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Chapter 9: Cellular Respiration
AP Biology
• Chemical E exists in the
arrangement of atoms in
organic compounds
• Cellular respiration is a
catabolic pathway
• To replenish ATP in cell,
food molecules are
constantly broken down
• Enzyme mediated reaction
Cellular Respiration
• Oxidation of food molecules,
releasing energy using an
inorganic substance as the
final electron acceptor.
• Usually oxygen is the final
electron acceptor (aerobic)
Respiration W/O O2
 Anaerobic respiration: uses nitrate
or sulfate as final electron acceptor
 Fermentation: the anaerobic
breakdown of food molecules in
which the final e- acceptor is an
organic molecule
Oxidation and Reduction
• E is gained by the transfer of e’s
• The relocation of e-’s releases the
stored E and the E ultimately
makes ATP
Redox Reactions
• Oxidation: the loss of e-’s
from a substance
• Reduction: the gain of e-’s
by a substance
• E must be added to pull e-’s
from an atom
Redox Reactions
• A redox reaction that
relocates an e- from a less
electronegative atom to a
more electronegative atom
loses potential E
Cellular Respiration
C6H12O6 + 6O2  6H2O + 6CO2 + E
• In most redox reactions, it is
not only the e- that is
transferred but in most
biological reactions the whole
hydrogen atom is transferred
Carbohydrates and Fats
• Contain high levels of hydrogen
and their electrons
• There is a barrier that keeps sugar
from combining immediately with
• This barrier is reduced inside the
body with the help of enzymes
• Main function: produce 3C
pyruvate molecule from
• Takes place in cytoplasm
(cytosol) of the cell
Steps of Glycolysis
Invest 2 molecules of ATP to donate two
phosphate groups to ends of glucose
molecule creating instability
2. 6C molecule split into two 3C molecules
with phosphate group attached(Pi comes
from ATP)
3. 3C molecule receives an additional Pi
group from cytosol, not from ATP
Steps of Glycolysis
Each 3C molecule gives up
H atom to form two NADH
5.Two 3C molecules lose all
phosphate groups to ADP
forming 4 ATP molecules
Results of Glycolysis
• Two 3C pyruvate molecules
• 2 NADH molecules
• Net of 2 ATP's (4 created - 2
invested in the beginning)
Mitochondria Structure
• Outer membrane very
• Inner membrane: selectively
permeable, pyruvate
molecules can diffuse in
• Matrix: inside inner membrane,
protein-rich solution. Many enzymes
of Citric Acid cycle dissolved in
matrix fluid, rest are attached to inner
membrane surface
• ETS molecules also located on inner
membrane surface
• Inner surface of mitochondria similar
to the plasma membrane of bacteria
 Series of redox reactions using 02 as
the final e- acceptor that breaks down
organic molecules and releases their
energy that was stored as covalent
• Occurs inside inner membrane of
mitochondria in eukaryotes and the
plasma membrane of prokaryotes
Preparation of Acetyl Coenzyme A
• 2 pyruvate molecules formed by
glycolysis enter mitochondrial matrix
• Steps
–Pyruvate loses one C and two O atoms
as a CO2 molecule
–Remaining 2C acetyl group is attached
to CoA molecule forming acetyl CoA
–Reduction of NAD+ molecule to
Citric Acid Cycle
• Also known as Krebs
Cycle, named after the
founder Sir Hans Krebs
who worked out the cycle
in 1937
Cyclic Nature of Citric Acid Cycle
• CoA transfers its 2C acetyl group
to a 4C molecule of oxaloacetate
forming a 6C citrate molecule
• Throughout the cycle, the 6C
citrate gives up two C atoms in the
form of CO2
• Remaining 4C molecule is
converted into oxaloacetate,
ready to accept another
acetyl group from CoA
Important Features
• H atoms are removed during the cycle
and picked up by NAD+ and FAD. H
are used in oxidative phosphorylation
to power formation of most ATP
• 1 ATP molecule is formed by
substrate level phosphorylation during
each turn of cycle (net per glucose = 2
• After 2 cycles, all 6 C
molecules of original glucose
