20.2 Energy and Biochemical Reactions
• Reactions in living organisms are no different from
reactions in a chemistry laboratory. Both follow the
same laws and have the same energy requirements.
• Spontaneous reactions release free energy, which is
available to do work. DG = DH – TDS
• Such favorable reactions, described as exergonic, are
the source of our biochemical energy.
• Exergonic applies to the release of free energy,
represented by a negative DG, Exothermic applies to
the release of heat, represented by a negative DH .
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• (a) In a favorable reaction, the products have
less energy than the reactants.
• (b) In an unfavorable reaction, the products
have more energy than the reactants.
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20.3 Cells and Their Structure
• There are two main categories of cells:
– prokaryotic cells, usually found in single celled
organisms including bacteria and blue-green algae
– eukaryotic cells, found in some single-celled
organisms and all plants and animals.
• Eukaryotic cells are about 1000 times larger
than bacterial cells, have a membrane
enclosed nucleus that contains their DNA, and
include several other kinds of internal
structures known as organelles—small,
functional units that perform specialized tasks.
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20.4 An Overview of Metabolism and Energy
Production
• Catabolism: Metabolic reaction pathways that break down food
molecules and release biochemical energy.
• Anabolism: Metabolic reactions that build larger biological
molecules from smaller pieces.
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20.5 Strategies of Metabolism: ATP
and Energy Transfer
Removal of one phosphate group from ATP by
hydrolysis gives adenosine diphosphate (ADP). The
reaction is exergonic; it releases chemical energy that
was held in the bond to the phosphate group.
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• ATP is an energy transporter because its production
from ADP requires an input of energy that is then
released wherever the reverse reaction occurs.
• Biochemical energy is gathered from exergonic
reactions that produce ATP. The ATP then travels to
where energy is needed, and ATP hydrolysis releases
the energy for whatever work must take place.
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20.6 Strategies of Metabolism: Metabolic
Pathways and Coupled Reactions
• Not every individual step in every metabolic pathway is
spontaneous.
• The metabolic strategy for dealing with what would be an
energetically unfavorable reaction is to couple it with an
energetically favorable reaction so that the overall energy
change for the two reactions is favorable.
• The principle of coupling must be put to use in the endergonic
synthesis of ATP from ADP which has DG = +7.3 kcal/mol .
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If the formation of ATP is coupled with the
hydrolysis of phosphoenolpyruvate, a phosphate
of higher energy than ATP, then the overall
reaction is transfer of a phosphoryl group from
phosphoenolpyruvate to ADP which is favorable
with DG = - 7.5 kcal/mol.
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20.7 Strategies of Metabolism: Oxidized and
Reduced Coenzymes
• The net result of catabolism is the
oxidation of food to release energy.
• Many metabolic reactions are
oxidation–reduction reactions.
• Oxidation can be loss of electrons, loss
of hydrogen, or gain of oxygen.
• Reduction can be gain of electrons,
gain of hydrogen, or loss of oxygen.
• Oxidation and reduction always occur
together.
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A steady supply of oxidizing and reducing agents must
be available, so a few coenzymes continuously cycle
between their oxidized and reduced forms.
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• The reduction of NAD+ occurs by addition of H- to
the ring in the nicotinamide part of the structure,
where the two electrons of H- form a covalent
bond.
• The second H atom removed from the oxidized
substrate enters the surrounding solution as H+.
• The product of NAD+ reduction is often
represented as NADH/ H+ to show that 2 H atoms
have been removed from the reactant.
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