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Free-Energy Change (G), Stability, and Equilibrium
• A living system’s free energy (G) is energy that can do work when temperature and pressure are uniform, as in a living cell.
• The free-energy change (G) of a reaction tells us whether or not the reaction occurs spontaneously.
© 2014 Pearson Education, Inc.
Figure 6.5b
(a) Gravitational motion
(c) Chemical reaction
(b) Diffusion
Exergonic and Endergonic Reactions in Metabolism
• An exergonic reaction proceeds with a net release of free energy and is spontaneous; ∆G is negative.
© 2014 Pearson Education, Inc.
Figure 6.6a
(a) Exergonic reaction: energy released, spontaneous
Amount ofenergy
released(G 0)
Reactants
ProductsEnergy
Progress of the reaction
Fre
e en
erg
y
• An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous; ∆G is positive.
© 2014 Pearson Education, Inc.
Figure 6.6b
(b) Endergonic reaction: energy required, nonspontaneous
Amount ofenergy
required(G 0)
Reactants
Products
Energy
Progress of the reaction
Fre
e en
erg
y
G 0 G 0
Equilibrium = Death
G 0 G 0
G 0
Food, or some other energy source like the sun.
Exergonic Endergonic
Figure 6.8a
(a) The structure of ATP
Phosphate groups
Adenine
Ribose
Figure 6.8b
(b) The hydrolysis of ATP
Energy
Adenosine triphosphate (ATP)
Adenosine diphosphate (ADP)Inorganic
phosphate
How the Hydrolysis of ATP Performs Work
• The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP.
• In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction.
• Overall, the coupled reactions are exergonic.
© 2014 Pearson Education, Inc.
Glutamic acid
GGlu 3.4 kcal/mol
GlutamineAmmonia
• Will this reaction happen spontaneously? – No
• Is this reaction catabolic or anabolic?– Anabolic
• Is this reaction exergonic or endergonic?– Endergonic
Figure 6.9b
(b) Conversion reaction coupled with ATP hydrolysis
Glutamic acid
GlutaminePhosphorylatedintermediate
Phosphorylatedintermediate
Figure 6.9c
(c) Free-energy change for coupled reaction
GGlu 3.4 kcal/mol
GATP −7.3 kcal/mol GGlu 3.4 kcal/mol
GATP −7.3 kcal/mol
G −3.9 kcal/mol Net
Figure 6.10
(a) Transport work: ATP phosphorylates transport proteins.
(b) Mechanical work: ATP binds noncovalently to motor proteinsand then is hydrolyzed.
Transport protein
Solute transported
Solute
Motor protein
Vesicle Cytoskeletal track
Protein andvesicle moved
Figure 6.11
Energy fromcatabolism(exergonic, energy-releasing processes)
Energy for cellularwork (endergonic, energy-consuming processes)
