Objectives Key Term How ATP Packs Energy - … to energy stored in another molecule. How ATP Packs Energy As you read in Chapter 6, ATP stands for adenosine triphosphate. The "adenosine"

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 thetorched peanut described in Concept 7.2. In fact, the carbohydrates, fats,and proteins obtained from food do not drive work in your cells in anydirect way. The chemical energy stored in these compounds must first beconverted to energy stored in another molecule.

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

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

Each phosphate group is negatively charged. Because like charges repel,

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the crowding of negative charge in the ATP tail contributes to thepotential energy stored in ATP. You can compare this to storing energyby compressing a spring. The tightly coiled spring has potential energy.When the compressed spring relaxes, its potential energy is released. Thespring's kinetic energy can be used to perform work such as pushing ablock attached to one end of the spring.

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

ATP and Cellular WorkDuring a chemical reaction that breaks one of ATP's bonds, the phosphategroup is transferred from ATP to another molecule. Specific enzymesenable this transfer to occur. The molecule that accepts the phosphateundergoes a change, driving the work.

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


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

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

Figure 7-11ATP is constantly recycled in yourcells.


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

Concept Check 7.31. 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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Objectives Key Term How ATP Packs Energy - … to energy stored in another molecule. How ATP Packs Energy As you read in Chapter 6, ATP stands for adenosine triphosphate. The "adenosine"