Ground Rules of Metabolism
Chapter 6 Part 1
Impacts, Issues:
A Toast to Alcohol Dehydrogenase
 In the liver, alcohol dehydrogenase helps break
down toxic alcohols, but at the expense of liver
function and energy metabolism
Fig. 6-1a, p. 92
Fig. 6-1b, p. 92
alcohol dehydrogenase
Fig. 6-1b, p. 92
6.1 Energy and the World of Life
 Assembly of the molecules of life starts with
energy input into living cells
Energy Disperses
 First law of thermodynamics
• Energy is neither created nor destroyed, but can
be transferred from one form to another
 Second law of thermodynamics
• Entropy (a measure of dispersal of energy in a
system) increases spontaneously
• The entropy of two atoms decreases when a
bond forms between them (endergonic reaction)
Motion: A Form Of Energy
Entropy
entropy
heat
energy
time
Fig. 6-3, p. 94
entropy
heat
energy
time
Stepped Art
Fig. 6-3, p. 94
One Way Flow of Energy
 The total amount of energy available in the
universe to do work is always decreasing
• Each time energy is transferred, some energy
escapes as heat (not useful for doing work)
 On Earth, energy flows from the sun, through
producers, then consumers
• Living things need a constant input of energy
Energy Conversion
 Only about 10% of the energy in food goes
toward building body mass, most is lost in
energy conversions
Energy Flow
energy input,
mainly from
sunlight
PRODUCERS
plants and other selffeeding organisms
ENERGY IN
Sunlight energy reaches
environments on Earth.
Producers of nearly all
ecosystems secure some
and convert it to stored
forms of energy. They
and all other organisms
convert stored energy to
forms that can drive
cellular work.
nutrient
cycling
CONSUMERS
animals, most fungi,
many protists, bacteria
energy output,
mainly heat
ENERGY OUT
With each conversion,
there is a one-way flow of
a bit of energy back to the
environment. Nutrients
cycle between producers
and consumers.
Fig. 6-5, p. 95
6.2 Energy in the Molecules of Life
 All cells store and retrieve energy in chemical
bonds of the molecules of life
 Free energy
• The amount of energy in a molecule that is
available to do work
Energy In, Energy Out
 Reaction
• A chemical change that occurs when atoms, ions,
or molecules interact
 Reactant
• Atoms, ions, or molecules that enter a reaction
 Product
• Atoms, ions, or molecules remaining at the end of
a reaction
Chemical Reactions
Reactants
2 H2
O2
+
(hydrogen)
(oxygen)
4 hydrogen
atoms
2 oxygen
atoms
Products
2 H2O
(water)
4 hydrogen atoms
+ 2 oxygen atoms
Fig. 6-6, p. 96
Reactions Require or Release Energy
 We can predict whether a reaction requires or
releases energy by comparing the bond
energies of reactants with those of products
 Endergonic (“energy in”)
• Reactions that require a net input of energy
 Exergonic (“energy out”)
• Reactions that end with a net release of energy
Endergonic and Exergonic Reactions
Free energy
2 H2 + O2
b Energy out
a Energy in
2 H2O
Fig. 6-7, p. 96
Why the World Doesn’t Go Up in Flames
 Activation energy
• The minimum amount of energy needed to get a
reaction started
• Some reactions require a lot of activation energy,
others do not
Activation Energy
Reactants:
2 H2 + O 2
Free energy
Activation energy
Difference in free
energy between
reactants and products
Products: 2 H2O
Time
Fig. 6-8, p. 97
ATP – The Cell’s Energy Currency
 ATP (adenosine triphosphate)
• A nucleotide with three phosphate groups
• Transfers a phosphate group and energy to other
molecules
 Phosphorylation
• A phosphate-group transfer
• ADP binds phosphate in an endergonic reaction
to replenish ATP (ATP/ADP cycle)
ATP
Fig. 6-9a, p. 97
adenine
three phosphate
groups
ribose
A Structure of ATP (adenine triphosphate).
Fig. 6-9a, p. 97
Fig. 6-9b, p. 97
adenine
ribose
AMP
P
ADP
P
ATP
P
B The molecule is called ATP when it has three phosphate groups.
After it loses one phosphate group, the molecule is called ADP
(adenosine diphosphate); after losing two phosphate groups it is
called AMP (adenosine monophosphate).
