Page 77 - Fundamentals of anatomy physiology
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64 Unit 1 Levels of Organization
2 This equation indicates that, in a sense, two reactions are tak- acidity, but such changes are deadly to cells. For example, every
ing place at the same time. One is a synthesis (A + B ¡ AB) day your cells break down complex sugars as part of your nor-
and the other is a decomposition (AB ¡ A + B). mal metabolism. Yet to break down a complex sugar in a labo-
ratory, you must boil it in an acidic solution. Your cells don’t
Recall from Chapter 1 that a state of equilibrium exists have that option! Temperatures that high and solutions that
when opposing processes or forces are in balance. At equilib- corrosive would immediately destroy living tissues. Instead,
rium, the rates of the two reactions are in balance. As fast as your cells use special proteins called enzymes to perform most
one molecule of AB forms, another degrades into A + B. of the complex synthesis and decomposition reactions in your
body.
What happens when equilibrium is disturbed—say, if you
add more AB? In our example, the rate of the synthesis reac- Enzymes promote chemical reactions by lowering the ac-
tion is directly proportional to the frequency of encounters be- tivation energy required (Figure 2–8). In doing so, they make
tween A and B. In turn, the frequency of encounters depends it possible for chemical reactions, such as the breakdown of
on the degree of crowding. (You are much more likely to bump sugars, to proceed under conditions compatible with life. Cells
into another person in a crowded room than in a room that make enzyme molecules, each of which promotes a specific
is almost empty.) So, adding more AB molecules will increase reaction.
the rate of conversion of AB to A and B. The amounts of A and
B will then increase, leading to an increase in the rate of the re- Enzymes belong to a class of substances called catalysts
verse reaction—the formation of AB from A and B. Eventually, (KAT-uh-lists; katalysis, dissolution), compounds that speed
a balance, or equilibrium, is again established. up chemical reactions without themselves being permanently
changed or consumed. Enzymatic reactions, which are revers-
&T i p s T r i c k s ible, can be written as
Jell-O provides an example of a physical reversible reaction.
Once Jell-O has been refrigerated, the gelatin sets up and A + B ∆enzyme AB
forms a solid, but if it sits without refrigeration for too long,
it goes back to a liquid again. An appropriate enzyme can accelerate, or speed up, a reac-
tion, but an enzyme affects only the rate of the reaction, not its
Checkpoint direction or the products that are formed. An enzyme cannot
bring about a reaction that would otherwise be impossible.
7. When using the rules for chemical notation, how is Enzymatic reactions are generally reversible, and they proceed
an ion’s electrical charge represented? until equilibrium is reached.
8. Using the rules for chemical notation, write the molecular The complex reactions that support life take place in a se-
formula for glucose, a compound composed of 6 carbon ries of interlocking steps, each controlled by a specific enzyme.
atoms, 12 hydrogen atoms, and 6 oxygen atoms. Such a reaction sequence is called a metabolic pathway. A syn-
thetic pathway can be diagrammed as
9. Identify and describe three types of chemical reactions
important to human physiology. A e¡nSztyemp 1e 1 B e¡nSztyemp 2e 2 C e¡nSztyemp 3e 3 and so on .
1 0. In cells, glucose, a six-carbon molecule, is converted Figure 2–8 Enzymes Lower Activation Energy. Enzymes lower
into two three-carbon molecules by a reaction that
releases energy. How would you classify this reaction? the activation energy required for a chemical reaction to proceed
readily (in order, from 1–4) under conditions in the body.
See the blue Answers tab at the back of the book.
Energy 1 Activation energy
2-4 Enzymes catalyze specific Reactant(s) required
biochemical reactions by lowering Without
the energy needed to start them enzyme
2
Learning Outcome Describe the crucial role of enzymes in metabolism.
With enzyme
Most chemical reactions do not take place spontaneously, or if
they do, they occur so slowly that they would be of little value 3
to living cells. Before a reaction can proceed, enough energy Stable
must be provided to activate the reactants. The amount of en-
ergy required to start a reaction is called the a ctivation energy. product(s)
Many reactions can be activated by changes in temperature or
4
Progress of reaction
