What Is Energy?
 Energy is the capacity to do work.
• Synthesizing molecules
• Moving objects
• Generating heat and light
 Types of energy
 First Law of Thermodynamics
• “Energy cannot be created nor destroyed,
but it can change its form.”
• Example: potential energy in gasoline can be
converted to kinetic energy in a car (but the
energy is not lost)
• Kinetic: energy of movement
• Potential: stored energy
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 Second Law of Thermodynamics
• “When energy is converted from one form
to another, the amount of useful energy
decreases.”
• No energy conversion is 100% efficient.
• Example: more potential energy is in the
gasoline than is transferred to the kinetic
energy of the car moving
• Some E is released as heat (a less useful
form ) but the total E is maintained.
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 In order to keep useful energy flowing in
ecosystems where plants and animals
produce more random forms of energy, new
energy must be brought in.
• i.e., in any system, in order to maintain order,
we need to continually input E
 SUN!!!!!!!!!
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 Matter tends to become less organized.
• There is a continual decrease in useful
energy, and a build up of heat and other nonuseful forms of energy.
• Entropy: spontaneous reduction in ordered
forms of energy, and an increase in
randomness and disorder as reactions
proceed
• Example: gasoline is made up of an eightcarbon molecule that is highly ordered
• When broken down to single carbons in CO2,
it is less ordered and more random.
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How Does Energy Flow In Chemical
Reactions?
 Chemical reaction: conversion of one set of
chemical substances (reactants) into
another (products)
A+B
C+D
• Two types of chemical reactions
1) Those that need E input (endergonic)
2) Those that release E (exergonic)
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1
 Endergonic reaction: a reaction that requires energy
input from an outside source; the product(s) contain
more energy than the reactants
Metabolism: All the chemical reactions of the
body
• Energy is used
• E.g., Dehydration synthesis
 Catabolism
• energy releasing (exergonic) decomposition reax
• Breaks apart bonds
• produces smaller molecules
energy
input
Protein
 Anabolism
• energy storing (endergonic) synthesis reax
• requires energy input
• production of protein or fat
• driven by energy that catabolism releases
A.A.
+
A.A.
+
H2 O
Products
reactants
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 Exergonic reaction: a reaction that releases E; the
products contain less energy than the reactants
Endergonic Reactions
 Photosynthesis requires energy.
• Energy is released
• Glucose + Oxygen have more energy than the reactants
energy
released
energy
C6H12O6 + 6 O2
(glucose) (oxygen)
6 CO2
+
(carbon
dioxide)
6 H2O
(water)
+
reactants
+
products
Fig. 5-5
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 Exergonic reaction: Burning glucose releases
energy .
Endergonic and Exergonic Reactions
• Mitochondria
• Produces ATP
 Endergonic reactions
require input of energy to
proceed
• Products contain more
energy than reactants
• Synthesis Reax
energy
released
C6H12O6
(glucose)
+
6 O2
 Exergonic reactions
release energy as they
proceed
• Products contain less
energy than reactants
• Decomposition Reax
(oxygen)
6 CO2
(carbon
dioxide)
+
6 H2O
(water)
Fig. 5-4
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4-26
2
Coupled Reactions: ATP
 Cells require constant
inputs of energy to buck
entropy and remain highly
organized
 Do this by coupling
endergonic reactions to
exergonic reactions
 ATP is the principal energy carrier in cells.
• ATP stores energy in its phosphate bonds
• ATP’s phosphate bonds can be broken yielding
ADP, phosphate, and energy.
• This energy is transferred to an energy-requiring
reaction (endergonic reaction)
Use ATP
1. Most endergonic
reactions in body make
ATP
2. An exergonic reaction
breaks down ATP - the
universal energy carrier
Make ATP
4-27
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 ATP is made from ADP (adenosine diphosphate) and
phosphate plus energy released from an exergonic
reaction (e.g., glucose breakdown) in a cell.
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 Breakdown of ATP releases energy.
energy
A
energy
P
P
P
ATP
A
P
P
A
P
P
ADP
A
P
ADP
P
+
P
phosphate
P
+
P
phosphate
ATP
Fig. 5-7
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Fig. 5-8
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Summary:
 Coupled reactions
 To summarize:
• Exergonic reactions (e.g., glucose breakdown) drive
endergonic reactions (e.g., the conversion of ADP to
ATP).
• ATP moves to different parts of cell and is broken
down exergonically to liberate its energy to drive
endergonic reactions.
glucose
A
exergonic
(glucose breakdown)
P
P
P
protein
endergonic
(ATP synthesis)
exergonic
(ATP breakdown)
endergonic
(protein synthesis)
CO2 + H2O + heat
A
P
P
+
P
amino
acids
Fig. 5-9
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How Energy Carried Between Coupled Reactions
 Common electron carriers are NAD + and FAD.
 Electron carriers also transport energy
within cells.
