Biology (Holt)

ATP

Recall that ATP (adenosine triphosphate) is a nucleotide with two

extra energy-storing phosphate groups. As shown in Figure 3, the

three phosphate groups in ATP form a chain that branches from a

five-carbon sugar called ribose (RIE bohs).This phosphate “tail” is

unstable because the phosphate groups are negatively charged and

therefore repel each other. The phosphate groups store energy like a

compressed spring does. This energy is released when the bonds

that hold the phosphate groups together are broken.

Breaking the outer phosphate bond requires an input of energy.

Much more energy is released, however, than is consumed by the

reaction. As shown in Figure 3, the removal of a phosphate group

from ATP produces adenosine diphosphate, or ADP. This reaction

releases energy in a way that enables cells to use the energy. The fol-

lowing equation summarizes this reaction:

ATP → ADP  P  energy

Cells use the energy released by this reaction to power metabo-

lism. In some chemical reactions, two phosphate groups are

removed from ATP instead of just one. This tends to make the reac-

tion irreversible because the pair of phosphate groups that is

removed is not available for the reverse reaction. Rather, the pair is

quickly split into two single phosphate groups.

96 CHAPTER 5Photosynthesis and Cellular Respiration

P P P P P

P

Base (adenine)
Phosphate groups

Sugar

(ribose)

AT P ADP

Energy

(Adenosine triphosphate) (Adenosine diphosphate)

Identifythe primary source of energy that flows

through most living systems. 9D

Comparethe metabolism of autotrophs with

that of heterotrophs. 9B

Describehow energy is released from ATP.

Critical ThinkingInferring Relationships

How can the energy in the food that a fox eats be

traced back to the sun? 9D

Critical ThinkingAnalyzing Patterns

Explain how life involves a continuous flow of

energy. 9D

A grasshopper obtains energy

by eating grass. A snake eats the grasshopper, and a

hawk then eats the snake. What is the original

source of energy for the hawk? 12E

Athe snake Cthe grass

Bthe grasshopper Dthe sun

TAKS Test PrepTAKS Test Prep

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Section 1 Review

When the outer phosphate group detaches from ATP, energy is released.

Figure 3 ATP releases energy

4B