The Cell's Energy Currency · ATPLab

Where Does a Firefly's Light Come From?

The lantern at the tip of a firefly's abdomen contains luciferin and luciferase. Grind the lanterns and add water: in the dark the extract gives off a faint yellow-green glow, which fades after about 15 minutes.

Once the glow has gone, add distilled water, glucose solution and ATP solution to different tubes. Which one lights up again? And what happens if a tube is boiled first and then given ATP?

On the right you can change the ATP concentration, the water-bath temperature and the pH, and watch the brightness and colour of the glow.

The Structure of ATP

ATP is short for adenosine triphosphate. Its structure can be written as A–P~P~P: A stands for adenosine (adenine + ribose), P for a phosphate group, and ~ for a special chemical bond.

Neighbouring phosphate groups all carry negative charges and repel each other, so this bond is unstable and the terminal phosphate group has a high transfer potential. When ATP is hydrolysed, the terminal phosphate group breaks off and energy is released.

Hydrolyse one more phosphate group and you get AMP — adenine ribonucleotide, one of the building blocks of RNA. Drag the scene to rotate the molecule.

Interconversion of ATP and ADP

Cells contain only a little ATP, but ATP and ADP are constantly converted into each other and kept in a dynamic balance.

Energy-absorbing reactions are usually linked to ATP hydrolysis. In the middle of the scene is a carrier protein for the active transport of Ca²⁺; it is also an enzyme that catalyses ATP hydrolysis. The terminal phosphate group of ATP is transferred to the carrier (phosphorylation), the carrier changes shape and releases Ca²⁺ outside the membrane.

Energy-releasing reactions are usually linked to ATP synthesis. The left side stands for cellular respiration: animals, humans, fungi and most bacteria get the energy for making ATP from respiration; green plants also get it from photosynthesis.

ATP synthase

Most of the ATP from aerobic respiration is made on the inner mitochondrial membrane. Respiration pumps H⁺ to the outer side of the inner membrane, building up an H⁺ gradient; H⁺ flows back down the gradient through ATP synthase into the matrix and turns its rotor.

Every 120° the rotor turns, one β subunit above releases one ATP; one full turn passes 8 H⁺ and makes 3 ATP. The thylakoid membranes of chloroplasts also have ATP synthase.

Make the gradient smaller, or add an uncoupler that lets H⁺ leak straight back, and see what the rotor does.

In-class testing

Look back over the four topics and answer the multiple-choice questions below.