Microfilaments differ from
microtubules in that microfilaments
A) are larger than microtubules.
B) are found only in plants whereas microtubules are
found in plants and animal cells.
C) are mainly composed of actin whereas
microtubules are composed of tubulin.
D) anchor organelles, whereas microtubules primarily
function to help cells change shape and move.
E) form the inner core of cilia and flagella whereas
microtubules regulate metabolism.
Cellular Energetics:
Thermodynamics, ATP, and Enzyme
catalysis
Campbell Biology
Chapter 5
Energy and Thermodynamics
Energy is the capacity to do work
• There are many forms
of energy:
–
–
–
–
Kinetic energy
Potential energy
Chemical energy
Electrical energy
All Living Things Require and Consume
Energy
• We get our energy from
food
• Ultimate source of
energy for all life on
earth is the sun
The First Law of Thermodynamics
• Energy cannot be
created or destroyed
• The amount of energy
in the universe is
constant
• Energy can be
interconverted from
one form to another:
– Potential energy
– Kinetic energy
– Radiant energy
Potential energy
• Energy is the ability to
do work
• Potential Energy of
position
• Gravitational potential
energy
• Chemical potential
energy
Kinetic energy
• Energy of motion
• KE= 1/2mv2
• Temperature is a
measure of molecular
kinetic energy
The 1st Law of Thermodynamics:
Energy can be interconverted from one
form to another
More energy interconversions
The 2nd Law of Thermodynamics
The Law of Entropy
• Interconversions of
energy are never 100%
efficient
• Entropy!
• Entropy is a measure of
disorder (i.e. chaos,
randomness)
• Each interconversion of
energy involves loss of
usable energy
:
Entropy in Action
Biochemical reactions are inefficient
The price of minimizing entropy is the
constant expenditure of free energy
Closed systems will deplete
themselves of usable (free) energy
• Given a finite amount of
energy, each energy
interconversion will
result in an everincreasing amount of
unusable energy
(entropy)
Recognizing Entropy in the world
Which system has more entropy?
A
B
Can living systems reduce entropy?
Recognizing Enthalpy
B
Enthalpy = Energy in chemical bonds
Which systems have more Enthalpy?
These?

Or
these?

