Coupled Reactions ATP

In order to remain alive, a cell must maintain its highly organized, low-entropy state at the expense of free energy in its environment. Accordingly, the cell contains many enzymes that catalyze exergonic reactions using substrates that come ultimately from the environment. The energy released by these exergonic reactions is used to drive the energy-requiring processes (endergonic reactions) in the cell. Since cells cannot use heat energy to drive energy-requiring processes, the chemical-bond energy that is released in exergonic reactions must be directly transferred to chemical-bond energy in the products of endergonic reactions. Energy-liberating reactions are thus coupled to energy-requiring reactions. This relationship is like that of two meshed gears; the turning of one (the energy-releasing exergonic gear) causes turning of the other (the energy-requiring endergonic gear). This relationship is illustrated in figure 4.14.

The energy released by most exergonic reactions in the cell is used, either directly or indirectly, to drive one particular endergonic reaction (fig. 4.15): the formation of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (abbreviated Pi).

The formation of ATP requires the input of a fairly large amount of energy. Since this energy must be conserved (first law of thermodynamics), the bond produced by joining Pi to ADP must contain a part of this energy. Thus, when enzymes reverse this reaction and convert ATP to ADP and Pi, a large amount of energy is released. Energy released from the breakdown of ATP

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Enzymes and Energy

C6H12Og+ 6 O2

Cellular oxidation

Cellular oxidation

Combustion

Cellular Respiration Release Energy

Energy

Total energy released

Combustion

Energy

■ Figure 4.13 A comparison of combustion and cell respiration. Since glucose contains more energy than six separate molecules each of carbon dioxide and water, the combustion of glucose is an exergonic reaction. The same amount of energy is released when glucose is broken down stepwise within the cell.

Products

Products

Endergonic Reaction
Reactants

Exergonic reactions Endergonic reactions

■ Figure 4.14 A model of the coupling of exergonic and endergonic reactions. The reactants of the exergonic reaction (represented by the larger gear) have more free energy than the products of the endergonic reaction because the coupling is not 100% efficient—some energy is lost as heat.

is used to power the energy-requiring processes in all cells. As the universal energy carrier, ATP serves to more efficiently couple the energy released by the breakdown of food molecules to the energy required by the diverse endergonic processes in the cell (fig. 4.16).

Adenosine diphosphate (ADP) +

Inorganic phosphate (P.)

Adenosine triphosphate (ATP) NH2

Adenosine Triphosphate Atp

■ Figure 4.15 The formation and structure of adenosine triphosphate (ATP). ATP is the universal energy carrier of the cell. High-energy bonds are indicated by a squiggle (~).

ATP produced _L

ATP used for cell work

ATP produced _L

ATP used for cell work

Coupled Reaction

■ Figure 4.16 A model of ATP as the universal energy carrier of the cell. Exergonic reactions are shown as gears with arrows going down (these reactions produce a decrease in free energy); endergonic reactions are shown as gears with arrows going up (these reactions produce an increase in free energy).

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Responses

  • carola
    Is combustion of glucose exergonic?
    8 years ago
  • angela
    Is the combustion of glucose exergonic or endergonic?
    8 years ago
  • heike rothstein
    Why are many reactions in cells coupled reactions?
    8 years ago
  • rosarmosario
    How is atp used in coupled reaction?
    8 years ago
  • Faryl Davidson
    What is formed in endergonic reaction ATP or ADP?
    8 years ago
  • cedivar
    How and why endergonic and exergonic reactions are coupled?
    8 years ago
  • lisa
    How can endergonic and exergonic energies be coupled to drive an endergonic reaction?
    8 years ago
  • maxima
    Is the burning of food an endergonic reaction?
    8 years ago
  • maxine
    WHAT REACTION IS JOING ATP TO ADP?
    8 years ago

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