Fig. 6-9b, p. 97
Fig. 6-9c, p. 97
ATP
endergonic reactions
reconstitute ATP
ATP drives other
endergonic reactions
ADP + phosphate
C ATP forms when an endergonic reaction drives the covalent bonding
of ADP and phosphate. ATP energy is transferred to another molecule
along with a phosphate group, and ADP forms again. Energy from such
transfers drives endergonic reactions that are the stuff of cellular work,
such as active transport and muscle contraction.
Fig. 6-9c, p. 97
Animation: Structure of ATP
Animation: Mitochondrial chemiosmosis
6.1-6.2 Key Concepts:
Energy Flow in the World of Life
 Energy tends to disperse spontaneously; each
time energy is transferred, some of it disperses
 Organisms maintain their organization only by
continuously harvesting energy
 ATP couples reactions that release usable
energy with reactions that require energy
6.3 How Enzymes
Make Substances React
 Enzyme
• A catalyst that makes a specific reaction occur
much faster than it would on its own
• Enzymes are not consumed or changed by
participating in a reaction
• Most are proteins, some are RNA
 Substrate
• The specific reactant acted upon by an enzyme
How Enzymes Work
 Enzymes lower the activation energy required to
bring on the transition state, when substrate
bonds break and reactions run spontaneously
 Active sites
• Locations on the enzyme molecule where
substrates bind and reactions proceed
• Complementary in shape, size, polarity and
charge to the substrate
Active Site of an Enzyme
Fig. 6-10a, p. 98
Fig. 6-10a (left), p. 98
active site
enzyme
reactant(s)
product(s)
A Hexokinase is an enzyme that attaches
phosphate groups to glucose and other
sugars with the help of ATP.
Fig. 6-10a (right), p. 98
Fig. 6-10b, p. 98
B A glucose and a phosphate
meet in hexokinase’s active site, the
microenvironment of which encourages these molecules to react.
Fig. 6-10b, p. 98
Fig. 6-10c, p. 98
C The glucose has bonded with
the phosphate. The product of this
reaction, glucose-6-phosphate, is
shown leaving the active site.
Fig. 6-10c, p. 98
Activation Energy
Transition state
Free energy
Activation energy
without enzyme
Activation energy
with enzyme
Reactants
Products
Time
Fig. 6-11, p. 98
Transition state
Free energy
Activation energy
without enzyme
Activation energy
with enzyme
Reactants
Products
Time
Stepped Art
Fig. 6-11, p. 98
Animation: Activation energy
Mechanisms of
Enzyme-Mediated Reactions
 Binding at enzyme active sites may bring on the
transition state by four mechanisms
• Helping substrates get together
• Orienting substrates in positions that favor
reaction
• Inducing a fit between enzyme and substrate
(induced-fit model)
• Shutting out water molecules
Effects of Temperature, pH, and Salinity
 Raising the temperature boosts reaction rates by
increasing a substrate’s energy
• But very high temperatures denature enzymes
 Each enzyme has an optimum pH range
• In humans, most enzymes work at ph 6 to 8
 Salt levels affect the hydrogen bonds that hold
enzymes in their three-dimensional shape
Enzymes and Temperature
Enzyme activity
normal
tyrosinase
temperaturesensitive
tyrosinase
20°C (68°F)
30°C (86°F)
40°C (104°F)
Fig. 6-12a, p. 99
Enzymes and pH
Fig. 6-13a, p. 99
glycogen
phosphorylase
Enzyme activity
trypsin
pepsin
1
a
2
3
4
5
6
7
8
9
10
11
pH
Fig. 6-13a, p. 99
Fig. 6-13b, p. 99
Help from Cofactors
 Cofactors
• Atoms or molecules (other than proteins) that are
necessary for enzyme function
• Example: Iron atoms in catalase
 Coenzymes
• Organic cofactors such as vitamins
• May become modified during a reaction
Catalase and Cofactors
 Catalase is an antioxidant that neutralizes free
radicals (atoms or molecules with unpaired
electrons that attack biological molecules)
 Catalase works by holding a substrate molecule
close to one of its iron atoms (cofactors)
• Iron pulls on the substrate’s electrons, bringing on
the transition state
6.3 Key Concepts:
How Enzymes Work
 On their own, reactions proceed too slowly to
sustain life
 Enzymes tremendously increase the rate of
metabolic reactions
 Environmental factors such as temperature, salt,
and pH influence enzyme function