• Besides ATP, other carrier molecules
transport energy within a cell.
• Electron carriers capture energetic electrons
transferred by some exergonic reaction.
• Energized electron carriers then donate these
energy-containing electrons to endergonic
reactions.
high-energy
reactants
energized
NADH
e–
e–
depleted
high-energy
products
NAD+ + H+
low-energy
products
low-energy
reactants
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 All reactions require an initial input of energy.
• The initial energy input to a chemical reaction is
called the activation energy.
 Metabolic pathways: sequence of cellular reactions
(e.g., photosynthesis and glycolysis)
Initial reactant
Final products
Intermediates
Activation energy needed
to ignite glucose
high
PATHWAY 1
Fig. 5-11
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A
B
C
D
E
Energy level of reactants
PATHWAY 2
F
energy
content
of
molecules
G
Activation
energy
captured
from
sunlight
glucose
glucose + O2
CO2 + H2O
Energy level of reactants
CO2 + H2O
low
progress of reaction
(a) Burning glucose (sugar): an exergonic reaction
progress of reaction
(b) Photosynthesis: an endergonic reaction
Fig. 5-12
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Fig. 5-6
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Enzymes!!
How Cells Control Their Metabolic Reactions
 At body temperature, many spontaneous reactions
proceed too slowly to sustain life.
• A reaction can be controlled by controlling its
activation energy (the energy needed to start the
reaction).
• At body temperature, reactions occur too slowly
because their activation energies are too high.
• Molecules called catalysts (enzymes) help lower
the activation energy needed for a reax
 Enzymes are catalysts that reduce activation energy
level.
• They speed up a chemical reactions
high
Activation energy
without catalyst
energy
content
of
molecules
Activation energy
with catalyst
reactants
products
low
progress of reaction
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4
Enzyme Structure and Action
 Three important principles about all
catalysts
 Substrate approaches active site on enzyme molecule
1. Enzymes speed up chemical reactions.
- reactions that would occur anyway, if their
activation energy could be surmounted.
2. Enzymes are specific – work on specific
molecules to produce a specific product
3. Catalysts are not altered by the reaction
- can be reused over and over
 Substrate binds to active site forming enzyme-substrate
complex
• highly specific fit – enzyme-substrate specificity
 Reaction products released
 Enzyme remains unchanged and is ready to repeat the
process
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2-26
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Enzymatic Reaction Steps
 How does an enzyme catalyze a reaction?
Sucrose (substrate)
1 Enzyme and
substrate
• Substrates enter the enzyme’s active site.
• Substrates enter an enzyme’s active site,
changing both of their shapes.
• The chemical bonds are altered in the
substrates, promoting the reaction.
• The substrates change into a new form that
will not fit the active site, and so are released.
O
Active site
Sucrase (enzyme)
2 Enzyme–substrate
complex
O
Glucose
Fructose
3 Enzyme
and reaction
products
Figure 2.27
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2-28
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How enzymes work
substrates
active site
of enzyme
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enzyme
1 Substrates enter
the active site in a
specific orientation
3 The substrates, bonded
together, leave the enzyme;
the enzyme is ready for a
new set of substrates
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2 The substrates and
active site change shape,
promoting a reaction
between the substrates
Fig. 5-14
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5
 Cells regulate metabolism by controlling enzymes.
• Allosteric regulation can increase or decrease
enzyme activity.
• In allosteric regulation, an enzyme’s activity is
modified by a regulator molecule.
• The regulator molecule binds to a special
regulatory site on the enzyme (separate from the
enzyme’s active site).
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 Binding of regulator molecule modifies the active site
on enzyme, causing the enzyme to become more or
less able to bind substrates.
 i.e., allosteric regulation can promote or inhibit enzyme
activity
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 Enzyme structure
 Allosteric inhibition
substrate
An allosteric regulator
molecule causes the
active site to change
shape, so the substrate
no longer fits
Many enzymes have
both active sites and
allosteric regulatory
sites
active site
enzyme
allosteric
regulatory site
(a) Enzyme structure
Fig. 5-15a
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Allosteric inhibition
allosteric
regulator
molecule
Fig. 5-15b
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Enzymatic Action: Important Points!!
 Reusability of enzymes
 Astonishing speed
• one enzyme molecule can consume millions of
substrate molecules per minute
 Factors that change enzyme shape
• pH and temperature
 Competitive inhibition
A competitive inhibitor molecule
occupies the active site and
blocks entry of the substrate
Fig. 5-16
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2-36
6
 Metabolic pathways: sequence of cellular reactions
(e.g., photosynthesis and glycolysis)
Initial reactant
PATHWAY 1
A
B
enzyme 1
D
C
enzyme 2
enzyme 3
enzyme 5
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E
enzyme 4
G
F
PATHWAY 2
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Final products
Intermediates
enzyme 6
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7