100 percent efficient. Therefore, the total amount of energy
in the universe is spontaneously flowing from forms
rich in energy (such as glucose) to forms having less and
less of it. This is the main point of the second law of
thermodynamics.
Examples of energy changes
When cells convert one form of energy to another, there is
a change in the amount of potential energy that is available
to them. Cells of photosynthetic organisms, notably green
plants, convert energy in sunlight into chemical energy,
which is stored in the bonds of organic compounds. The
outcome is a net increase in energy in the product molecule
(such as glucose), as diagrammed in Figure A.2. A reaction
in which there is a net increase in energy in the product
compound is an endergonic reaction (meaning energy
in). By contrast, reactions in cells that break down glucose
(or another energy-rich compound) release energy. They
are called exergonic reactions (meaning energy out).
The role of enzymes in metabolic reactions
The catalytic molecules called enzymes are crucial actors
in metabolism. To better understand why, it helps to begin
with the idea that in cells, molecules, or ions of substances
are always moving at random. As a result of this random
motion, they are constantly colliding. Metabolic reactions
may take place when participating molecules collide—but
only if the energy associated with the collisions is great
enough. This minimum amount of energy required for a
chemical reaction is called activation energy. Activation
energy is a barrier that must be surmounted one way or
another before a reaction can proceed.
Nearly all metabolic reactions are reversible. That is, they
can run “forward,” from starting substances to products,
or in “reverse,” from a product back to starting substances.
Which way such a reaction runs depends partly on the ratio
of reactant to product. When there is a high concentration of
reactant molecules, the reaction is likely to run strongly in
the forward direction. On the other hand, when the product
concentration is high enough, more molecules or ions of the
product are available to revert spontaneously to reactants.
Any reversible reaction tends to run spontaneously toward
chemical equilibrium—the point at which it will be run-
ning at about the same pace in both directions.
As just described, before reactants enter a metabolic
reaction they must be activated by an energy input; only
then will the steps leading to products proceed. And while
random collisions might provide the energy for reactions,
our survival depends on thousands of reactions taking
place with amazing speed and precision. This is the key
function of enzymes, for enzymes lower the activation
energy barrier (Figure A.3). As Section 3.13 described,
substrates and enzymes interact at the enzyme’s active
site. According to the induced fit model, a surface region
of each substrate has chemical groups that are almost but
not quite complementary to chemical groups in an active
site. However, as substrates settle into the site, the contact
strains some of their bonds, making them easier to break.
There also are interactions among charged or polar groups
that prime substrates for conversion to an activated state.Figure A.2 Energy inputs and outputs in chemical reactions.
1 Endergonic reactions convert molecules with lower energy
to molecules with higher energy, so they require a net
energy input in order to proceed.
2 Exergonic reactions convert molecules with higher energy
to molecules with lower energy, so they end with a net
energy output.Figure A.3 An enzyme enhances the rate of a reaction by
lowering its activation energy. (© Cengage Learning)2H 2 O2H 2 1 O 2energy outenergy inFree energy 12TimeFree energyReactantsTransition stateProductsActivation energy
with enzymeActivation energy
without enzyme© Cengage LearningA-2 Appendix i
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