Human Physiology, 14th edition (2016)

Enzymes and Energy 99

Figure 4.13 A comparison of combustion and cell respiration. Because glucose contains more energy than six separate
molecules each of carbon dioxide and water, the combustion of glucose is an exergonic process. The same amount of energy is
released when glucose is broken down stepwise within the cell. Each step represents an intermediate compound in the aerobic
respiration of glucose.

Free energy

6 CO 2 + 6 H 2 O

Total

energy

released

Combustion

C 6 H 12 O 6 + 6 O 2

Energy

Cellular

oxidation

Energy

Energy

Energy

Energy

Energy

The energy released by most exergonic reactions in the

cell is used, either directly or indirectly, to drive one particu-

lar endergonic reaction ( fig. 4.15 ): the formation of adenosine

triphosphate (ATP) from adenosine diphosphate (ADP) and

inorganic phosphate (abbreviated P i ).

The formation of ATP requires the input of a fairly large

amount of energy. Because this energy must be conserved (first

law of thermodynamics), the bond produced by joining P i to

ADP must contain a part of this energy. Thus, when enzymes

reverse this reaction and convert ATP to ADP and P i , a large

amount of energy is released. Energy released from the break-

down of ATP 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 ender-

gonic processes in the cell ( fig. 4.16 ).

Coupled Reactions: Oxidation-Reduction

When an atom or a molecule gains electrons, it is said to become

reduced; when it loses electrons, it is said to become oxidized.

Reduction and oxidation are always coupled reactions:

an atom or a molecule cannot become oxidized unless it

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.

Reactants

Reactants

Exergonic reactions Endergonic reactions

Products

Products

Free energy

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 rela-
tionship is illustrated in figure 4.14.