t

1. BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 5
The Working Cell
Modules 5.5 – 5.9
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

2. HOW ENZYMES WORK
5.5 Enzymes speed up the cell’s chemical reactions
by lowering energy barriers
• For a chemical reaction to begin, reactants must
absorb some energy
– This energy is called the energy of activation
(EA)
– This represents the energy barrier that prevents
molecules from breaking down spontaneously
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

3. • A protein catalyst called an enzyme can
decrease the energy barrier
EA
Enzyme
barrier
Reactants
1 Products 2
Figure 5.5A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

4. EA
without
EA
with enzyme
Reactants enzyme Net
change
in energy
Products
Figure 5.5B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

5. 5.6 A specific enzyme catalyzes each cellular
reaction
• Enzymes are selective
– This selectivity determines which chemical
reactions occur in a cell
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

6. • How an Enzyme
(sucrase)
Active
site Substrate
(sucrose)
enzyme
works Glucose Fructose 1
4
Enzyme available
Products are with empty active
released site
3
Substrate is 2
converted to
products Substrate
binds to
enzyme with
Figure 5.6
induced fit
• The enzyme
is unchanged and can repeat the process
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

7. 5.7 The cellular environment affects enzyme
activity
• Enzyme activity is influenced by
– temperature
– salt concentration
– pH
• Some enzymes require nonprotein cofactors
– Some cofactors are organic molecules called
coenzymes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

8. 5.8 Enzyme inhibitors block enzyme action
• Inhibitors interfere with enzymes
– A competitive
inhibitor takes Substrate Active
site
the place of a
Enzyme
substrate in the
active site NORMAL BINDING OF SUBSTRATE
– A noncompetitive Competitive
inhibitor
Noncompetitive
inhibitor
inhibitor alters an
enzyme’s function
by changing its shape ENZYME INHIBITION
Figure 5.8
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

9. 5.9 Connection: Some pesticides and antibiotics
inhibit enzymes
• Certain pesticides are toxic to insects because
they inhibit key enzymes in the nervous system
• Many antibiotics inhibit enzymes that are
essential to the survival of disease-causing
bacteria
– Penicillin inhibits an enzyme that bacteria use in
making cell walls
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings