How fireflies glow

Section 2 of 4

On summer evenings fireflies flash yellow-green over fields and gardens. Each flash is a reaction in the firefly's tail, run by an enzyme.

Fireflies are beetles, and more than 2,000 kinds are known. A firefly makes its light in a lantern on the underside of the last few segments of its body. Light made by a living thing is called bioluminescence.

The light comes from luciferin, a small molecule made in the lantern's cells, and luciferase, the enzyme that makes it react. The names come from the Latin lucifer, light-bringer. The reaction also needs oxygen from the air and ATP, the molecule every living cell uses to carry energy in small amounts.

Firefly luciferin, C₁₁H₈N₂O₃S₂The molecule fireflies react with oxygen to make light. A benzene ring carrying an -OH group shares an edge with a five-membered ring of carbon, nitrogen and sulfur. That ring joins a second five-membered ring of carbon, nitrogen and sulfur, which carries a -COOH group.SSOHOOHNN
Firefly luciferin, C₁₁H₈N₂O₃S₂. A benzene ring with an -⁠OH group shares an edge with a five-membered ring holding a nitrogen and a sulfur atom, and a second ring of the same atoms carries the -⁠COOH group. Alternating double bonds run from the benzene ring into the second ring.

Two steps inside the enzyme

Luciferase holds luciferin and works on it in two steps. First it joins luciferin's -⁠COOH group to part of an ATP molecule, which readies the group to react. Then oxygen joins on, and the molecule closes a small ring of two carbons and two oxygens, held by a weak O–⁠O bond, much like the ring in a glow stick.

The ring splits off a carbon dioxide molecule, and the rest of the molecule, now called oxyluciferin, is left excited. Its electron falls back and gives out yellow-green light, around 560 nm. Luciferin has been oxidised: it has lost a carbon as carbon dioxide and gained a C=O.

What goes into a firefly's light and what comes out Luciferin, ATP and oxygen go into a box labelled luciferase, the enzyme. Out of it come oxyluciferin, carbon dioxide, and the two pieces of the ATP, AMP and pyrophosphate. A wavy arrow from oxyluciferin shows yellow-green light of about 560 nanometres. luciferin ATP oxygen, O₂ luciferase the enzyme oxyluciferin CO₂ AMP and pyrophosphate yellow-green light, about 560 nm
Luciferin, ATP and oxygen go into luciferase. Oxyluciferin, carbon dioxide and the two pieces of the ATP, AMP and pyrophosphate, come out, along with a flash of light.

In a 2008 measurement, about 40 of every 100 luciferin molecules that reacted gave out light. The rest lost their energy as a little heat, too little to warm the lantern. Many kinds of firefly flash in patterns of their own, and males and females answer each other's flashes to find a partner. A firefly switches its light on and off by controlling how much oxygen reaches the luciferase.

Quick check

What does luciferase do in a firefly's lantern?