1. METABOLISM
Chapter 6

2. WHAT IS ENERGY? Energy is the capacity to cause change
Energy exists in various forms, some of which can perform work 2

3. FORMS OF ENERGY Kinetic energy energy associated with motion
Thermal energy
kinetic energy associated with random movement of atoms or molecules
Heat
thermal energy in transfer from one object to another 3

4. FORMS OF ENERGY Potential energy
energy that matter possesses because of its location or structure
Chemical energy
potential energy available for release in a chemical reaction
Energy can be converted from one form to another 4

5. The Laws of Energy Transformation
Thermodynamics
the study of energy transformations
An isolated system, such as that approximated by liquid in a thermos, is isolated from its surroundings
In an open system, energy and matter can be transferred between the system and its surroundings
Organisms are open systems 5

6. (a) An isolated hydroelectric system
Figure 6.7a
G  0
G  0
Figure 6.7a Equilibrium and work in isolated and open systems (part 1: isolated system)
(a) An isolated hydroelectric system 6

7. (b) An open hydroelectric system G  0 Figure 6.7b
Figure 6.7b Equilibrium and work in isolated and open systems (part 2: open system) 7

8. The First Law of Thermodynamics
According to the first law of thermodynamics, the energy of the universe is constant
Energy can be transferred and transformed, but it cannot be created or destroyed
The first law is also called the principle of conservation of energy 8

9. (a) First law of thermodynamics
Figure 6.3a
Chemical
energy
Figure 6.3a The two laws of thermodynamics (part 1: conservation of energy)
(a) First law of thermodynamics 9

10. The Second Law of Thermodynamics
During every energy transfer or transformation, some energy is unusable and is often lost as heat
According to the second law of thermodynamics
Every energy transfer or transformation increases the entropy of the universe
Entropy is a measure of disorder, or randomness 10

11. (b) Second law of thermodynamics
Figure 6.3b
Heat
Figure 6.3b The two laws of thermodynamics (part 2: entropy)
(b) Second law of thermodynamics 11

12. ONE-WAY FLOW OF ENERGY The sun is life’s primary energy source
Producers trap energy from the sun and convert it into chemical bond energy
All organisms use the energy stored in the bonds of organic compounds to do work 12

13. Overview: The Energy of Life
The living cell is a miniature chemical factory where thousands of reactions occur
The cell extracts energy and applies energy to perform work
Metabolism is the totality of an organism’s chemical reactions
Metabolism is an emergent property of life that arises from interactions between molecules within the cell 13

14. Metabolic Pathways
A metabolic pathway begins with a specific molecule and ends with a product
Each step is catalyzed by a specific enzyme
Figure 6.UN01 In-text figure, metabolic pathway schematic, p. 116
Enzyme 1
Enzyme 2
Enzyme 3
Starting
molecule A B C D
Product
Reaction 1
Reaction 2
Reaction 3 14

16. Free-Energy Change (G), Stability, and Equilibrium
A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell
The change in free energy (∆G) during a chemical reaction is the difference between the free energy of the final state and the free energy of the initial state
∆G = Gfinal state – Ginitial state
Only processes with a negative ∆G are spontaneous
Spontaneous processes can be harnessed to perform work 16

17. Metabolic Pathways 2 types Catabolic degradative process
exergonic reaction
respiration
products have less energy than starting substance
Metabolic Pathways
glucose, a high energy starting substance
+ 6O2
ENERGY OUT 6 6
low energy products 17

18. Exergonic Reactions in Metabolism
An exergonic reaction proceeds with a net release of free energy and is spontaneous; ∆G is negative
The magnitude of ∆G represents the maximum amount of work the reaction can perform 18

19. Amount of energy released (G  0)
Figure 6.6a
(a) Exergonic reaction: energy released, spontaneous
Reactants
Amount of
energy
released
(G  0)
Energy
Free energy
Products
Figure 6.6a Free energy changes (ΔG) in exergonic and endergonic reactions (part 1: exergonic)
Progress of the reaction 19