are given off as CO2. 1C is
lost from each pyruvate to
make acetyl groups and 2C’s
from Krebs Cycle
Oxidative Phosphorylation
• Some E has been made by substrate
level phosphorylation of ATP during
glycolysis and the Citric Acid Cycle
• Most E is now in the form of H atoms
carried by NADH and FADH2
• Oxidative Phosphorylation
couples the oxidation of H to
ATP synthesis by using an
electrochemical gradient as an
E intermediate
Electron Transport Chain
• Complexes mostly made of proteins
that are embedded on the
mitochondrial inner membrane
• Multiprotein complexes numbered IIV
• Prosthetic groups: non-protein
components essential to certain
• ETC makes no ATP directly
Actions of the ETC
• Electron carriers of the ETC are
reduced and oxidized as they
accept and donate e-‘s
• 1st carrier of chain has lowest
e- affinity and last carrier has
the highest
• NADH passes e- to 1st
complex of the ETC called a
• FADH2 passes e- to 2nd
complex of ETC (1/3 less E
than NADH)
• Most electron carriers after
Complex I are called
• When the carriers of the 4
complexes are reduced they
attract H+ from the matrix and
transfer them into the
intermembrane space
• Use of a H+ gradient across a
membrane used to drive cellular
• Consists of a series of carrier molecules
in the inner mitochondrial membrane.
Some carry H atoms, some only carry
• FMN and CoQ carry 2 H atoms each
• Cytochromes: proteins containing heme
carry only e-'s
• Groups of carrier molecules
have heigthening levels of eaffinity ending at O which has
the highest e- affinity of all
• Oxygen dissolves into the
mitochondria and becomes
sandwiched b/w a heme ion and
copper ion in the last
cytochrome of the ETS
H+ Gradient & Phosphorylation
 The H+ gradient is a store of
elctrochemical potential E
 Chemical: concentration
difference of H+
• Electrical: difference of charges
on either side of a membrane
 Protonmotive force: H+ moving
down the [gradient]
 Chemiosmotic ATP synthesis:
coupling of H+ flowing down
[gradient] and phosphorylation of
• H+ [gradient] powers ATP
synthesis indirectly by freeing
ATP sythetase's active site to make
more ATP
• H+ potential E is stockpiled inside the
inner membrane of mitochondria via
redox reactions
• E-'s from H atoms are passed to ETS and
uses the energy to pump H+ through
membrane against its [gradient]
• At end, e-'s are passed to O2 which picks
up some H+ to form H2O
• Potential E of [gradient] used
to make ATP
• H+ recrosses membrane via
channel proteins connected to
ATP synthetase
Energy Yield of Glucose
 Glycolysis: 2 ATP
 Citric Acid Cycle: 2 ATP
• Equals: 4 ATP produced by
substrate level phosphorylation
 Pair of e-'s from NADH 3
 Pair of e-'s from FADH2 
 ETS: 34 ATP created by
oxidative phosphorylation
• Maximum total: 38ATP
 If a cell runs out of O2, all the ecarriers are stuck in reduced form,
halting system
• Pyruvate produced by glycolysis
acts as alternative acceptor of H
from NADH, keeping glycolysis
going to allow small ATP
Alcoholic Fermentation
• Yeasts break down sugar into
• Each pyruvate is dismantled into a
molecule of CO2 and a 2C compound
• Acetaldehyde is reduced by accepting
2H's from NADH and H+ forming 2C
alcohol ethanol (ethyl alcohol)
Lactic Acid Fermentation
• Occurs during strenuous exercise
• Pyruvate from glycolysis is reduced
by accepting hydrogens from NADH
and H+
• Pyruvate converted into 3C
compound, lactate
There are three metabolic pathways that provide
the energy for all human action. These “metabolic
engines” are known as the phosphagen pathway,
the glycolytic pathway, and the oxidative
pathway.The first, the phosphagen, dominates the
highest-powered activities, those that last less
than about ten seconds. The second pathway, the
glycolytic, dominates moderate-powered
activities, those that lastup to several minutes.
The third pathway, the oxidative, dominates lowpowered activities, those that last in excess of
several minutes.