Transcript Document

Energy and Metabolism
Chapter 6
Flow of Energy
Energy: the capacity to do work
-kinetic energy: the energy of motion
-potential energy: stored energy
Energy can take many forms:
mechanical
electric current
heat
light
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Flow of Energy
Most forms of energy can be converted to
heat energy.
Heat energy is measured in kilocalories.
One calorie = the amount of heat required
to raise the temp of water by 1oC
1 kilocalorie (kcal) = 1000 calories
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Flow of Energy
Potential energy stored in chemical bonds
can be transferred from one molecule to
another by way of electrons.
oxidation: loss of electrons
reduction: gain of electrons
redox reactions are coupled to each other.
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Laws of Thermodynamics
First Law of Thermodynamics – energy
cannot be created or destroyed
-energy can only be converted from one
form to another
For example:
sunlight energy
chemical energy
photosynthesis
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Laws of Thermodynamics
Second Law of Thermodynamics: disorder
is more likely than order
entropy: disorder in the universe
The 2nd Law of Thermodynamics states that
entropy is always increasing.
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Laws of Thermodynamics
Free energy: the energy available to do
work
-denoted by the symbol G (Gibb’s free
energy)
enthalpy: energy contained in a molecule’s
chemical bonds
free energy = enthalpy – (entropy x temp.)
G = H - TS
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Laws of Thermodynamics
• Chemical reactions can create changes in
free energy:
 DG = DH - T DS
• When products contain more free energy
than reactants – DG is positive.
• When reactants contain more free energy
than products – DG is negative.
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Laws of Thermodynamics
Chemical reactions can be described by the
transfer of energy that occurs:
endergonic reaction: a reaction requiring
an input of energy
- DG is positive
exergonic reaction: a reaction that
releases free energy
- DG is negative
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Laws of Thermodynamics
Most reactions require some energy to get
started.
activation energy: extra energy needed to
get a reaction started
-destabilizes existing chemical bonds
-required even for exergonic reactions
catalysts: substances that lower the
activation energy of a reaction
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Energy Currency of Cells
ATP = adenosine triphosphate
-the energy “currency” of cells
ATP structure:
-ribose, a 5-carbon sugar
-adenine
-three phosphates
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Energy Currency of Cells
ATP stores energy in the bonds between
phosphates.
Phosphates are highly negative, therefore:
-the phosphates repel each other
-much energy is required to keep the
phosphates bound to each other
-much energy is released when the bond
between two phosphates is broken
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Energy Currency of Cells
When the bond between phosphates is
broken:
ATP
ADP + Pi
energy is released
ADP = adenosine diphosphate
Pi = inorganic phosphate
This reaction is reversible.
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Energy Currency of Cells
The energy released when ATP is broken
down to ADP can be used to fuel
endergonic reactions.
The energy released from an exergonic
reaction can be used to fuel the production
of ATP from ADP + Pi.
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Enzymes
Enzymes: molecules that catalyze reactions
in living cells
-most are proteins
-lower the activation energy required for a
reaction
-are not changed or consumed by the
reaction
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Enzymes
Enzymes interact with substrates.
substrate: molecule that will undergo a
reaction
active site: region of the enzyme that binds
to the substrate
Binding of an enzyme to a substrate causes
the enzyme to change shape, producing a
better induced fit between the molecules.
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Enzymes
Multienzyme complexes offer certain
advantages:
1. The product of one reaction can be
directly delivered to the next enzyme.
2. The possibility of unwanted side reactions
is eliminated.
3. All of the reactions can be controlled as a
unit.
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Enzymes
Not all enzymes are proteins.
Certain reactions involving RNA molecules
are catalyzed by the RNA itself.
ribozymes: RNA with enzymatic abilities
For example, the ribosome is a ribozyme.
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Enzymes
Enzyme function is affected by its
environment.
Factors that can change an enzyme’s 3dimensional shape can change its function.
-for example, pH, temperature, regulatory
molecules
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Enzymes
Temperature
-enzyme activity may be increased with
increasing temp, up to the temp optimum
-temperatures too far above the temp
optimum can denature the enzyme,
destroying its function
pH – most enzymes prefer pH values from 6
to 8.
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Enzymes
Inhibitors are molecules that bind to an
enzyme to decrease enzyme activity.
-competitive inhibitors compete with the
substrate for binding to the same active
site
-noncompetitive inhibitors bind to sites
other than the enzyme’s active site
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Enzymes
Allosteric enzymes exist in either an active
or inactive state.
-possess an allosteric site where
molecules other than the substrate bind
-allosteric inhibitors bind to the allosteric
site to inactivate the enzyme
allosteric activators bind to the allosteric
site to activate the enzyme
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Metabolism
Metabolism: all chemical reactions
occurring in an organism
Anabolism: chemical reactions that expend
energy to make new chemical bonds
Catabolism: chemical reactions that harvest
energy when bonds are broken
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Metabolism
Some enzymes require additional molecules
for proper enzymatic activity.
These molecules could be:
-cofactors: usually metal ions, found in the
active site participating in catalysis
-coenzymes: nonprotein organic molecules,
often used as an electron donor or
acceptor in a redox reaction
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Metabolism
Biochemical pathways are a series of
reactions in which the product of one
reaction becomes the substrate for the
next reaction.
Biochemical pathways are often regulated
by feedback inhibition in which the end
product of the pathway is an allosteric
inhibitor of an earlier enzyme in the
pathway.
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