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Cellular Respiration
Section 5-3
Cellular Respiration Produces ATP
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Before you can use the energy you
obtain from food, it has to be transferred
to ATP.
Glucose’s energy (and other organic
compounds) is transferred to ATP
through cellular respiration.
Oxygen makes the production of ATP
more efficient, but some ATP is produced
without oxygen.
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Aerobic Respiration – metabolic
processes that require oxygen
Anaerobic respiration – metabolic
processes that do not require
oxygen
Overview of Cellular Respiration
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Figure 5-10 p. 104
STAGE 1: Glucose is converted to pyruvate, producing a
small amount of ATP and NADH.
STAGE 2: When oxygen is present, pyruvate and NADH
are used to make a large amount of ATP (aerobic
respiration). Aerobic respiration occurs in the
mitochondria in eukaryotic cells and in the cell
membrane of prokaryotic cells. When oxygen is not
present, pyruvate is converted to either lactate or
ethanol and CO2 (anaerobic respiration).
STAGE 1: Glycolysis
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Glycolysis – the breakdown of glucose
The primary fuel for cellular respiration is
glucose.
Glucose formed when carbohydrates
(starch and sucrose) are broken down.
If too few carbohydrates are available,
other molecules such as fats can be broken
down to make ATP. (1 gram of fat = 2
grams carbohydrates)
STAGE 1: Glycolysis
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Glycolysis – glucose is being broken
down in the cytoplasm. It is an enzymeassisted anaerobic process that breaks
down one 6-carbon molecule of glucose
to two 3-carbon pyruvates.
Pyruvate – the ion of a 3-carbon organic
acid called pyruvic acid
As glucose is broken down, some of its
hydrogen atoms are transferred to an
electron acceptor called NAD+. This
forms the electron carrier called NADH.
STAGE 1: Glycolysis
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For cellular respiration to
continue, the electrons carried by
NADH are eventually donated to
other organic compounds. This
recycles NAD+ making it
available to accept more
electrons.
STAGE 1: Glycolysis
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Summary of Glycolysis Figure 5-11 p. 105
Step 1: Phosphate groups from 2 ATP molecules
transferred to a glucose molecule
Step 2: Resulting 6-carbon compound broken
down to 2 3-carbon compounds, each with a
phosphate group.
Step 3: 2 NADH molecules produced. 1 more
phosphate group transferred to each 3-carbon
compound.
Step 4: Each 3-carbon compound is converted to
a 3-carbon pyruvate. Produces 4 ATP molecules.
Process uses 2 ATP, produces 4 ATP, so… NET
GAIN OF 2 ATP.
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Glycolysis is followed by another set
of reactions that use the energy
temporarily stored in NADH to make
more ATP.
STAGE 2: Aerobic Respiration
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When oxygen is present, pyruvate
produced during glycolysis enters a
mitochondrion. Then pyruvate is
converted to a 2-carbon compound.
When pyruvate is converted to a 2-carbon
compound – 1 CO2, 1 NADH, and 2carbon acetyl group is produced.
Acetyl group is attached to a molecule
called coenzyme A (CoA) – forms acetylCoA
STAGE 2: Aerobic Respiration –
Kreb’s Cycle
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Acetyl-CoA enters a series of enzymeassisted reactions – Kreb’s cycle
Step 1: Acetyl-CoA combines with a 4carbon compound, forming a 6-carbon
compound and releasing coenzyme-A
Step 2: CO2 released from 6-carbon
compound, forming 5-carbon compound.
Electrons transferred to NAD+, making a
molecule of NADH.
STAGE 2: Aerobic Respiration –
Kreb’s Cycle
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Step 3: CO2 released from 5-carbon
compound, forming a 4-carbon
compound. 1 ATP and 1 NADH made.
Step 4: 4-carbon compund converted to a
new 4-carbon compound. Electrons
transferred to an electron acceptor FAD,
making a molecule of FADH2 (another
type of electron carrier).
Step 5: New 4-carbon compound then
converted to the 4-carbon compound that
began cycle. Another NADH is produced.
STAGE 2: Aerobic Respiration –
Kreb’s Cycle
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NADH and FADH2 now contain much
of the energy that was previously
stored in glucose and pyruvate.
Recycles the 4-carbon compound.
STAGE 2: Aerobic Respiration –
Electron Transport Chain
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The electrons donated by NADH and
FADH2 pass through an electron transport
chain.
Figure 5-13 p. 107
Occurs in the inner membranes of
mitochondria
Energy of the electrons used to pump H+
out of inner mitochondrial compartments
H+ accumulates in outer compartment –
produces a concentration gradient across
the inner membrane
STAGE 2: Aerobic Respiration –
Electron Transport Chain
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H+ diffuses back into inner compartment
through a carrier protein that adds a
phosphate group to ADP to make ATP.
At end of electron transport chain, H+
and spent electrons combine with O2 to
form H2O – oxygen is final electron
acceptor.
Fermentation
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Fermentation follows glycolysis in the
absence of oxygen. – anaerobic
respiration
When enough oxygen is not present for
aerobic respiration to occur, electron
transport chain does not function. Why?
Oxygen is not able to serve as final
electron acceptor. Also, NADH electrons
not transferred, so NAD+ cannot be
recycled.
Fermentation
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Anaerobic Respiration – NAD+ recycled in
a different way. Electrons carried by
NADH are transferred to pyruvate that is
produced in glycolysis. This recycles
NAD+ so it can continue making ATP
through glycolysis.
Fermentation – recycling NAD+ using an
organic hydrogen acceptor
Lactic Acid Fermentation
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3-carbon pyruvate converted to a 3carbon lactate. NAD+ recycled.
Lactate – ion of organic acid called lactic
acid
During vigorous exercise, pyruvate in
muscles converted to lactate when
muscles operate without enough oxygen.
Causes soreness because it builds up in
muscles – blood does not remove it fast
enough
Alcoholic Fermentation
3-carbon pyruvate converted to a 2carbon ethanol molcule. CO2 is released.
 2 Step Process:
1. Pyruvate converted to a 2-carbon
compound – CO2 released.
2. Electrons transferred from NADH to
the 2-carbon compound producing
ethanol.
o NAD+ recycled
o Yeast or fungi – foods and beverages
(wine, beer, rising of bread dough)
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