Y

Adenosine

AMP (Adenosine monophosphate)

TY"

ADP (Adenosine diphosphate)

ATP (Adenosine triphosphate)

Atp Adp Amp Adenosine

6.5 ATP (a) ATP is richer in energy than its relatives ADP and AMP The hydrolysis of ATP releases this energy. (b) Fireflies use ATP to initiate the oxidation of luciferin.This converts chemical energy into light energy, emitting rhythmic flashes that signal the insect's readiness to mate. Very little of the energy in this conversion is lost as heat.

6.5 ATP (a) ATP is richer in energy than its relatives ADP and AMP The hydrolysis of ATP releases this energy. (b) Fireflies use ATP to initiate the oxidation of luciferin.This converts chemical energy into light energy, emitting rhythmic flashes that signal the insect's readiness to mate. Very little of the energy in this conversion is lost as heat.

phate groups is much higher than the energy of the H—O bond that forms after hydrolysis. So some usable energy is released upon hydrolysis. Second, because phosphates are negatively charged and so repel each other, it takes energy to get phosphates near enough to each other to make the cova-lent bond that links them together (e.g., to add a phosphate to ADP to make ATP).

ATP couples exergonic and endergonic reactions

As we have just seen, the hydrolysis of ATP is exergonic and yields ADP, Pj, and free energy. The reverse reaction, the formation of ATP from ADP and Pj, is endergonic and consumes as much free energy as is released by the breakdown of ATP:

Many different exergonic reactions in the cell can provide the energy to convert ADP to ATP. In eukaryotes, the most important of these reactions is cellular respiration, in which some of the energy released from fuel molecules is captured in ATP. The formation and hydrolysis of ATP constitute what might be called an "energy-coupling cycle," in which ADP picks up energy from exergonic reactions to become ATP, which donates energy to endergonic reactions.

How does this ATP cycle trap and release energy? An ex-ergonic reaction is coupled to the endergonic reaction that forms ATP from ADP and Pj (Figure 6.6). Coupling of exer-gonic and endergonic reactions is very common in metabolism. When it forms, ATP captures free energy and retains it like a compressed spring. ATP then diffuses to another site in the cell, where its hydrolysis releases free energy to drive an endergonic reaction.

A specific example of this energy-coupling cycle is shown in Figure 6.7. The formation of the amino acid glutamine has a positive AG (is endergonic) and will not proceed without the input of free energy from ATP hydrolysis, which has a negative AG (is exergonic). The total AG for the coupled reactions is negative (the two AGs are added together). Hence the reactions proceed exergonically when they are coupled, and glutamine is synthesized.

An active cell requires millions of molecules of ATP per second to drive its biochemical machinery. An ATP molecule

Exergonic reaction:

(releases energy)

• Cell respiration

• Catabolism

Exergonic reaction:

(releases energy)

• Cell respiration

• Catabolism

Exergonic Drives Endergonic Metabolism

Synthesis of ATP from ADP and Pi requires energy.

Endergonic reaction:

(requires energy)

• Active transport

• Cell movements

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