BIOL 600 Lecture Notes - Lecture 2: Oxidative Phosphorylation, Exergonic Reaction, Metabolic Pathway
26 May 2018
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AP Bio Chapter 9 Cellular Respiration: Harvesting Chemical Energy
Lecture Outline
Overview: Life Is Work
To perform their many tasks, living cells require energy from outside sources.
Energy enters most ecosystems as sunlight and leaves as heat.
Photosynthesis generates oxygen and organic molecules that the mitochondria of
eukaryotes use as fuel for cellular respiration.
Cells harvest the chemical energy stored in organic molecules and use it to regenerate
ATP, the molecule that drives most cellular work.
Respiration has three key pathways: glycolysis, the citric acid cycle, and oxidative
phosphorylation.
Concept 9.1 Catabolic pathways yield energy by oxidizing organic fuels
The arrangement of atoms of organic molecules represents potential energy.
Enzymes catalyze the systematic degradation of organic molecules that are rich in energy
to simpler waste products with less energy.
Some of the released energy is used to do work; the rest is dissipated as heat.
Catabolic metabolic pathways release the energy stored in complex organic molecules.
One type of catabolic process, fermentation, leads to the partial degradation of sugars in
the absence of oxygen.
A more efficient and widespread catabolic process, cellular respiration, consumes oxygen
as a reactant to complete the breakdown of a variety of organic molecules.
o In eukaryotic cells, mitochondria are the site of most of the processes of cellular
respiration.
Cellular respiration is similar in broad principle to the combustion of gasoline in an
automobile engine after oxygen is mixed with hydrocarbon fuel.
o Food is the fuel for respiration. The exhaust is carbon dioxide and water.
The overall process is:
o organic compounds + O2 --> CO2 + H2O + energy (ATP + heat).
Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to
consider glucose.
C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy (ATP + heat)
The catabolism of glucose is exergonic with a ? G of ?686 kcal per mole of glucose.
o Some of this energy is used to produce ATP, which can perform cellular work.
Redox reactions release energy when electrons move closer to electronegative atoms.
Catabolic pathways transfer the electrons stored in food molecules, releasing energy that
is used to synthesize ATP.
Reactions that result in the transfer of one or more electrons from one reactant to another
are oxidation-reduction reactions, or redox reactions.
o The loss of electrons is called oxidation.
o The addition of electrons is called reduction.
The formation of table salt from sodium and chloride is a redox reaction.
o Na + Cl --> Na+ + Cl?
o Here sodium is oxidized and chlorine is reduced (its charge drops from 0 to ?1).
More generally: Xe? + Y --> X + Ye?
o X, the electron donor, is the reducing agent and reduces Y.
o Y, the electron recipient, is the oxidizing agent and oxidizes X.
Redox reactions require both a donor and acceptor.
Redox reactions also occur when the transfer of electrons is not complete but involves a
change in the degree of electron sharing in covalent bonds.
o In the combustion of methane to form water and carbon dioxide, the nonpolar
covalent bonds of methane (C—H) and oxygen (O=O) are converted to polar
covalent bonds (C=O and O—H).
o When methane reacts with oxygen to form carbon dioxide, electrons end up
farther away from the carbon atom and closer to their new covalent partners, the
oxygen atoms, which are very electronegative.
o In effect, the carbon atom has partially “lost” its shared electrons. Thus, methane
has been oxidized.
The two atoms of the oxygen molecule share their electrons equally. When oxygen reacts
with the hydrogen from methane to form water, the electrons of the covalent bonds are
drawn closer to the oxygen.
o In effect, each oxygen atom has partially “gained” electrons, and so the oxygen
molecule has been reduced.
o Oxygen is very electronegative, and is one of the most potent of all oxidizing
agents.
Energy must be added to pull an electron away from an atom.
The more electronegative the atom, the more energy is required to take an electron away
from it.
An electron loses potential energy when it shifts from a less electronegative atom toward
a more electronegative one.
A redox reaction that relocates electrons closer to oxygen, such as the burning of
methane, releases chemical energy that can do work.
The “fall” of electrons during respiration is stepwise, via NAD+ and an electron
transport chain.
Cellular respiration does not oxidize glucose in a single step that transfers all the
hydrogen in the fuel to oxygen at one time.
Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a
specific enzyme.
o At key steps, electrons are stripped from the glucose.
o In many oxidation reactions, the electron is transferred with a proton, as a
hydrogen atom.
The hydrogen atoms are not transferred directly to oxygen but are passed first to a
coenzyme called NAD+ (nicotinamide adenine dinucleotide).
How does NAD+ trap electrons from glucose?
o Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose),
oxidizing it.
o The enzyme passes two electrons and one proton to NAD+.
o The other proton is released as H+ to the surrounding solution.
By receiving two electrons and only one proton, NAD+ has its charge neutralized when it
is reduced to NADH.
o NAD+ functions as the oxidizing agent in many of the redox steps during the
catabolism of glucose.
The electrons carried by NADH have lost very little of their potential energy in this
process.
Each NADH molecule formed during respiration represents stored energy. This energy is
tapped to synthesize ATP as electrons “fall” from NADH to oxygen.
How are electrons extracted from food and stored by NADH finally transferred to
oxygen?
o Unlike the explosive release of heat energy that occurs when H2 and O2 are
combined (with a spark for activation energy), cellular respiration uses an electron
transport chain to break the fall of electrons to O2 into several steps.
The electron transport chain consists of several molecules (primarily proteins) built into
the inner membrane of a mitochondrion.
Electrons released from food are shuttled by NADH to the “top” higher-energy end of the
chain.
At the “bottom” lower-energy end, oxygen captures the electrons along with H+ to form
water.
Electron transfer from NADH to oxygen is an exergonic reaction with a free energy
change of ?53 kcal/mol.
Electrons are passed to increasingly electronegative molecules in the chain until they
reduce oxygen, the most electronegative receptor.
In summary, during cellular respiration, most electrons travel the following “downhill”
route: food --> NADH --> electron transport chain --> oxygen.
These are the stages of cellular respiration: a preview.
Respiration occurs in three metabolic stages: glycolysis, the citric acid cycle, and the
electron transport chain and oxidative phosphorylation.
Glycolysis occurs in the cytoplasm.
o It begins catabolism by breaking glucose into two molecules of pyruvate.
The citric acid cycle occurs in the mitochondrial matrix.
o It completes the breakdown of glucose by oxidizing a derivative of pyruvate to
carbon dioxide.
Several steps in glycolysis and the citric acid cycle are redox reactions in which
dehydrogenase enzymes transfer electrons from substrates to NAD+, forming NADH.
NADH passes these electrons to the electron transport chain.
In the electron transport chain, the electrons move from molecule to molecule until they
combine with molecular oxygen and hydrogen ions to form water.
Document Summary
Ap bio chapter 9 cellular respiration: harvesting chemical energy. To perform their many tasks, living cells require energy from outside sources. Energy enters most ecosystems as sunlight and leaves as heat. Photosynthesis generates oxygen and organic molecules that the mitochondria of eukaryotes use as fuel for cellular respiration. Cells harvest the chemical energy stored in organic molecules and use it to regenerate. Atp, the molecule that drives most cellular work. Respiration has three key pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation. Concept 9. 1 catabolic pathways yield energy by oxidizing organic fuels. The arrangement of atoms of organic molecules represents potential energy. Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler waste products with less energy. Some of the released energy is used to do work; the rest is dissipated as heat. Catabolic metabolic pathways release the energy stored in complex organic molecules.
