1. An Introduction to Metabolism
Chapter 8
An Introduction to Metabolism

2. 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.
Some organisms even convert energy to light, as in bioluminescence.
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3. What causes the bioluminescence in these fungi?
Figure 8.1

4. Metabolism is ALL / the totality of an organism’s chemical reactions.
An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics
Metabolism is ALL / the totality of an organism’s chemical reactions.
Metabolism is an emergent property of life that arises from interactions between molecules within the cell.
Metabolism has two basic subdivisions:
Anabolism = building / synthesis
Catabolism = breaking down
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5. Organization of the Chemistry of Life into Metabolic Pathways
A metabolic pathway begins with a specific molecule / substrate and ends with a product.
Each step is catalyzed by a specific enzyme in a specific sequence.
Metabolic pathways are regulated by various mechanisms.
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6. Metabolic Pathways are organized into Specific Sequences
Enzyme 1
Enzyme 2
Enzyme 3 A B C D
Reaction 1
Reaction 2
Reaction 3
Starting
molecule
Product

7. Catabolic pathways will release energy by breaking down complex molecules into simpler compounds.
Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism.
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8. The synthesis of protein from amino acids is an example of anabolism.
Anabolic pathways consume / use energy to build complex molecules from simpler ones.
The synthesis of protein from amino acids is an example of anabolism.
Bioenergetics is the study of how organisms manage their energy resources.
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9. Forms of Energy Energy is the capacity to cause change.
Energy is the ability to do work.
Energy exists in various forms, some of which can perform work.
Energy can be converted from one form to another.
The Laws of Thermodynamics govern all energy transformations.
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10. Kinetic energy is energy associated with motion.
Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules.
Potential energy is energy that matter possesses because of its location or structure.
Chemical energy is potential energy available for release in a chemical reaction.
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11. Transformations between potential and kinetic energy
A diver has more potential
energy on the platform
than in the water.
Diving converts
potential energy to
kinetic energy.
Figure 8.2
Climbing up converts the kinetic
energy of muscle movement
to potential energy.
A diver has less potential
energy in the water
than on the platform.

12. The Laws of Energy Transformation: Thermodynamics
A closed 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, and they perform energy transformations to grow and survive.
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13. The First Law of Thermodynamics
The first law of thermodynamics: the energy of the universe is a constant quantity:
– 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.
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14. The Second Law of Thermodynamics
During every energy transfer or transformation, some energy is unusable, and is often lost as heat.
The second law of thermodynamics:
– Every energy transfer or transformation increases the entropy (disorder) of the universe
The quality of the energy changes - becomes more disordered (1st law states the quantity is constant)
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15. The two laws of thermodynamics
Heat
CO2 +
Chemical
energy
H2O
Figure 8.3
(a) First law of thermodynamics
(b) Second law of thermodynamics

16. Biological Order and Disorder
Cells create ordered structures from less ordered materials.
Organisms also replace ordered forms of matter and energy with less ordered forms.
Energy flows into an ecosystem in the form of light and exits in the form of heat.
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17. The evolution of more complex organisms does not violate the second law of thermodynamics.
Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases.
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18. The free-energy change of a reaction G
Biologists want to know which reactions occur spontaneously and which require the input of energy.
For this, they need to determine energy changes that occur in chemical reactions.
A living system’s free energy, G, is energy that can do work when temperature and pressure are uniform, as in a living cell.
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19. The change in free energy (∆G) during a process is related to:
the change in enthalpy, or change in total energy (∆H)
change in entropy (∆S), disorder,
and temperature in Kelvin (T):
∆G = ∆H – T∆S
Only processes with a negative ∆G are spontaneous.
Spontaneous processes can be harnessed to perform work.
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20. Free Energy, Stability, and Equilibrium
Free energy is a measure of a system’s instability, its tendency to change to a more stable state.
During a spontaneous change, free energy decreases, - ∆G ,and the stability of a system increases.
Equilibrium is a state of maximum stability.
A process is spontaneous and can perform work only when it is moving toward equilibrium.
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21. The relationship of free energy to stability, work capacity, and spontaneous change
More free energy (higher G)
Less stable
Greater work capacity
In a spontaneous change
The free energy of the system
decreases (∆G < 0)
The system becomes more stable
The released free energy can
be harnessed to do work
Less free energy (lower G)
More stable
Less work capacity
Figure 8.5
(a) Gravitational motion
(b) Diffusion
(c) Chemical reaction

