1. CHAPTER 5 The Working Cell: Energy and Enzymes
Modules

2. Cool “Fires” Attract Mates and Meals
Fireflies use light, instead of chemical signals, to send signals to potential mates
Females can also use light flashes to attract males of other firefly species — as meals, not mates

3. The light comes from a set of chemical reactions, the luciferin-luciferase system
Fireflies make light energy from chemical energy
Life is dependent on energy conversions

4. ENERGY AND THE CELL
Living cells are compartmentalized by membranes
Membranes are sites where chemical reactions can occur in an orderly manner
Living cells process energy by means of enzyme-controlled chemical reactions

5. 5.1 Energy is the capacity to perform work
Energy is defined as the capacity to do work
All organisms require energy to stay alive
Energy makes change possible

6. Kinetic energy is energy that is actually doing work
Figure 5.1A
Potential energy is stored energy
Figure 5.1B

7. 5.2 Two laws govern energy conversion
First law of thermodynamics
Energy can be changed from one form to another
However, energy cannot be created or destroyed
Figure 5.2A

8. Second law of thermodynamics
Energy changes are not 100% efficient
Energy conversions increase disorder, or entropy
Some energy is always lost as heat
Figure 5.2B

9. 5.3 Chemical reactions either store or release energy
Cells carry out thousands of chemical reactions
The sum of these reactions constitutes cellular metabolism

10. Potential energy of molecules
There are two types of chemical reactions:
Endergonic reactions absorb energy and yield products rich in potential energy
Products
Amount of energy INPUT
Potential energy of molecules
Reactants
Figure 5.3A

11. Amount of energy OUTPUT Potential energy of molecules
Exergonic reactions release energy and yield products that contain less potential energy than their reactants
Reactants
Amount of energy OUTPUT
Potential energy of molecules
Products
Figure 5.3B

12. 5.4 ATP shuttles chemical energy within the cell
In cellular respiration, some energy is stored in ATP molecules
ATP powers nearly all forms of cellular work
ATP molecules are the key to energy coupling

13. Adenosine triphosphate Adenosine diphosphate (ADP)
When the bond joining a phosphate group to the rest of an ATP molecule is broken by hydrolysis, the reaction supplies energy for cellular work
Adenine
Phosphate groups
Hydrolysis
Energy
Ribose
Adenosine triphosphate
Adenosine diphosphate (ADP)
Figure 5.4A

14. Potential energy of molecules
How ATP powers cellular work
Reactants
Products
Potential energy of molecules
Protein
Work
Figure 5.4B

15. Dehydration synthesis
The ATP cycle
Hydrolysis
Dehydration synthesis
Energy from exergonic reactions
Energy for endergonic reactions
Figure 5.4C

16. For a chemical reaction to begin, reactants must absorb some energy
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

17. A protein catalyst called an enzyme can decrease the energy barrier
EA barrier
Enzyme
Reactants 1
Products 2
Figure 5.5A

18. EA without enzyme EA with enzyme Reactants Net change in energy
Products
Figure 5.5B

19. 5.6 A specific enzyme catalyzes each cellular reaction
Enzymes are selective
This selectivity determines which chemical reactions occur in a cell

20. The enzyme is unchanged and can repeat the process
How an enzyme works
Active site
Enzyme (sucrase)
Substrate (sucrose)
Glucose
Fructose 1 4
Enzyme available with empty active site
Products are released 3 2
Substrate is converted to products
Substrate binds to enzyme with induced fit
Figure 5.6
The enzyme is unchanged and can repeat the process

21. 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

22. 5.8 Enzyme inhibitors block enzyme action
Inhibitors interfere with enzymes
A competitive inhibitor takes the place of a substrate in the active site
A noncompetitive inhibitor alters an enzyme’s function by changing its shape
Substrate
Active site
Enzyme
NORMAL BINDING OF SUBSTRATE
Competitive inhibitor
Noncompetitive inhibitor
ENZYME INHIBITION
Figure 5.8

23. 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