Chemical Energy and ATP

Why is ATP useful to cells?

Energy is the ability to do work. Nearly every activity in modern society depends upon energy. When a car runs out of fuel—more precisely, out of the chemical energy in gasoline—it comes to a sputtering halt. Without electrical energy, lights, appliances, and computers stop working. Living things depend on energy, too. Sometimes the need for energy is easy to see. It takes plenty of energy to play soccer or other sports. However, there are times when that need is less obvious. Even when you are sleeping, your cells are quietly busy using energy to build new molecules, contract muscles, and carry out active transport. Simply put, without the ability to obtain and use energy, life would cease to exist.

Energy comes in many forms, including light, heat, and electricity. Energy can be stored in chemical compounds, too. For example, when you light a candle, the wax melts, soaks into the wick, and is burned. As the candle burns, chemical bonds between carbon and hydrogen atoms in the wax are broken. New bonds then form between these atoms and oxygen, producing CO₂ and H₂O (carbon dioxide and water). These new bonds are at a lower energy state than the original chemical bonds in the wax. The energy lost is released as heat and light in the glow of the candle's flame.

Living things use chemical fuels as well. One of the most important compounds that cells use to store and release energy is adenosine triphosphate (uh DEN uh seen try FAHS fayt), abbreviated ATP. As shown in Figure 8–1, ATP consists of adenine, a 5-carbon sugar called ribose, and three phosphate groups. As you'll see, those phosphate groups are the key to ATP's ability to store and release energy.

FIGURE 8–1 ATP ATP is the basic energy source used by all types of cells.

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