4.4: ATP molecules are like free-floating rechargeable batteries in all living cells.

Much of the work that cells do requires energy. But even though light from the sun carries energy, as do molecules of sugar, fat, and protein, none of this light energy can be used directly to fuel chemical reactions in organisms’ cells. First it must be captured in the bonds of a molecule called adenosine triphosphate (ATP), a free-floating molecule found in cells that acts like a rechargeable battery, temporarily storing energy that can then be used for cellular work in plants, animals, bacteria, and all the other organisms on earth. The use of ATP solves an important timing and coordination problem for living cells: a supply of ATP guarantees that the energy required for energy-consuming reactions will be available when it’s needed.

ATP is a simple molecule with three components (FIGURE 4-7). At the center of the ATP molecule are two of these components: a small sugar molecule attached to a molecule called adenine. But it is the third component that makes ATP so effective in carrying and storing energy for a short time: attached to the sugar and adenine is a chain of three negatively charged phosphate groups (hence the “tri” in “triphosphate”). Because the bonds between these three phosphate groups must hold the groups together in the face of the three negative electrical charges that all repel one another, each of these bonds contains a large amount of energy and is stressed and unstable. The instability of these high-energy bonds makes the three phosphate groups like a tightly coiled spring or a twig that is bent almost to the point of breaking. With the slightest push, one of the phosphate groups will pop off, displaced by water. And in the process, a little burst of energy is released that the cell can use.

It is precisely because each molecule of ATP is always on the brink of ejecting one of its phosphate groups that ATP is such an effective energy source inside a cell. As long as plenty of ATP molecules are around, they can energize the chemical reactions that make it possible for the cell to carry out the processes that require work, such as building muscle, repairing wounds, or growing roots. Each time a cell expends one of its ATP molecules to pay for an energetically expensive reaction, a phosphate is broken off and energy is released. What is left is a molecule with two phosphates, called ADP (adenosine diphosphate), and a separate phosphate group (labeled P_i).

An organism can then use ADP, a free-floating phosphate, and an input of kinetic energy to rebuild its ATP stocks (FIGURE 4-8). The kinetic energy is converted to potential energy when a free phosphate group attaches to the ADP molecule and makes ATP. In this manner, ATP functions like a rechargeable battery. Where does the input of energy for recharging ATP come from? When we discuss photosynthesis, we’ll see that plants, algae, and some bacteria can directly use light energy from the sun to make ATP from ADP and free-floating phosphate groups. These photosynthetic organisms can also use the energy generated from breaking down sugar and other molecules. That’s the energy source that animals use, generating ATP from the energy contained in the bonds of their food molecules.

Whether it comes from the sun or from the breakdown of molecules such as sugar, the energy is used to re-create the unstable bond in the triphosphate chain of ATP. When energy is needed, the organism can again release it by breaking the bond holding the phosphate group to the rest of the molecule. Our bodies recycle ATP molecules in this way tens of thousands of times a day.

Here’s the ATP story in a nutshell. Breaking down a molecule of sugar—in a glass of orange juice, for example—leads to a miniature burst of energy in one of your cells. The energy from the mini-explosion is put to work building the unstable high-energy bonds that attach phosphate groups to ADP molecules, creating new molecules of ATP. Later—perhaps only a fraction of a second later—when an energy-consuming reaction is needed, your cells can release the energy stored in the new ATP molecules.