20. Biosynthetic pathways involve the building of complicated molecules
Anabolic
Biosynthetic pathways
involve the building of complicated molecules
endergonic reaction
“uphill” reaction
photosynthesis
glucose, a high energy product
+ 6O2
ENERGY IN
low energy starting substances 6 6 20

21. Endergonic Reactions in Metabolism
An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous; ∆G is positive
The magnitude of ∆G is the quantity of energy required to drive the reaction 21

22. Amount of energy required (G  0)
(b) Endergonic reaction: energy required,
nonspontaneous
Products
Amount of
energy
required
(G  0)
Energy
Free energy
Reactants
Figure 6.6b Free energy changes (ΔG) in exergonic and endergonic reactions (part 2: endergonic)
Progress of the reaction 22

23. Equilibrium and Metabolism
Reactions in a closed system eventually reach equilibrium and then do no work
Cells are not in equilibrium; they are open systems experiencing a constant flow of materials
A defining feature of life is that metabolism is never at equilibrium 23

24. 24

25. 25

26. ATP A cell does three main kinds of work Chemical Transport Mechanical
To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one
Most energy coupling in cells is mediated by ATP 26

27. The Structure and Hydrolysis of ATP
ATP (adenosine triphosphate) is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups
In addition to its role in energy coupling, ATP is also used to make RNA
ATP Formation 27

28. (a) The structure of ATP
Adenine
Phosphate groups
Ribose
Figure 6.8a The structure and hydrolysis of adenosine triphosphate (ATP) (part 1: structure)
(a) The structure of ATP 28

29. Adenosine triphosphate (ATP)
Figure 6.8b The structure and hydrolysis of adenosine triphosphate (ATP) (part 2: hydrolysis)
Energy
Inorganic
phosphate
Adenosine diphosphate (ADP)
(b) The hydrolysis of ATP 29

30. How the Hydrolysis of ATP Performs Work
The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP
In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction
Overall, the coupled reactions are exergonic 30

31. GGlu  3.4 kcal/mol Glutamic acid Ammonia Glutamine
(a) Glutamic acid conversion to glutamine
Figure 6.9a How ATP drives chemical work: energy coupling using ATP hydrolysis (part 1: a nonspontaneous reaction) 31

32. Phosphorylated intermediate Phosphorylated intermediate
Glutamic acid
Phosphorylated
intermediate
Figure 6.9b How ATP drives chemical work: energy coupling using ATP hydrolysis (part 2: phosphorylation)
Phosphorylated
intermediate
Glutamine
(b) Conversion reaction coupled with ATP hydrolysis 32

33. (c) Free-energy change for coupled reaction G  −3.9 kcal/mol Net
GGlu  3.4 kcal/mol
GGlu  3.4 kcal/mol
GATP  −7.3 kcal/mol
GATP  −7.3 kcal/mol 
Figure 6.9c How ATP drives chemical work: energy coupling using ATP hydrolysis (part 3: coupled free energy)
(c) Free-energy change for coupled reaction
G  −3.9 kcal/mol
Net 33

34. The recipient molecule is now called a phosphorylated intermediate
ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant
The recipient molecule is now called a phosphorylated intermediate
ATP hydrolysis leads to a change in a protein’s shape and often its ability to bind to another molecule 34

35. Protein and vesicle moved
Transport protein
Solute
Solute transported
(a) Transport work: ATP phosphorylates transport proteins.
Vesicle
Cytoskeletal track
Figure 6.10 How ATP drives transport and mechanical work
Motor protein
Protein and
vesicle moved
(b) Mechanical work: ATP binds noncovalently to motor proteins
and then is hydrolyzed. 35

36. Mitochondrial chemiosmosis
ATP/ADP Cycle
Mitochondrial chemiosmosis

37. The Regeneration of ATP
ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)
phosphorylation
The energy to phosphorylate ADP comes from catabolic reactions in the cell
The ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways 37

38. Energy from catabolism (exergonic, energy- releasing processes)
Energy for cellular
work (endergonic,
energy-consuming
processes)
Figure 6.11 The ATP cycle 38