22. More free energy (higher G) Less stable Greater work capacity
In a spontaneous change
The free energy of the system
decreases (∆G < 0)
The system becomes more
stable
The released free energy can
be harnessed to do work
Figure 8.5 The relationship of free energy to stability, work capacity, and spontaneous change
Less free energy (lower G)
More stable
Less work capacity

23. (a) Gravitational motion (b) Diffusion (c) Chemical reaction
Spontaneous
change
Spontaneous
change
Spontaneous
change
Figure 8.5 The relationship of free energy to stability, work capacity, and spontaneous change
(a) Gravitational motion
(b) Diffusion
(c) Chemical reaction

24. Free Energy and Metabolism: Exergonic and Endergonic Reactions
An exergonic reaction proceeds with a net release of free energy and is spontaneous ∆G
An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous +∆G
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25. ΔG in exergonic endergonic Free energy changes and reactions Reactants
Amount of
energy
released
(∆G < 0)
Energy
Free energy
Products
Progress of the reaction
(a) Exergonic reaction: energy released
Products
Figure 8.6
Amount of
energy
required
(∆G > 0)
Energy
Free energy
Reactants
Progress of the reaction
(b) Endergonic reaction: energy required

26. Amount of energy released (∆G < 0)
Reactants
Amount of
energy
released
(∆G < 0)
Free energy
Energy
Products
Figure 8.6a Free energy changes (ΔG) in exergonic and endergonic reactions
Progress of the reaction
(a) Exergonic reaction: energy released

27. Amount of energy required (∆G > 0)
Products
Amount of
energy
required
(∆G > 0)
Energy
Free energy
Reactants
Figure 8.6b Free energy changes (ΔG) in exergonic and endergonic reactions
Progress of the reaction
(b) Endergonic reaction: energy required

28. Equilibrium and Metabolism
Reactions in a closed system eventually reach equilibrium and then do no work ∆G = 0
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.
A catabolic pathway in a cell releases free energy in a series of reactions.
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29. ATP powers cellular work by coupling exergonic reactions to endergonic reactions = Energy Coupling
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.
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30. The Structure and Hydrolysis of ATP
ATP (adenosine triphosphate) is the cell’s energy shuttle
ATP is composed of:
ribose (a pentose sugar)
adenine (a nitrogenous base)
and three phosphate groups
For the Cell Biology Video Space Filling Model of ATP (Adenosine Triphosphate), go to Animation and Video Files.
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31. The structure of adenosine triphosphate ATP
Adenine
3 Phosphate groups
Figure 8.8
Ribose

32. The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis.
Energy is released from ATP when the terminal phosphate bond is broken.
This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.
For the Cell Biology Video Stick Model of ATP (Adenosine Triphosphate), go to Animation and Video Files.
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33. + H2O P P P Adenosine triphosphate (ATP) P + P P Energy
Hydrolysis of ATP releases energy to do work P P P
Adenosine triphosphate (ATP)
H2O
Figure 8.9 The hydrolysis of ATP P + P P +
Energy i
Inorganic phosphate
Adenosine diphosphate (ADP)

34. Overall, the coupled reactions are exergonic.
How 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.
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35. Energy coupling How ATP drives chemical work: using ATP hydrolysis +
NH2
NH3 +
∆G = +3.4 kcal/mol
Glu
Glu
Glutamic
acid
Ammonia
Glutamine
(a) Endergonic reaction 1
ATP phosphorylates
glutamic acid,
making the amino
acid less stable. P +
ATP +
ADP
Glu
Glu
NH2 P 2
Ammonia displaces
the phosphate group,
forming glutamine.
NH3 + + P i
Glu
Glu
Figure 8.10
(b) Coupled with ATP hydrolysis, an exergonic reaction
(c) Overall free-energy change

36. ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant.
The recipient molecule is now phosphorylated, energy rich and unstable.
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37. How ATP drives transport and mechanical work Membrane protein Solute
Solute transported
(a) Transport work: ATP phosphorylates
transport proteins
ADP
ATP + P i
Vesicle
Cytoskeletal track
Figure 8.11
ATP
Motor protein
Protein moved
(b) Mechanical work: ATP binds noncovalently
to motor proteins, then is hydrolyzed

38. The Regeneration of ATP
ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)
ADP + P --> ATP
The energy to phosphorylate ADP comes from catabolic reactions in the cell.
The chemical potential energy temporarily stored in ATP drives most cellular work.
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39. The ATP cycle H2O + ADP + ATP P Energy from Energy for cellular
catabolism (exergonic,
energy-releasing
processes)
Energy for cellular
work (endergonic,
energy-consuming
processes)
Figure 8.12
ADP + P i

40. Enzymes speed up the rate of metabolic reactions by lowering energy barriers
A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction
An enzyme is a catalytic protein
Enzymes are specific. Enzymes have a shape-match with their substrates.
Enzymes are named after their substrate and generally end in -ase.
Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction.
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41. Sucrose (C12H22O11) Sucrase Glucose (C6H12O6) Fructose (C6H12O6)
Example of an
enzyme-catalyzed
reaction:
hydrolysis of
sucrose by
sucrase
Sucrose (C12H22O11)
Sucrase
Figure 8.13
Glucose (C6H12O6)
Fructose (C6H12O6)

42. The Activation Energy EA 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 heat from the surroundings.
Enzymes lower the amount of EA needed, so the reaction rate is faster.
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43. Exergonic Reaction A B C D Transition state A B EA C D Free energy
Reactants A B
Figure 8.14 Energy profile of an
∆G < O C D
Products
Progress of the reaction

44. How Enzymes Lower the EA Barrier
Enzymes catalyze reactions by lowering the EA barrier.
Enzymes do not affect the change in free energy (∆G); instead, they hasten / speed up reactions that would occur eventually.
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45. The effect of an enzyme on activation energy
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
Products
Progress of the reaction

46. 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 (Reaction occurs)
The active site is the region on the enzyme where the substrate binds (shape match).
Induced fit of a substrate brings chemical groups of the active site into positions (orientation) that enhance their ability to catalyze the reaction.
For the Cell Biology Video Closure of Hexokinase via Induced Fit, go to Animation and Video Files.
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47. Induced fit between an enzyme and its substrate
Active site
Figure 8.16
Enzyme
Enzyme-substrate
complex
(a)
(b)

48. Catalysis in the Enzyme’s Active Site
In an enzymatic reaction, the substrate binds to the active site of the enzyme
The enzyme’s active site can lower an EA barrier by
Orienting substrates correctly
Straining substrate bonds / tension
Providing a favorable microenvironment
Covalently bonding to the substrate
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49. Enzyme-substrate Complex: Reaction occurs
Substrates enter active site; enzyme
changes shape such that its active site
enfolds the substrates (induced fit). 1
Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds. 2
Substrates
Enzyme-substrate
Complex:
Reaction occurs
Active site can lower EA
and speed up a reaction. 3
Active
site is
available
for two new
substrate
molecules. 6
Figure 8.17 The active site and catalytic cycle of an enzyme
Enzyme 5
Products are
released.
Substrates are
converted to
products. 4
Products

