Biology (Holt)

Enzymes

Like engines, cells consume fuel because they need energy to func-

tion. Just as an engine requires a spark of energy to begin burning

gasoline, most biochemical reactions—chemical reactions that

occur in cells—require activation energy to begin. The chemical

reactions in cells occur quickly and at relatively low temperatures

because of the action of many enzymes. are substances

that increase the speed of chemical reactions. Most enzymes are

proteins. Enzymes are catalysts (KAT uh lists), which are substances

that reduce the activation energy of a chemical reaction. As shown

in Figure 14, an enzyme increases the speed of a chemical reaction

by reducing the activation energy of the reaction.

Enzymes help organisms maintain homeostasis. Without

enzymes, chemical reactions would not occur quickly enough to

sustain life. For example, consider a reaction that takes place in

your blood. Blood carries carbon dioxide, CO 2 , (a waste product

made by cells) to your lungs, where it is eliminated as you breathe

out. In the lungs, carbon dioxide reacts with water, H 2 O, to form

carbonic acid, H 2 CO 3 , as shown by the following equation:

carbonic anhydrase

CO 2  H 2 O ←→ H 2 CO 3

The reverse reaction occurs in your lungs, converting carbonic acid

back to carbon dioxide and water. Most enzyme-assisted reactions

are reversible, meaning they can proceed in the opposite direction.

Without an enzyme, the reaction that produces carbonic acid is

very slow; only about 2000 molecules of carbonic acid are produced

in an hour. This rate is not fast enough for your blood to carry away

the carbon dioxide released by millions of cells. Fortunately, your

blood contains the enzyme carbonic anhydrase (an HIED rays). In

the presence of carbonic anhydrase, carbon dioxide and water react

to form about 600,000 molecules of carbonic acid per second! The

enzyme increases the reaction rate about one million times,

enabling your body to eliminate carbon dioxide efficiently.

Enzymes

40 CHAPTER 2Chemistry of Life

Reactants

Products

Activation energy

without an enzyme

Activation energy

with an enzyme

Energyreleased

Energyabsorbed

Reaction progress

Activation Energy With and Without an Enzyme

Figure 14 Enzymes lower
activation energy.Enzymes
decrease the amount of energy
needed to start a chemical
reaction. Enzymes do not
change the amount of energy
contained in either the reac-
tants or the products.

Reading Effectively

As you read, notice that the

names of most enzymes,

such as amylase and cata-

lase, end with -ase.This will

help you identify other

enzymes you will encounter

in this book.