Biochemical reactions are
spontaneous only if ∆G is negative
• Reactions which release
energy are exergonic
• Reactions which require
energy are endergonic
∆ G = ∆H - T∆S
• Only exergonic processes
with a negative ∆G are
spontaneous
• Spontaneous processes
can be harnessed to
perform work
If ΔG < 0, the reaction is spontaneous
(it will happen)
C6H12O6(s) + 6O2(g)  6CO2(g)+ 6H2O(l)
Will the Reaction happen?
Well, is heat given off?
Does entropy increase?
G = ∆H - T∆S
∆H = enthalpy (heat in chemical bonds)
∆S= Degree of entropy (chaos) created by Rxn
T= Temperature at which Rxn occurs
Important: Spontanous ≠ fast
LE 5-2b
Heat
Chemical reactions
Carbon dioxide
Glucose
ATP
ATP
Water
Oxygen
Energy for cellular work
Which of these diagrams depicts an
endergonic reaction?
Reactants
Energy required
Reactants
A
Amount of
energy
required
Potential energy of molecules
Potential energy of molecules
Products
Amount of
energy
released
Energy released
Products
B
LE 8-7a
G < 0
A closed hydroelectric system
G = 0
LE 8-7c
G < 0
G < 0
G < 0
A multistep open hydroelectric system
In living things, a state of equilibrium
most often means ___________.
A)
B)
C)
D)
E)
Efficiency is optimized
The reaction is Endothermic
Enthalpy is increased
Entropy is minimized
You are dead
ATP
A steer must eat over 100 pounds of
grain to gain less than 10 pounds of
• muscle tissue. This illustrates
A) the first law of thermodynamics.
B) the second law of thermodynamics.
C) that some energy is destroyed in every energy
conversion.
D) that energy transformations are typically
100% efficient.
E) None of the choices are correct.
Living cells manage to perform
endergonic activities
• How is this possible?
ATP hydrolysis can be coupled to endergonic reactions
to power cellular work
• A cell does three main kinds of work:
– Mechanical
– Transport
– Chemical
• To do work, cells manage energy resources by energy
coupling, the use of an exergonic process to drive an
endergonic one
The Structure and Hydrolysis of ATP
• ATP (adenosine triphosphate) is the cell’s energy
shuttle
• ATP provides energy for cellular functions
• ATP is a nucleic acid monomer
ATP is the energy currency of all living
things
Adenine
Phosphate groups
Ribose
ATP: Adenosine Triphosphate
LE 8-9
P
P
P
Adenosine triphosphate (ATP)
H2O
Pi
Inorganic phosphate
+
P
P
Adenosine diphosphate (ADP)
+
Energy
Phosphorylation can change the
conformation of proteins
LE 8-10
Anabolic (building up) reactions are usually endergonic
NH2
Glu
+
NH3
Ammonia
Glutamic
acid
G = +3.4 kcal/mol
Glu
Glutamine
Breakdown of ATP is exergonic
ATP
+
H2O
ADP
+
P i
G = –7.3 kcal/mol
Coupled reactions: Overall G is negative;
together, reactions are spontaneous G = –3.9 kcal/mol
How ATP Performs Work
• ATP drives endergonic reactions by phosphorylation,
transferring a phosphate group to some other
molecule, such as a reactant
• The recipient molecule is now phosphorylated
Three types of
cellular work are
powered by ATP
hydrolysis
•Mechanical
•Transport
•Chemical
The Regeneration of ATP
• ATP is regenerated by addition of a phosphate group to
ADP
• The energy to phosphorylate ADP comes from food
• The chemical potential energy temporarily stored in
ATP drives most cellular work
LE 8-12
ATP
Energy for cellular work
(endergonic, energyconsuming processes)
Energy from catabolism
(exergonic, energyyielding processes)
ADP +
P
i
Enzymes
At which level of protein structure
are interactions between R groups
most important?
A) primary
B) secondary
C) tertiary
D) quaternary
E) the R groups are not related to the overall
structure of a protein
Sugar is an energy-rich molecule
• Breakdown of sugar is
spontaneous
• C6H12O6(s) + 6O2(g) 
6CO2(g)+ 6H2O(l)
Wood and paper are made of
cellulose
• Cellulose is a polymer of
glucose
• Why doesn’t our jar of
sugar burst into flame?
Exergonic reactions still require
activation energy
• Spontaneous ≠ fast
• Ea is dependent on
temperature
• At high temperatures,
reactions happen faster
Jumping bean analogy
• Molecules are like
jumping beans
• Temperature ≈ height of
jump
• Living things cannot wait
for a good jump
• After a long time, where
will the beans be?
• All of them?
• Will they ever stop
jumping?
Living things can use enzymes to
speed up reactions
• Enzymes speed up
reactions by lowering
energy of activation
• They are catalysts
Catalysts speed up reactions
• Platinum is used in
catalytic converters
• 2CO + 02 2CO2
• Catalysts are not
consumed in a
reaction
• They cannot add
energy to a reaction
Enzymes are protein catalysts
• Catalysts- things added to
chemical reactions which
speed up those reactions
• Catalysts are not consumed
in a reaction
• Catalysts cannot add energy
to a reaction
• -ase: The enzyme suffix
Catalase
Enzymes can dramatically lower the energy
of activation for a reaction
no enzyme
with enzyme
Ea
Energy
Ea
reactants
products
Reaction Course
Note that the equilibrium of the reaction is unaffected
12
How enzymes work
 Structure aids
function
 An active site
naturally fits
substrate
 Enzyme
specificity
depends on
shape
Substrate Binding and Reaction
Some important Enzymes
Cellulase
ATP
synthase

Nitrogenase
Catalysis in the Enzyme’s Active Site
• In an enzymatic reaction, the substrate binds to the
active site
• The active site can lower an EA barrier by
– Orienting substrates correctly
– Straining substrate bonds
– Providing a favorable microenvironment
– Covalently bonding to the substrate
LE 5-6
Enzyme available
with empty active
site
Active site
Substrate
(sucrose)
Substrate binds
to enzyme with
induced fit
Glucose
Enzyme
(sucrase)
Fructose
H2O
Products are
released
Substrate is
converted to
products
Factors Affecting Enzyme Activity
1.
2.
3.
4.
Salts
Temperature
pH
Inhibitors and Activators
Effects of Temperature and pH
• Each enzyme has an
optimal temperature in
which it can function
• Each enzyme has an
optimal pH in which it
can function
• Tertiary structure can
be radically altered by
changes in pH
LE 8-18
Optimal temperature for
typical human enzyme
0
Optimal temperature for
enzyme of thermophilic
(heat-tolerant
bacteria)
40
60
Temperature (°C)
20
80
100
Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach enzyme)
0
1
2
3
Optimal pH
for trypsin
(intestinal
enzyme)
4
5
pH
Optimal pH for two enzymes
6
7
8
9
10
Enzyme Inhibition
• Competitive inhibitors
bind to the active site of
an enzyme, competing
with the substrate
• Noncompetitive
(allosteric) inhibitors
bind to another part of
an enzyme, causing the
enzyme to change shape
(allostery) and making
the active site less
effective
Many drugs are enzyme inhibitors
• Protease inhibitors fight
HIV
Can enzymes catalyze endothermic
reactions?
• If so, how?
• If not, why not?
Inhibition of an enzyme is irreversible
when
A) a competitive inhibitor is involved.
B) a noncompetitive inhibitor is involved.
C) the shape of the enzyme is changed.
D) bonds form between inhibitor and enzyme.
E) None of the choices are correct.