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Cellular Respiration:
Harvesting Chemical
Energy
Chapter 9
Energy in Natural Systems
• Energy stored in organic molecules (food)
ultimately comes from the sun
Catabolic Pathways and ATP
• Organic compounds have potential
energy because of the arrangement of
their atoms
• Compounds in exergonic reactions can
be seen as fuels
• Some of those fuels can be used to do
cellular work and some is lost as heat
Catabolic Pathways and ATP
• Aerobic Respiration – oxygen is
consumed as a reactant along with the
sugar
– Most prevalent and efficient
– Most eukaryotes and some prokaryotes
• C6H1206 + 602  6CO2 + 6H2O + Energy
(ATP + Heat)
• Glucose is fuel used most often (can also
be fueled by other carbs, lipids, etc.)
• Exergonic reaction
Catabolic Pathways and ATP
• Anaerobic Respiration – an item other
than oxygen in consumed
• Fermentation – catabolic process that
partially degrades sugar without the use
of oxygen
– Yeast
• Cellular Respiration – includes both
aerobic and anaerobic respiration
• These catabolic pathways regenerate
ATP to complete cellular work
Redox Reactions
• Redox reactions occur during a
chemical reaction when one reactant
transfers electrons to another reactant
– Oxidation – the loss of electrons from one
item (the electron donor is called the
reducing agent)
– Reduction – the addition of electrons to
another (the electron acceptor is called
the oxidizing agent)
• Some redox reactions change the
degree of sharing in covalent bonds
Redox Reactions
Redox Reactions
• Those items that are more electronegative
will be stronger oxidizing agents (ie. Oxygen)
• Energy must be added to pull an electron
away
• More energy is required the more
electronegative the atom
• The move of an electron towards a more
electronegative item decreases the amount of
potential energy and releases chemical
energy that can be put to work
Redox Reactions
• In biology, cellular respiration is a redox
reaction
– Glucose is oxidized to carbon dioxide
– Oxygen is reduced to water
• The oxidation of glucose transfers
electrons to a lower energy state,
liberating energy that is now available
for ATP synthesis
• Hydrogen is an excellent source for
these electrons; therefore carbohydrates
and fats are our best fuel sources
Respiration and Electrons
• Cellular respiration oxidizes glucose
and other organic fuels through
several steps where each is catalyzed
by an enzyme.
• At key steps, electrons are stripped
from the glucose via hydrogen atoms
– Hydrogen is passed via an electron
carrier (coenzyme) called NAD+ which
acts as an oxidizing agent
Functioning of NAD+
• NAD+ traps electrons via enzymes called dehydrogenases
which remove Hydrogen atoms from the glucose (or other
substrate) oxidizing it.
• NAD+ receives 2 electrons and 1 proton (the other proton is
released) creating NADH which helps to store energy for
making ATP later
Electron Transport
• Hydrogen atoms are harnessed via NADH for their
electrons, but to help control the reaction between hydrogen
(electrons) and oxygen an Electron Transport Chain is used.
• Electron transfer from NADH to oxygen is exergonic
• Electrons cascade down the chain from one carrier
molecule to another; each step is slightly more
electronegative
Cellular Respiration
(Aerobic)
• 3 stages:
– Glycolysis
– The citric acid cycle / Krebs Cycle
– Oxidative phosphorylation: Electron Transport
Chain and chemiosmosis
Glycolysis
• Process of “splitting sugar” into 2
molecules of pyruvate
– First the glucose is split into 2 3-carbon
sugars which are then oxidized into
pyruvate
• Occurs in the cytosol
• Begins cellular respiration (occurs in
both aerobic and anaerobic pathways)
Glycolysis
• Can be split into 2
phases
– Energy
Investment: uses 2
ATP
– Energy Payoff:
creates 4 ATP
• uses substrate
level
phosphorylation
• NAD+ gains
electrons
becoming NADH
Pyruvate and Acetyl CoA
• Glycolysis only releases about ¼ of chemical energy
stored in glucose. The rest is still in pyruvate.
• In aerobic respiration, pyruvate will enter
mitochondria and be converted to Acetyl CoA
– Carboxyl oxidized releasing carbon dioxide
– Two carbon fragment oxidized forming acetate and
transferring electrons to NADH
– Coenzyme A is attached to the acetate
Citric Acid Cycle / Krebs
• Acetyl CoA has high
potential energy
• Pyruvate (2 per glucose)
is broken down to 3
carbon dioxides
• Produces 1 ATP per turn
• Most energy is
transferred to NAD+ and
FAD
– NADH and FADH2 are
then transferred to the
ETC
Electron Transport Chain
• Collection of molecules
embedded in the inner
membrane of the
mitochondria
– Largely proteins, but also
have prosthetic groups which
are nonprotein compounds
• Each component becomes
reduced as it accepts
electrons from its ‘uphill’
neighbor…. Each step down
is more electronegative
Chemiosmosis
• In the inner membrane of the
mitochondria are many copies of
the protein complex ATP
synthase (ATP synthetase)
– Enzyme that makes ATP from ADP
• Power source for the ATP
synthase is a difference in
concentration of H+ on opposite
sides of the mitochondrial
membrane
• This is chemiosmosis (referring to
the flow of H+ across a
membrane)
Chemiosmosis
Energy Yield
Anaerobic Respiration vs.
Fermentation
• Anaerobic Respiration – uses an
electron transport chain, but does not
use oxygen as a final electron
acceptor
– May use sulfate ion
• Fermentation – harvests energy
without using oxygen or an ETC
– An extension of glycolysis that allows
ATP to be continually made
– Replaces NAD+ for glycolysis to continue
Fermentation
• Alcoholic – pyruvate is
converted to ethyl alcohol
– Releases carbon dioxide
– Used in winemaking, brewing
and baking
– Bacteria (yeast)
• Lactic Acid – pyruvate is
reduced directly by NADH
to form lactate
– No release of carbon dioxide
– Used to make yogurt and
cheese
– Fungi and bacteria
Beyond Glucose
• Your body can use a variety
of carbohydrates to
complete cellular respiration
– Must be broken down to
glucose (monomer)
• Proteins must be broken
down to amino acids and
amino groups must be
removed
• Fats broken into glycerol
and fatty acid
– Fatty acid must go through
beta oxidation to break down
Feedback
Mechanisms
in Cellular
Respiration