• Endergonic reaction: requires energy, usually ATP

• Exergonic reaction: releases energy, occurs spontaneously

Endergonic and Exergonic Reactions

+

+reactants products

energy

+

+

reactants

productsenergy

∆G > 0

∆G < 0

If exergonic reactions occur spontaneously, why isn’t the paper in front

of you on fire?

• An exergonic reaction

Just as the person may need a push to get started down a hill, many of the exergonic reactions that living

organisms depend on for survival may need a “push”.

∆G < 0

Figure 6.12 Energy profile of an exergonic reaction

EA = activation energy

Example of an enzyme-catalyzed reaction: Hydrolysis of sucrose

+ energy

In living organisms the “somethings” that provide that push are called enzymes

Without enzyme:

With enzyme:

Figure 6.13 Enzymes lower the barrier of activation energy

Most enzymes are protein molecules with a specific 3-dimensional shape

enzyme “induced fit”

substrate

Active site

Enzymes catalyze reactions in living organisms: 1. Bring reactant molecules close together2. Make bonds easier to break/form

Notes about enzymes

• Enzymes only increase the rate of exergonic reactions, they can’t cause non-spontaneous reactions to occur

• Only the substrate will be altered, the enzyme will be released to work again

• Enzymes are one type of catalyst

What determines how well an enzyme works?

• Binding of enzyme and substrate

• Temperature at which the enzyme functions

Can a species’ enzymes be shaped by natural selection?

• Does the same enzyme show variation from one individual to another?

• Is the form of enzyme an individual produce a heritable trait?

• Does the form of enzyme individual produce affect its survival/reproduction?

Figure 6.16 Environmental factors affecting enzyme activity