39. reactions that release energy reactions that require energy
ATP/ADP Cycle
ATP
cellular work
(e.g., synthesis,
breakdown, or rearrangement
of substances;
contraction of
muscle cells;
active transport
across a cell
membrane)
ATP
reactions that release energy
reactions that require energy
ADP + Pi 39

40. ENZYME STRUCTURE AND FUNCTION
A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction
An enzyme is a catalytic protein 40

41. 4 FEATURES OF ENZYMES
1) Enzymes do not make anything happen that could not happen on its own. They just make it happen much faster. 2) Reactions do not alter or use up enzyme molecules. 3) The same enzyme usually works for both the forward and reverse reactions. 4) Each type of enzyme recognizes and binds to only certain substrates. 41

42. The Activation Energy Barrier
Every chemical reaction between molecules involves bond breaking and bond forming
The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)
Activation energy is often supplied in the form of thermal energy that the reactant molecules absorb from their surroundings 42

43. Progress of the reactionB C D
Transition state A B EA
Free energy C D
Reactants A B
Figure 6.12 Energy profile of an exergonic reaction
G  0 C D
Products
Progress of the reaction 43

44. How Enzymes Speed Up Reactions
Enzymes catalyze reactions by lowering the EA barrier
Enzymes do not affect the change in free energy (∆G); instead, they hasten reactions that would occur eventually
© 2014 Pearson Education, Inc. 44

45. Activation Energy
Activation energy

46. Progress of the reaction
Course of
reaction
without
enzyme EA
without
enzyme
EA with
enzyme
is lower
Reactants
Free energy
Course of
reaction
with enzyme
G is unaffected
by enzyme
Figure 6.13 The effect of an enzyme on activation energy
Products
Progress of the reaction 46

47. Substrate Specificity of Enzymes
The reactant that an enzyme acts on is called the enzyme’s substrate
The enzyme binds to its substrate, forming an enzyme-substrate complex
The active site is the region on the enzyme where the substrate binds
Enzyme specificity results from the complementary fit between the shape of its active site and the substrate shape 47

48. Enzymes change shape due to chemical interactions with the substrate
This induced fit of the enzyme to the substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction 48

49. Catalysis in the Enzyme’s Active Site
In an enzymatic reaction, the substrate binds to the active site of the enzyme
The active site can lower an EA barrier by
Orienting substrates correctly
Straining substrate bonds
Providing a favorable microenvironment
Covalently bonding to the substrate 49

50. Substrates are 2 Substrates enter 1 held in active site by
weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Figure The active site and catalytic cycle of an enzyme (steps 1-2) 50

51. Substrates are 2 Substrates enter 1 held in active site by
weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Figure The active site and catalytic cycle of an enzyme (step 3) 3
Substrates are
converted to
products. 51

52. 2 Substrates are held in active site by weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Figure The active site and catalytic cycle of an enzyme (step 4)
Products are
released. 4 3
Substrates are
converted to
products.
Products 52

53. 2 Substrates are held in active site by weak interactions. 1
Substrates enter
active site.
Substrates
Enzyme-substrate
complex
Active
site is
available
for new
substrates. 5
Figure The active site and catalytic cycle of an enzyme (step 5)
Enzyme
Products are
released. 4 3
Substrates are
converted to
products.
Products 53

54. ENZYME HELPERS Cofactors Coenzymes (organic) NAD+, NADP+, FAD
Accept electrons and hydrogen ions; transfer them within cell
Derived from vitamins
Metal ions
Ferrous iron in cytochromes 54

55. Enzyme Inhibitors Competitive inhibitors
bind to the active site of an enzyme, competing with the substrate
Noncompetitive inhibitors
bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective
Examples of inhibitors include toxins, poisons, pesticides, and antibiotics
Competitive vs Non-Competititve Inhibition 55

56. (b) Competitive inhibition (c) Noncompetitive inhibition
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive
inhibition
Substrate
Active site
Competitive
inhibitor
Enzyme
Figure 6.17 Inhibition of enzyme activity
Noncompetitive
inhibitor 56

