Overview: The Energy of Life
• The living cell is a miniature chemical factory
where thousands of chemical reactions occur
• The cell moves energy around and changes its
form as it applies energy to perform work:
energy transduction.
• Examples: Some organisms light energy to
chemical energy, as in photosynthesis. Others
convert chemical energy to light, as in
bioluminescence
Concept 8.1: An organism’s metabolism transforms
matter and energy, subject to the laws of
thermodynamics
• One way to look at metabolism is to examine the
chemical reactions that occur.
• Reactions are organized in a series called a
metabolic pathway.
• A metabolic pathway begins with a specific
molecule and ends with a product
• The reactions need a catalyst to speed them up
• Each reaction is catalyzed by a specific enzyme
Diagram of a simple metabolic pathway
Enzyme 1
A
Reaction 1
Starting
molecule
Enzyme 2
B
Enzyme 3
C
Reaction 2
D
Reaction 3
Product
• Catabolic pathways release energy by
breaking down complex molecules into simpler
compounds (catabolism)
• Cellular respiration, the breakdown of glucose
in the presence of oxygen, is an example of a
pathway of catabolism
• Anabolic pathways consume energy to build
complex molecules from simpler ones
(anabolism)
• The synthesis of protein from amino acids is an
example of anabolism
Energy flow and transfer in metabolism
• Another way to look at metabolism is to follow
energy flow (bioenergetics)
• Energy is defined as the capacity to cause
change
• Energy exists in various forms, some of which
can perform work
Main Types of Energy in Biological Systems
• 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
Energy can be converted from one form to anotherthis is called energy transduction
Two Key Laws of Energy Transduction
• Thermodynamics is the study of energy
transformations
• 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
• During every energy transfer or transformation,
some energy is unusable (often lost as waste
heat)
• TANSTAAFL
• According to the second law of
thermodynamics:
– Every energy transfer or transformation
increases the entropy (disorder) of the
universe
Living systems have to obey these two laws.
• They survive because they are open systems.
• A closed system is isolated from its
surroundings
• But in an open system, energy and matter can
be transferred between the system and its
surroundings
Organisms import and export energy
(including entropy) and matter.
• The functioning of individual cells does not
violate the second law.
• The evolution of more complex organisms does
not violate the second law of thermodynamics
Entropy (disorder) may decrease within the
boundaries of a cell or an organism or a species
as long as the universe’s total entropy increases
Concept 8.2: The free-energy change of a reaction
tells us whether or not the reaction occurs
spontaneously
• To understand how living systems operate,
biologists want to know which reactions occur
spontaneously and which require input of
energy
• To do so, they need to determine energy
changes that occur in chemical reactions
• 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
process is related to the change in enthalpy, or
change in total energy (∆H) after the entropy
change (∆S) has been subtracted:
∆G = ∆H – T∆S
• Only processes with a negative ∆G are
spontaneous
• Spontaneous negative ∆G processes can be
harnessed to perform work
Overall free energy content(G not ∆G) is a
measure of a system’s instability, its tendency to
change to a more stable state
• During a spontaneous change, free energy
decreases and the stability of a system
increases
• Equilibrium is a state of maximum stability so a
process always has a -∆G as it moves toward
equilibrium
• A process is spontaneous and can perform
work only when it is moving toward equilibrium
Examples
Spontaneous
change
(a) Gravitational motion
Spontaneous
change
(b) Diffusion
Spontaneous
change
(c) Chemical reaction
The concept of free energy can be applied to the
chemistry of life’s processes-allows definition of 2
types of reactions
• An exergonic reaction proceeds with a net
release of free energy and is spontaneous
• An endergonic reaction absorbs free energy
from its surroundings and is nonspontaneous
Reactants
Free energy
Amount of
energy
released
(∆G < 0)
Energy
Products
Progress of the reaction
(a) Exergonic reaction: energy released
Products
Free energy
Graphic
Explanation
Amount of
energy
required
(∆G > 0)
Energy
Reactants
Progress of the reaction
(b) Endergonic reaction: energy required
∆G < 0
(a) An isolated hydroelectric system
∆G = 0
∆G < 0
(b) An open hydroelectric system
∆G < 0
∆G < 0
∆G < 0
(c) A multistep open hydroelectric system: cells and organisms
have a series of molecular machines to capture some of
the energy released
Reactions in a closed system eventually reach
equilibrium and then do no work
• But cells and organisms are not in equilibrium;
they are open systems experiencing a constant
flow of materials and energy
Something that is alive is
never at equilibrium
Concept 8.3: ATP powers cellular work by
coupling exergonic reactions to endergonic
reactions
• 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 ATPbecause ATP hydrolysis is highly exergonic
The Structure and Hydrolysis of ATP
• ATP (adenosine triphosphate) is the cell’s
energy shuttle
• ATP is composed of ribose (a sugar), adenine
(a nitrogenous base), and three phosphate
groups
Structure of ATP
Adenine (a base) + ribose (a sugar) = adenosine
+3 phosphates = triphosphate
Adenine
Phosphate groups
Ribose
• The bonds between the phosphate groups of
ATP’s tail (“phosphate bonds”) can be broken
by hydrolysis
• Energy is released from ATP when the terminal
phosphate bond is broken (rarely the other
bonds)
• The difference between products and reactants
under cellular conditions is the key
P
P
P
Adenosine triphosphate (ATP)
Hydrolysis of
ATP to ADP
and Pi
Pi
Delta G varies
with
conditions
H2O
+
Inorganic phosphate
P
P
+
Adenosine diphosphate (ADP)
Energy
Cellular work is powered primarily 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
• One of the ways that ATP shuttles energy is by
transfer of the terminal phosphate group
(phosphorylation reaction)
Example of
endergonic
and exergonic
reactions
coupled by
ATP
NH2
Glu
Glutamic
acid
NH3
+
∆G = +3.4 kcal/mol
Glu
Ammonia
Glutamine
(a) Endergonic reaction
1 ATP phosphorylates
glutamic acid,
making the amino
acid less stable.
