Biology » Cell Metabolism » ATP: Adenosine Triphosphate

By the end of this lesson and the next, you should be able to:

• Explain the role of ATP as the cellular energy currency
• Describe how energy is released through hydrolysis or breakdown of ATP

Even exergonic, energy-releasing reactions require a small amount of activation energy in order to proceed. However, consider endergonic reactions, which require much more energy input, because their products have more free energy than their reactants. Within the cell, where does energy to power such reactions come from? The answer lies with an energy-supplying molecule called adenosine triphosphate, or ATP.

### What is ATP?

ATP is a small, relatively simple molecule (see image below), but within some of its bonds, it contains the potential for a quick burst of energy that can be harnessed to perform cellular work. This molecule can be thought of as the primary energy currency of cells in much the same way that money is the currency that people exchange for things they need. ATP is used to power the majority of energy-requiring cellular reactions.

ATP is the primary energy currency of the cell. It has an adenosine backbone with three phosphate groups attached. Image Attribution: OpenStax Biology

### Structure of ATP

As its name suggests, adenosine triphosphate is comprised of adenosine bound to three phosphate groups (see image above). Basically, adenosine is a nucleoside consisting of the nitrogenous base adenine and a five-carbon sugar, ribose. The three phosphate groups, in order of closest to furthest from the ribose sugar, are labeled alpha, beta, and gamma. Together, these chemical groups constitute an energy powerhouse. However, not all bonds within this molecule exist in a particularly high-energy state.

Both bonds that link the phosphates are equally high-energy bonds (phosphoanhydride bonds). In fact, when they are broken, they release sufficient energy to power a variety of cellular reactions and processes. These high-energy bonds are the bonds between the second and third (or beta and gamma) phosphate groups and between the first and second phosphate groups.

### Hydrolysis of ATP

The reason that these bonds are considered “high-energy” is because the products of such bond breaking—adenosine diphosphate (ADP) and one inorganic phosphate group (Pi)—have considerably lower free energy than the reactants: ATP and a water molecule. Because this reaction takes place with the use of a water molecule, scientists refer to it as a hydrolysis reaction. In other words, ATP is hydrolyzed into ADP in the following reaction:

$$\mathrm{ATP} + \mathrm{H_2O} →$$$$\mathrm{ADP} + \mathrm{P_i} + \mathrm{free \; energy}$$

Like most chemical reactions, the hydrolysis of ATP to ADP is reversible. The reverse reaction regenerates ATP from ADP + Pi. Indeed, cells rely on the regeneration of ATP just as people rely on the regeneration of spent money through some sort of income. Since ATP hydrolysis releases energy, ATP regeneration must require an input of free energy. The formation of ATP is expressed in this equation:

$$\mathrm{ADP} + \mathrm{P_i} + \mathrm{free \; energy}$$ $$→ \mathrm{ATP} + \mathrm{H_2O}$$