1. Free Energy, ATP and Energy Coupling
Chapter 6
Free Energy, ATP and Energy Coupling

2. 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.
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3. (a) Gravitational motion (b) Diffusion (c) Chemical reaction
Figure 6.5b
The change in free energy (∆G) during a chemical reaction is the difference between the free energy of the final state and the free energy of the initial state.
∆G = Gfinal state – Ginitial state
Only processes with a negative ∆G are spontaneous.
Spontaneous processes can be harnessed to perform work.
(a) Gravitational
motion
(b) Diffusion
(c) Chemical
reaction 3

4. Exergonic and Endergonic Reactions in Metabolism
An exergonic reaction proceeds with a net release of free energy and is spontaneous; ∆G is negative.
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5. Amount of energy released (G  0)
Figure 6.6a
(a) Exergonic reaction: energy released, spontaneous
Reactants
Amount of
energy
released
(G  0)
Energy
Free energy
Products
Figure 6.6a Free energy changes (ΔG) in exergonic and endergonic reactions (part 1: exergonic)
Progress of the reaction 5

6. An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous; ∆G is positive.
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7. Amount of energy required (G  0)
Figure 6.6b
(b) Endergonic reaction: energy required,
nonspontaneous
Products
Amount of
energy
required
(G  0)
Energy
Free energy
Reactants
Figure 6.6b Free energy changes (ΔG) in exergonic and endergonic reactions (part 2: endergonic)
Progress of the reaction 7

8. Equilibrium = Death G  0 G  0
Reactions in a closed system eventually reach equilibrium and then do no work.
Cells are not in equilibrium; they are open systems experiencing a constant flow of materials.
A defining feature of life is that metabolism is never at equilibrium. 8

9. Food, or some other energy source like the sun.
Exergonic
To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one.
Most energy coupling in cells is mediated by ATP.
Endergonic 9

11. (a) The structure of ATP
Figure 6.8a
Adenine
Phosphate groups
Ribose
ATP (adenosine triphosphate) is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups.
(a) The structure of ATP 11

12. Adenosine triphosphate (ATP)
Figure 6.8b
Adenosine triphosphate (ATP)
The bonds between the phosphate groups of ATP can be broken by hydrolysis.
Energy is released from ATP when the terminal phosphate bond is broken.
This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.
Energy
Inorganic
phosphate
Adenosine diphosphate (ADP)
(b) The hydrolysis of ATP 12

13. 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.
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14. Will this reaction happen spontaneously?
Glutamic acid
GGlu  3.4 kcal/mol
Glutamine
Ammonia
Will this reaction happen spontaneously? No
Is this reaction catabolic or anabolic?
Anabolic
Is this reaction exergonic or endergonic?
Endergonic

15. Phosphorylated intermediate Phosphorylated intermediate
Figure 6.9b
Glutamic acid
Phosphorylated
intermediate
ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant.
The recipient molecule is now called a phosphorylated intermediate.
ATP hydrolysis leads to a change in a protein’s shape and often its ability to bind to another molecule.
Phosphorylated
intermediate
Glutamine
(b) Conversion reaction coupled with ATP hydrolysis 15

16. (c) Free-energy change for coupled reaction G  −3.9 kcal/mol Net
Figure 6.9c
GGlu  3.4 kcal/mol
GGlu  3.4 kcal/mol
GATP  −7.3 kcal/mol
GATP  −7.3 kcal/mol 
Figure 6.9c How ATP drives chemical work: energy coupling using ATP hydrolysis (part 3: coupled free energy)
(c) Free-energy change for coupled reaction
G  −3.9 kcal/mol
Net 16

17. Protein and vesicle moved
Figure 6.10
Transport protein
Solute
Solute transported
(a) Transport work: ATP phosphorylates transport proteins.
Vesicle
Cytoskeletal track
Figure 6.10 How ATP drives transport and mechanical work
Motor protein
Protein and
vesicle moved
(b) Mechanical work: ATP binds noncovalently to motor proteins
and then is hydrolyzed. 17

18. Energy from catabolism (exergonic, energy- releasing processes)
Figure 6.11
Energy from
catabolism
(exergonic, energy-
releasing processes)
Energy for cellular
work (endergonic,
energy-consuming
processes)
Figure 6.11 The ATP cycle 18