50. Effects of Local Conditions on Enzyme Activity
An enzyme’s activity / reaction rate / can be affected by environmental factors such as:
temperature, pH, and concentration of enzyme or substrate.
Chemicals that specifically influence the enzyme.
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51. Effects of Temperature and pH
Each enzyme has an optimal temperature in which it functions best / forming the maximum enzyme-substrate complexes --> highest rate of reaction.
Each enzyme has an optimal pH in which it functions best / forming the maximum enzyme-substrate complexes --> highest rate of reaction.
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52. Environmental factors affecting enzyme activity
Optimal temperature for
typical human enzyme
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria
Rate of reaction 20 40 60 80
100
Temperature (ºC)
(a) Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach enzyme)
Optimal pH
for trypsin
(intestinal
enzyme)
Figure 8.18
Rate of reaction 1 2 3 4 5 6 7 8 9 10 pH
(b) Optimal pH for two enzymes

53. Cofactors are nonprotein enzyme helpers.
Affect enzyme action
Cofactors:
Cofactors are nonprotein enzyme helpers.
Cofactors may be inorganic (such as a metal in ionic form) or organic.
Organic cofactors are called coenzymes.
Coenzymes include vitamins.
Inorganic cofactors include minerals and salts.
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54. 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 able to bind with the substrate.
Examples of inhibitors include toxins, poisons, pesticides, and antibiotics.
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55. Inhibition of enzyme activity
Substrate
Active site
Competitive
inhibitor
Enzyme
Figure 8.19
Noncompetitive inhibitor
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive inhibition

56. Regulation of enzyme activity helps control metabolism
Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated.
A cell regulates metabolism by:
switching on or off the genes that encode specific enzymes.
or by regulating the activity of enzymes.
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57. Allosteric Regulation of Enzymes
Allosteric regulation may either inhibit or stimulate an enzyme’s activity.
Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site.
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58. 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.
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59. Allosteric Regulation of enzyme activity Figure 8.20 Allosteric enyzme
with four subunits
Active site
(one of four)
Regulatory
site (one
of four)
Activator
Active form
Stabilized active form
Oscillation
Non-
functional
active
site
Inhibitor
Inactive form
Stabilized inactive
form
Figure 8.20
(a) Allosteric activators and inhibitors
Substrate
Inactive form
Stabilized active
form
(b) Cooperativity: another type of allosteric activation

60. Cooperativity is a form of allosteric regulation that can amplify enzyme activity.
In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits.
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61. Identification of Allosteric Regulators
Allosteric regulators are attractive drug candidates for enzyme regulation.
Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses.
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62. Feedback Inhibition: Allosteric Enzyme Regulation
In feedback inhibition, the end product of a metabolic pathway shuts down the pathway. End product builds up and becomes and allosteric inhibitor.
Allosteric inhibitor binds to an allosteric enzyme early in the pathway, shutting down the pathway.
Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed.
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63. Feedback Inhibition Initial substrate (threonine) Active site
available
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Isoleucine
used up by
cell
Intermediate A
Feedback
inhibition
Enzyme 2
Active site of
enzyme 1 no
longer binds
threonine;
pathway is
switched off.
Intermediate B
Enzyme 3
Intermediate C
Figure 8.22 Feedback inhibition in isoleucine synthesis
Isoleucine
binds to
allosteric
site
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)

64. Specific Localization of Enzymes Within the Cell
Structures within the cell help bring order to metabolic pathways.
Some enzymes act as structural components of membranes.
In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria.
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65. Organelles and structural order in metabolism
Mitochondria
Enymes are embedded in the membrane
and are organized in a specific sequence
for an efficient metabolic pathway.
Figure 8.23
1 µm

66. You should now be able to:
Distinguish between the following pairs of terms: catabolic and anabolic pathways; kinetic and potential energy; open and closed systems; exergonic and endergonic reactions.
In your own words, explain the second law of thermodynamics and explain why it is not violated by living organisms.
Explain in general terms how cells obtain the energy to do cellular work.
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67. Explain how ATP performs cellular work.
Explain why an investment of activation energy is necessary to initiate a spontaneous reaction.
Describe the mechanisms by which enzymes lower activation energy.
Describe how allosteric regulators may inhibit or stimulate the activity of an enzyme.
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