An Introduction to Metabolism

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Biologists want to know which reactions occur spontaneously and which require input of energy

To do so, they need to determine energy changes that occur in chemical reactions

Slide 20

Free-Energy Change, ∆G

A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell

Slide 21

The change in free energy (∆G) during a process is related to the change in enthalpy, or change in total energy (∆H), change in entropy (∆S), and temperature in Kelvin (T):

∆G = ∆H – T∆S

Only processes with a negative ∆G are spontaneous

Spontaneous processes can be harnessed to perform work

Slide 22

Free Energy, Stability, and Equilibrium

Free energy is a measure of a system’s instability, its tendency to change to a more stable state

During a spontaneous change, free energy decreases and the stability of a system increases

Equilibrium is a state of maximum stability

A process is spontaneous and can perform work only when it is moving toward equilibrium

Slide 23

Fig. 8-5

(a) Gravitational motion

(b) Diffusion

(c) Chemical reaction

More free energy (higher G)

Less stable

Greater work capacity

In a spontaneous change

The free energy of the system

decreases (∆G < 0)

The system becomes more

stable

The released free energy can

be harnessed to do work

Less free energy (lower G)

More stable

Less work capacity

Slide 24

Fig. 8-5a

Less free energy (lower G)

More stable

Less work capacity

More free energy (higher G)

Less stable

Greater work capacity

In a spontaneous change

The free energy of the system

decreases (∆G < 0)

The system becomes more

stable

The released free energy can

be harnessed to do work

Slide 25

Fig. 8-5b

Spontaneous

change

Spontaneous

change

Spontaneous

change

(b) Diffusion

(c) Chemical reaction

(a) Gravitational motion

Slide 26

Free Energy and Metabolism

The concept of free energy can be applied to the chemistry of life’s processes

Slide 27

Exergonic and Endergonic Reactions in Metabolism