Neshaminy High School

Objectives

• Describe the structure of ATP and how it stores energy.
• Give examples of work that cells perform.
• Summarize the ATP cycle.

Key Term

• ATP

It's a good thing that food doesn't fuel your cells by burning like the torched peanut described in Concept 7.2. In fact, the carbohydrates, fats, and proteins obtained from food do not drive work in your cells in any direct way. The chemical energy stored in these compounds must first be converted to energy stored in another molecule.

How ATP Packs Energy
As you read in Chapter 6, ATP stands for adenosine triphosphate. The "adenosine" part consists of a nitrogen-containing compound called adenine and a five-carbon sugar called ribose (Figure 7-9). The triphosphate "tail" consists of three phosphate groups. The tail is the "business" end of ATP—it is the source of energy used for most cellular work.

Figure 7-9 An ATP molecule contains potential energy, much like a compressed spring. When a phosphate group is pulled away during a chemical reaction, energy is released.

Each phosphate group is negatively charged. Because like charges repel, the crowding of negative charge in the ATP tail contributes to the potential energy stored in ATP. You can compare this to storing energy by compressing a spring. The tightly coiled spring has potential energy. When the compressed spring relaxes, its potential energy is released. The spring's kinetic energy can be used to perform work such as pushing a block attached to one end of the spring.

The phosphate bonds are symbolized by springs in Figure 7-9. When ATP is involved in a chemical reaction that breaks one or both of these phosphate bonds, potential energy is released. In most cases of cellular work, only one phosphate group is lost from ATP. Then the tail of the molecule has only two phosphate groups left. The resulting molecule is called adenosine diphosphate, or ADP.

ATP and Cellular Work
During a chemical reaction that breaks one of ATP's bonds, the phosphate group is transferred from ATP to another molecule. Specific enzymes enable this transfer to occur. The molecule that accepts the phosphate undergoes a change, driving the work.

Your cells perform three main types of work: chemical work, mechanical work, and transport work (Figure 7-10). An example of chemical work is building large molecules such as proteins. ATP provides the energy for the dehydration synthesis reaction that links amino acids together. An example of mechanical work is the contraction of a muscle. In your muscle cells, ATP transfers phosphate groups to certain proteins. These proteins change shape, starting a chain of events which cause muscle cells to contract. An example of transport work is pumping solutes such as ions across a cellular membrane. Again, the transfer of a phosphate group from ATP causes the receiving membrane protein to change shape, enabling ions to pass through.

Figure 7-10 The energy in ATP drives three main types of cellular work.

The ATP Cycle
ATP is continuously converted to ADP as your cells do work. Fortunately, ATP is "recyclable." For example, ATP can be restored from ADP by adding a third phosphate group (Figure 7-11). Like compressing a spring, adding the phosphate group requires energy. The source of this energy is the organic molecules from food. Thus, ATP operates in a cycle within your cells. Work consumes ATP, which is then regenerated from ADP and phosphate.

Figure 7-11 ATP is constantly recycled in your cells.

The ATP cycle churns at an astonishing pace. A working muscle cell recycles all of its ATP molecules about once each minute. That's 10 million ATP molecules spent and regenerated per second! The next concept focuses on how your cells keep pace with this incredible demand for ATP.

Concept Check 7.3
1. In what way is ATP like a compressed spring?
2. List three main types of cellular work.
3. What is the source of energy for regenerating ATP from ADP?

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