P
+
Glu
ATP
Glu
+ ADP
NH2
2 Ammonia displaces
the phosphate group,
forming glutamine.
P
Glu
+
NH3
Glu
+ Pi
(b) Coupled with ATP hydrolysis, an exergonic reaction
(c) Overall free-energy change
ATP is a renewable resource that is regenerated by addition
of a phosphate group to adenosine diphosphate (ADP)
ATP + H2O
Energy from
catabolism (exergonic,
energy-releasing
processes)
ADP + P i
Energy for cellular
work (endergonic,
energy-consuming
processes)
Endergonic and exergonic reactions are linked or “coupled” through ATP
NOTE CARD MARCH 4
Review:
Explain the difference between free energy and entropy.
Concept 8.4: Enzymes speed up metabolic
reactions by lowering energy barriers
• Even spontaneous reactions do not
automatically happen quickly
• A catalyst is a chemical agent that speeds up
a reaction but is not consumed by the reaction
• An enzyme is a catalytic protein
• Enzymes increase the rate of metabolic
reactions.
• Hydrolysis of sucrose by the enzyme sucrase
is an example of an enzyme-catalyzed reaction
Sucrose (C12H22O11)
Sucrase
Glucose (C6H12O6)
Fructose (C6H12O6)
• 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)
• Enzymes speed up reactions by lowering the
energy of activation
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
∆G is unaffected
by enzyme
Products
Progress of the reaction
• 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
• Enzymes change reaction rate-not reaction
energy
• 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
• Induced fit of a substrate brings chemical
groups of the active site into positions that
enhance their ability to catalyze the reaction
• Induced fit helps the reactants move into the
transition state
1 Substrates enter active site; enzyme
changes shape such that its active site
enfolds the substrates (induced fit).
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Substrates
Enzyme-substrate
complex
3 Active site can lower EA
and speed up a reaction.
6 Active
site is
available
for two new
substrate
molecules.
Enzyme
5 Products are
released.
4 Substrates are
converted to
products.
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html
• An enzyme’s activity can be affected by
general environmental factors
– temperature or pH
– Chemicals that specifically influence the
enzyme
– Availability of substrate
• Each enzyme has a temperature at which it
functions best
• This is called the temperature optimum for the
enzyme
• Each enzyme has an optimal pH at which it
functions best
• This is called the pH optimum for the enzyme
Temperature and pH affect an enzyme by altering its shape or configuration
Some chemicals can specifically influence
enzymes by aiding the reaction
• Cofactors are nonprotein enzyme helpers
• The provide additional chemical flexibility and
they take part in the enzyme’s reaction
• Cofactors may be inorganic (such as a metal in
ionic form) or organic
• An organic cofactor is called a coenzyme
• Frequently, coenzymes are produced from
vitamins in the diet
Some chemicals can specifically influence
enzymes without participating in the
reaction
• 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- alpha amanitin from
the death cap mushroom is a specific example
Competitive and Noncompetitive inhibition
Competitive-blocks the active site
“If the substrate won’t fit-then the enzyme must quit”
Substrate
Active site
Competitive
inhibitor
Enzyme
Noncompetitive inhibitor
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive inhibition
Noncompetitive-works by changing the conformation of the enzyme
Some inhibitors are permanent and some are reversible
Concept 8.5: Regulation of enzyme activity helps
control metabolism
• Chemical chaos would result if a cell’s
metabolic pathways were not tightly regulated
• Metabolic reactions and pathways are
conrolled by enzymes
• Therefore living systems can regulate their
metabolism by regulating their enzymes
• A fundamental way to turn off an enzyme
reaction is by not having the enzyme around.
• Turning off enzyme synthesis = enzyme
repression; turning it on = enzyme induction
(both are slow)
• Another fundamental way to regulate is to
control the amount of substrate
• No substrate = no reaction; increasing
substrate = increasing reaction-but only up to a
point where the enzyme is working as fast as it
can: saturation.
1. Induction/repression = slow
2. Substrate concentration = limited
3. pH and temperature = not always practical or
possible
4. Inhibitors = hard to control and may not be
reversible
• Better method needed for quick and flexible
control of enzyme reactions
Allosteric regulation: control of enzyme
reaction by reversible shape change
• Allosteric regulation may either inhibit or
stimulate an enzyme’s activity
(positive/negative allosteric regulation)
• Allosteric regulation occurs when a regulatory
molecule binds to a protein at one site and
affects the protein’s function at another site
Cooperativity is one important form of allosteric
regulation that can amplify enzyme activity
Substrate
Inactive form
Stabilized active
form
(b) Cooperativity: another type of allosteric activation
In cooperativity, binding by a substrate to one
active site stabilizes favorable conformational
changes at all other subunits
• Feedback inhibition, is a metabolic control
strategy that usually involves allosteric
regulation
• In feedback inhibition, the end product of a
metabolic pathway shuts down the pathway
• Feedback inhibition prevents a cell from
synthesizing more product than the cell needthis avoids wasting of chemical resources
Initial substrate
(threonine)
Example of
feedback
inhibition
Active site
available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Isoleucine
binds to
allosteric
site
Enzyme 2
Active site of
enzyme 1 no
longer binds Intermediate B
threonine;
pathway is
Enzyme 3
switched off.
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
Animations
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter
2/animation__how_enzymes_work.html
• http://www.shmoop.com/energy-flowenzymes/resources.html
• http://www.youtube.com/watch?v=PILzvT3spCQ