identify the main source of the ATP used for activating luciferin, as shown in Figure 1, and describe the effect of the release of NO on
oxygen levels in the photocyte during flashing.