57. The Evolution of Enzymes
Enzymes are proteins encoded by genes
Changes (mutations) in genes lead to changes in amino acid composition of an enzyme
Altered amino acids in enzymes may alter their substrate specificity
Under new environmental conditions a novel form of an enzyme might be favored 57

58. Regulation of enzyme activity helps control metabolism
Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated
A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes 58

59. Allosteric Regulation of Enzymes
may either inhibit or stimulate an enzyme’s activity
occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site 59

60. Allosteric Activation and Inhibition
Most allosterically regulated enzymes are made from polypeptide subunits
Each enzyme has active and inactive forms
The binding of an activator stabilizes the active form of the enzyme
The binding of an inhibitor stabilizes the inactive form of the enzyme 60

61. Allosteric Activation

62. ALLOSTERIC ACTIVATION
allosteric activator
enzyme active site
vacant
allosteric binding site
active site cannot bind substrate
active site altered, can bind substrate 62

63. Allosteric Inhibition

64. ALLOSTERIC INHIBITION
allosteric inhibitor
allosteric binding site vacant; active site can bind substrate
active site altered, can’t bind substrate 64

65. Feedback Inhibition Feedback Inhibition
the end product of a metabolic pathway shuts down the pathway
prevents a cell from wasting chemical resources by synthesizing more product than is needed 65

67. END PRODUCT (tryptophan)
FEEDBACK INHIBITION
enzyme 2
enzyme 3
enzyme 4
enzyme 5
A cellular change, caused by a specific activity, shuts down the activity that brought it about
enzyme 1
END PRODUCT (tryptophan)
SUBSTRATE 67

68. ENZYME WEBSITES Enzyme Definitions Active Site Enzymes and Reactions
Enzyme Action (fats, carbs, proteins)
peristalsis / chyme 68

69. FACTORS INFLUENCING ENZYME ACTIVITY
Temperature pH Salt concentration Allosteric regulators Coenzymes and cofactors 69

70. EFFECT OF TEMPERATURE
Small increase in temperature increases molecular collisions, reaction rates
High temperatures disrupt bonds and destroy the shape of active site 70

71. Optimal temperature for typical human enzyme (37C)
enzyme of thermophilic
(heat-tolerant)
bacteria (77C)
Rate of reaction
Figure 6.16a Environmental factors affecting enzyme activity (part 1: temperature) 20 40 60 80
100
120
Temperature (C)
Optimal temperature for two enzymes 71

72. Enzymes and temperature
Effect of Temperature
Enzymes and temperature

73. Optimal pH for two enzymes
Optimal pH for pepsin
(stomach
enzyme)
Optimal pH for trypsin
(intestinal
enzyme)
Rate of reaction
Figure 6.16b Environmental factors affecting enzyme activity (part 2: pH) 1 2 3 4 5 6 7 8 9 10 pH
Optimal pH for two enzymes 73

74. ALCOHOL, ENZYMES, AND YOUR LIVER
Catalase is an enzyme that helps the body break down toxic substances in alcoholic drinks 74

75. The liver plays a central role in alcohol metabolism
Consumption of too much alcohol, as in binge drinking, can lead to alcoholic hepatitis or alcoholic cirrhosis 75

76. Alcohol, Enzymes, and Your Liver

77. REDOX REACTIONS
Cells release energy efficiently by electron transfers, or oxidation-reduction reactions (“redox” reactions)
Oxidation (oxidized)
one molecule gives up electrons
Reduction (reduced)
another molecule gains electrons
Hydrogen atoms are commonly released at the same time, thus becoming H+ 77

78. ELECTRON TRANSFER CHAINS
Arrangement of enzymes, coenzymes, at cell membrane
As one molecule is oxidized, next is reduced
Function in aerobic respiration and photosynthesis 78

79. UNCONTROLLED VS. CONTROLLED ENERGY RELEASEH2
1/2 O2
Explosive release of energy as
heat that cannot be harnessed
for cellular work
H2O 79